Immersion Heater - PV electricity

[methley]

If you turn your argument on it's head, you could just as easily run upstair and turn the immersion on every time the sun shines!

 

[inie meanie]

There are several simple controllers, like the SolarHeat controller I am marketing, which operate on PV output and a modulating one is being supplied with a monitoring system and PV panels by Engensa. I expect that others will be available later this year as Paul's colleagues on OpenEnergyMonitor seem to have cracked it. They all seem to use CT's or similar but the tricky bit is getting the switching or modulation right as reducing output by half reduces the heat by a quarter (IxIxR). Not much point in stopping the output to the grid if it doesn't actually do anything useful.

Err are you suggestig that if we reduce the power to the immersion heater by half that we will only get a quarter of the heat? If you're not then I apologise for the rest of this comment. You use I x I X R for heat disapation but we are not controling on current we are controling power. If you only put 1500 watts into the water it will take directly twice as long to heat up as 3000 watts, not 4 times. If it did take 4 times as long where did the extra energy go? You can not create or destroy energy only convert it.

 

[echase]

I have been using this switch ( Business Building Kits items in Fair Trade Hub Shop store on eBay!) Alternately, when i want to turn on the high loads like washing machine during the day time, the inside neon switch to the immersion is turned off. Will get a full tank with 3 or 4 hours of sun shine and I'm saving on gas bill as well.

These people are claiming far more than they can deliver. As I said before, even if you track the export properly by using proportional control you only get about £50 - £60 off your annual gas bill, assuming a 3-4kW PV system and an efficient gas water heating system (no pilot light, short pipes and condensing boiler). Let’s say it is £100 for all systems averaged, including the inefficient gas systems like mine and oil boilers. Any of these sort of switching units can’t come close to saving £100. If you use a 1kW element there will be many times you are underusing the available export and some times you import. You have to pay £60 or more for the element on top of controller cost. If you use a 3kW element you will be importing so much that the benefit will be negligible or even negative.

Plus who wants to bother with turning off the immersion every time they boil a kettle, etc?

Let’s have the data to prove otherwise if you really can prove that these switching units can recover their cost in less than 10 years. I have 9 months of data measured every 5 minutes for 2 samples of mine which shows a much shorter payback but I don’t make any specific payback claim as people’s houses are so different.

 

[TechyTim]

Inie,
If you are controlling power directly, eg with a Triac, then you are correct. If you are controlling output by reducing the voltage (as people have done with transformers) the the output is a square law as the heater resistance is fixed.
I am told that The biggest issue with Traics in commercial kit is the EMC regulations and harmonics generated which can be solved at a cost but with annual savings of £50 -£100 it may not be cost effective.

 

[TechyTim]

echase:
"Let’s have the data to prove otherwise if you really can prove that these switching units can recover their cost in less than 10 years. I have 9 months of data measured every 5 minutes for 2 samples of mine which shows a much shorter payback but I don’t make any specific payback claim as people’s houses are so different. "

I think, and this wil only be proved when I have some data in a couple of months, that you have mised the interaction between the PV output and the size of panel. What you say would be true if the output was fixed at 1kW but if the controller is on a 2.9kW system, the output in the summer will continue to increase once it has passed the threshold, reducing the import. I am suggesting that these switches are used in the summer only.

I wonder how the cost of your proportional system compares with the potential annual saving of £50 - £100?

 

[robassais]

so the 4p/kWh is 6.7 p/kWh.

for a system on the south coast which have much higher output figures than the MCS values based on the UK average,

QUOTE]

Let’s use your 6.7p then. With the 10 panel system you quote the MCS approved annual generation is about 2000kWhr. If all that went into heating water you have displaced 2000kWhr of gas at about 6.7p per unit = £134. Due to the house load only about half is available to the immersion so it’s down to £67. Your unit does not track the export and is turned off in winter so halve it again to £33, e.g. for significant periods of time the PV output will be above the house base load of a few hundred watts but below 1.5kW so that energy is exported.

So about 2000/2/2= 500kWhr has gone into the heater which has run for 500/1.5 hours = 333 hours at 1.5kW. Then you are importing electricity to make up the remaining 1.5kw. So every time you have the immersion on you are importing full price electricity up to 1.5kw at a value = 12pence, which would otherwise have been heated by gas at 6.7p. So it’s an extra charge of 333 x (12-6.7) x 1.5 = £26. So the net benefit is £33-£26=£7. That is a bit pessimistic as if the spare electricity is >>1.5Kw less is imported, but does a 10 panel system often exceeds 1.5kW by much?

I live near the south coast and my system does not exceed the MCS by a huge amount. Maybe 10-20%.

ANSWER = YES A 3.3 Kw system should achieve 1.6Kw average in December the worst month and 13Kw average in June the best month assuming system losses of 25%

 

[echase]

ANSWER = YES A 3.3 Kw system should achieve 1.6Kw average in December the worst month and 13Kw average in June the best month assuming system losses of 25%

Answer to what question please? You have your units wrong. Did you mean kWhr average per day? What system losses (inverter, immersion, panels, wiring?)?

Crudely calculated the average of 1.6 kWhr and 13 kWhr is 5.7 kWhr; times 365 = 2080 kWhr per year. Seems about right according to government. A 1kW switching unit is very unlikely to deliver more than a quarter of that to the immersion with the rest exported or used in house, so it’s maybe saved gas worth 2080 x 0.067/5 = £34pa. A 3kW switch will loose all that benefit in the many periods it is importing to part feed the immersion, unless you have huge panel area. A proportional controller is required.

A 3.3kW system only delivers more than 1.5kw for maybe 2-4 hours in middle of each sunny summer day so immersion is not going to be on for much time. But I stress I am guessing here. Need to check my recorded data. Definitely not 13kW.

 

[echase]

echase:

I wonder how the cost of your proportional system compares with the potential annual saving of £50 - £100?

No more expensive than ones like yours, once cost of adding a 1kW element is added in. If there is any small extra cost it would easily pay back in a year due to increased power into the immersion. But all other proportional designs that I know of are significantly more expensive because they use different design concepts to mine, which needs the use of more expensive components and/or a more complex installation. But except for the expensive EMMA I am sure their payback period is shorter than any switched design. I ignore here any one offs made using salvaged/eBay components as clearly that is a way to potentially cut costs.

Having said all that I am pulling out of delivering kits as the design has been handed over for commercial production by a firm to be announced. This was always my intention as this firm encouraged me to do this development on the back of an interest in taking it on if I succeeded. It does not incorporate any circuit design ideas picked up here, apart from using current transformers mentioned here. So very sorry to those who contacted me very recently but I can not do new kit deliveries beyond the ones I have already promised. (Those of you here promoting other designs may now rejoice!)

I will continue to support my existing units out there. These have been useful Beta test units to prove the design works well and is reliable. No one has ever said they regret their choice. I will be contacting early users soon to offer the latest design update.

Thanks for the encouragement/answers from you all. Particular thanks to Paul for starting this very successful thread.

 

[Whip1971]

Good luck. Hope you do well. I'm in the process of making pmcalli circuit.

 

[echase]

A 3.3kW system only delivers more than 1.5kw for maybe 2-4 hours in middle of each sunny summer day so immersion is not going to be on for much time. But I stress I am guessing here. Need to check my recorded data. Definitely not 13kW.

My 2-4 hours guess was not right for mostly sunny days but is for mostly cloudy days where often there are quite short spikes in middle of day (see last 2 graphs). Only about 10% of spring/summer days are nearly fully sunny.

Here are 2 plots of PV o/p for above average spring/summer UK southern weather. First one is a few days ago and had it been fully sunny 1.5kW would have been exceeded 07.30 to 14.00 (6.5 hours). The clouds though have limited this to about 5 hours. The total power delivered is 14.7kWhr according to the software. Turning a 1kW element on for 5 hours uses 5kWhr. A good proportional controller will use everything above the 500W line if base load plus controller threshold is assumed to be 500W on average. The graph area below 500W is about 5 kWhr which goes to house or export and 14.7 – 5 = 9.7 goes to immersion. That is twice that of stepped controller.


View attachment 11377
Second one is for June (2011), the best month, and notice that whilst the hours above 1.5kW are slightly wider it’s not that dramatic, perhaps one more hour. Leave it to you to work out how much different controllers divert to an immersion.

Don’t take too much notice of the absolute time and kW here as the monitoring system was not set for BST properly and the inverter was uprated between the 2 graphs, so top one is with more efficient/powerful inverter. My 3.7kW faces ESE so peaks at 11am GMT and probably is nearly equivalent to a 3.3kW south facing system.


View attachment 11378

Here is my pick of an average day throughout the year. 1.5kW is exceeded rarely and erratically meaning any mechanical relay has to wear itself out by coming on and off regularly, or stay off to save wear and so miss the short peaks. A solid state proportional relay can track reliably and quickly.

 

[dennis2011]

I have read the whole of this thread and found it fascinating. I am not an electrician but have a scientific background. It occurs to me that with a maximum saving of only around 50p per day, during the summer months, the level of sophistication you need with any system depends not only your HW usage but also what kind of installation is possible at an economic price.
We have a recently installed 3.92 kW SE facing PV system, with a newly acquired ‘freebie’ display unit. (Referral gift). There are only two of us in the house most of the time. We are retired and are at home quite a lot especially in the winter. Our ‘background’ power usage is around 300w. We probably use more than half of the electricity we generate during the winter. We heat hot water using gas CH boiler linked to a DPS Pandora Heatbank system which gives mains pressure HW to all taps and showers.
We have no immersion heater whatsoever. There is not even a power point in the airing cupboard; neither are there any spare ‘ways’ on the consumer unit. Changing this for a larger one would be quite expensive. However looking on the bright side, when I ordered my thermal store a few years ago I had the cost of a standard immersion heater deducted of the bill and they fitted a blanking plug instead. The tank is 150 litres in capacity and does have an immersion heater point half way up the tank – so a short 280mm heater, like the ones from the SureJust range, (probably go for 1000w), could be fitted horizontally and heat half the tank at least. I have checked with DPS and they see no problem – so just drain, fit heater, refill, add inhibitor and plumbing side of job done.
I have checked for a suitable method of getting power to the airing cupboard and the utility room below has power sockets on a 32A short ring main serving just the utility, dining room and one bedroom – so I propose to take a spur from one of the sockets then to a switched FCU and then up the wall, through the ceiling and up through one of the floorboards into the airing cupboard finishing with a twin 13amp socket on the inside of airing cupboard wall.
I need some advice please as to what I put in between the FCU in the utility and the twin socket in the airing cupboard.I am thinking along the following lines –
from the FCU go to a socket box mounted timeswitch, like Grasslin 24 hr, then next up to something like Legrand 20A dual bath/sink immersion switch. This could both isolate the immersion heater (irrespective of timeswitch setting), and also divert the electricity supply direct to the immersion heater, 1000w, in the ‘bath’ position or alternatively to the next box up on my wall container a ‘power halver’ which would operate at 500w, in the ‘sink’ position. I am still waiting to hear back from Abeltronics but my wife is hoping for the sake of appearance/neatness that I can put the ‘power halver’ in something like a galvanised cooker back box, 47mm deep, mounted vertically and covered with a standard brushed steel blanking plate. (Drilling ventilation holes top and bottom if needed)
My way of operating this proposed ‘system’ could only generously be described at ‘semi-automatic’. I would set the time clock to come on say 1 hour after dawn and go off say 3 hours before sunset. The system would start each day on 500w and when the inverter was producing over 1500w then manually switch the heater to 1000W. If it became cloudy then manually back down to 500w. If we went out for the day unless certain to be cloud free would leave on 500w.
You might be wondering why the twin socket in the airing cupboard and not simply a wired in connection. Am I correct in thinking that immersion heaters 2000w or less can be fitted with a 3 pin plug and run from a plug socket? I may use the other plug socket for circulation of water in the tank so that the immersion heater could possibly heat the whole tank.
Due to several bad admin errors when I ordered my DPS system I had fitted on the input side of my heatstore cylinder a ‘quick recovery system’ instead of the more usual indirect coil. As compensation I was not charged extra for this. It is basically another plate exchanger on the input side operated by a Wylo pump. DPS say they see no reason why I couldn’t utilise this pump, if needed, to circulate the HW heated electrically. Whether I need to this I don’t know yet as I could run the top part of the tank at 85 C if needed and it would still come out at 50 C through the mixing valve and this might give us enough hot water.
So what do you think of my system? The inputs of Mr Robinson and Mr Heath are quite evident! but I am somewhat ‘snookered’ in that I can’t easily install a fully automatic system, at least I can’t at an economic price.
What I like about my design is –
I will have for first time an emergency use immersion heater

Total installation cost is quite cheap
I can control it from down stairs without having to use my savings towards buying a new stair carpet!
What I don’t like about my system is –
Too many sockets/boxes on the wall
Need to change time clock at least every month to allow for different day lengths.
Danger of leaving set at end of day in 1000w position so starts next day at 1000w! Could really do with a warning light when in 1000w ‘bath’ but nothing in 500w ‘sink’ position.
All comments and suggestions most welcome – and thanks for your time reading this.

 

[inie meanie]

Inie,
If you are controlling power directly, eg with a Triac, then you are correct. If you are controlling output by reducing the voltage (as people have done with transformers) the the output is a square law as the heater resistance is fixed.
I am told that The biggest issue with Traics in commercial kit is the EMC regulations and harmonics generated which can be solved at a cost but with annual savings of £50 -£100 it may not be cost effective.

Yes but we are all dealing with power measurement and control, we are not spinning a motor.
I would have thought that every body here is using triacs. You can purchase a two stage filter up to 20 amps for £10 so not a big deal. If you make your controler effective it will have a shorter payback. A little bit more effort in the design will reap rewards in the long term. Buy cheap buy twice!

 

[dennis2011]

Just looked at this Fair Trade Gadget - at £168 it doesn't look good value. With a 3 kW system lots of time with cloud around you will often be in the 1000-1200 range and your 1kw heater won't work but you could run 500 heater. With the system as sold, every time you make a hot drink, make toast, use your microwave etc you will have to turn your immersion off. I hope your switch is downstairs, otherwise the temptation will be not to bother and then you will be heating your water at full electric prices. If this device could send all power in excess of say 600w to your immmersion heater by a variable control up to say 1kw then it would have some value, especially for people out at work all day.

 

[sharpener]

I have checked with DPS and they see no problem – so just drain, fit heater, refill, add inhibitor and plumbing side of job done.
I have checked for a suitable method of getting power to the airing cupboard and the utility room below has power sockets on a 32A short ring main serving just the utility, dining room and one bedroom – so I propose to take a spur from one of the sockets then to a switched FCU and then up the wall, through the ceiling and up through one of the floorboards into the airing cupboard finishing with a twin 13amp socket on the inside of airing cupboard wall.
...I am still waiting to hear back from Abeltronics but my wife is hoping for the sake of appearance/neatness that I can put the ‘power halver’ in something like a galvanised cooker back box, 47mm deep, mounted vertically and covered with a standard brushed steel blanking plate. (Drilling ventilation holes top and bottom if needed)
...Could really do with a warning light when in 1000w ‘bath’ but nothing in 500w ‘sink’ position.
All comments and suggestions most welcome – and thanks for your time reading this.

A couple of quick comments:

It used to be common practice to connect 3kW immersion heaters to an airing cupboard socket on the ring but the 17th Edition On-site guide now says they should have a dedicated circuit, from memory there is no mention of the power level either way.

The inhibitor goes in the indirect side so you don't need to add any when fitting an immersion heater.

AFAIR the Abletronics power halver is a series diode or similar arrangement. As I have pointed out before this will put DC back down the supply wires which as well as being illegal will upset the trip threshold of an ordinary ELCB so you will not get the shock protection it would normally provide.

If you are going to the trouble of fitting a supply to the airing cupboard and all the time switches and controls you describe then IMHO you would be better off waiting for a fully proportional controller to become available because it will be a truly fit-and-forget solution. I leave mine on all the time and even in January there is a 50% likelihood of not needing any night-time top-up. On dull days there is still enough power to make good the losses and keep the tank up to temperature.

 

[suntrap]

The inhibitor goes in the indirect side so you don't need to add any when fitting an immersion heater.

I imagine this system is a heat store like a hot water cylinder "in reverse", which is filled from the primary circuit and which heats domestic hot water instantanously by passing mains water through a coil.

 

[echase]

All comments and suggestions most welcome – and thanks for your time reading this.

I’d say don’t do it this way if you value your marriage and your mental health. You will end up always worrying about whether is should be 500W or 1000W and what time to set and beating yourself up for getting it wrong. And, like some friends of mine, the couple will argue about it constantly.

Look at my graphs from yesterday to see how fickle the UK weather is and it’s actually worse than this because some clouds could have gone by between the 5 minute samples points shown. You can’t possibly second guess this with a time clock.

 

[dennis2011]

Thanks for replies so far. Firstly as concisely as possible re my HW system.
Yes, my heat store is like a hot water system ‘in reverse’, NOT filled from the primary circuits, but mains water does in effect pass through it by means of a very efficient plate heat exchanger not a coil. A dedicated pump comes on whenever you turn on a HW tap that both circulates water in the tank, (from top to bottom), and also through this plate exchanger to heat the cold mains water. The tank originally comes pre-dosed with inhibitor and only needs a slight draining off and topping up with 1 litre of inhibitor every 5 years. The water in the tank is sealed, apart from an expansion vessel in the top, and does not mix at all with the water in the central heating system. The effective usable HW capacity of the tank is governed by its size, the temperature of the water in the store, (it can be set as high a 90 C), and also how fast it can be re-heated.
Quoting from DPS blurb –
Rapid Indirect Recovery Options:
Instead of using a coil to allow connection to a boiler, the Pandora can be supplied with a plate heat exchanger (PHE) to reheat the store using everything the boiler can provide. This reduces heat up times dramatically, and reduces the size of the store required.
So my HW system has all this, and so in total has 2 pumps and 2 plate heat exchangers.
Right – on to the electrics!

I really do need an emergency HW system. Since posting my first message my CH system has gone wrong, the CH pump keeps running – I think one of the motorised valves has failed in the open position – plumber hopefully coming soon!
So at the end of the day if all I can get is a 1kw immersion heater with just an on/off switch downstairs then I will happily settle for that. I am aware of Sharpener’s point that ideally an immersion heater should be on a separate circuit, but I do think it would be a bit of an overkill for me to go to the trouble and expense replacing the consumer unit with a larger one, with a separate 16A mcb protected circuit just to provide power to a 1K heater drawing at most 4A intermittently, on a lightly loaded ring main circuit.

I would be very interested in a fully proportional controller but I have already been in touch with Rudge Systems and I am presuming all the other systems work along similar lines in that they require a dedicated immersion heater circuit, because sometimes they turn this circuit off. I don’t have a dedicated circuit - I am coming off a ring main and therein lies the problem. I think I need to be coming direct from the consumer unit for any system to measure how much electricity is being used in the house at any point in time?

We have been married 46 years so our marriage is quite stable. I will not lose any sleep if I wake up one morning and ‘the sun don’t shine’ and the heater has been on 500w for 2 hours. OK it will cost me 15p – I can live with that. Most of the time especially in the summer months I will be generating more than enough power to utilise 500w one hour after dawn. (In the winter I would forget the time clock and just switch on/off manually). I would be quite happy to leave it on 500w a lot of the time rather than chasing the sun. The benefit of even halving an immersion heater when I had nothing before would still make it worthwhile for us.

So if I understand correctly, then the ‘Power Halver’ is not recommended for my situation. Perhaps ok to use one on a dedicated 16A immersion heater circuit but not on a ring main spur as I would lose some protection for all the sockets on this ring main. So how then could I safely, easily and conveniently, reduce the power of my immersion heater, running it from a fused spur?


For example I have seen several 1kw light dimmers on the market – but many have rotary control – might be difficult to work out exactly where the 500w setting was – many seem to be rotary with no markings. That’s why I liked the simplicity of the ‘power halver’ - flick the switch one way 500w, switch the other way for 1000w. I even thought about wiring up a small 2 x neon indicator panel, green for 500w, red for 1000w.

Perhaps I should forget the idea of trying to devise a dual 500w/1000w immersion heater? SureJust do make a 750w model. I could just fit that, forget power halvers, crossover switches etc and just set my timeclock to catch just the peak 4 hours or so per day during the summer months. For emergency use, if left on continuously, a 750w heater should still heat up my tank and provide a reasonable amount of hot water?


Thanks for all comments and advice – more please.

 

[Whip1971]

Hi Dennis
I've read on this thread that people are using 110v site transformers to drop the wattage on 3kW immersion heaters.

3kW = 240v / 12.5A / 19.2ohms
630W = 110v / 5.7A / 19.2ohms

Also you would be able to plug it into a ring circuit. It's just the cost of a transformer.

 

[dennis2011]

Hi whip1971 - it is not just the cost of a transformer. As I mentioned in my first post I don't have any immersion heater at all. I cannot really see the point of buying a 3kW immersion heater just to derate it to 630w with a transformer when I can just buy a 750w or 1000w immersion 'off the shelf' especially as Sharpener points out that it is not best practice to run an immersion off a ring main anyway. So why make things potentially worse by having 3kW potentially available?

 

[pmcalli]

Hi whip1971 - it is not just the cost of a transformer. As I mentioned in my first post I don't have any immersion heater at all. I cannot really see the point of buying a 3kW immersion heater just to derate it to 630w with a transformer when I can just buy a 750w or 1000w immersion 'off the shelf' especially as Sharpener points out that it is not best practice to run an immersion off a ring main anyway. So why make things potentially worse by having 3kW potentially available?

After reading all this distressing stuff I have to take pity on you. Send me a pm with your email and I will send you a circuit and parts list to control up to 12kW. You can plug it into the ring main all you would need to do Is run a screened wire back to a ct on your meter tail.

 

[echase]

as Sharpener points out that it is not best practice to run an immersion off a ring main anyway.

My house is wired that way and its never been much of a problem. It is even the same ring main as the TV uses and the TV never complains about any interference from the SSR.

 

[Andysimm2000]

Non electrical question...

I currently have a combi boiler with no hot water tank / immersion heater. I can see that I could install a pressurised tank, Immersion fed from solar surplus, Combisol Mixer valve (to either send hot water to taps or preheat boiler water feed) but can not see an economic feedback - I hate to see me "wasting" solar PV but can anyone persuade me that my thoughts are wrong?
TIA
Andy

 

[pmcalli]

Non electrical question...

I currently have a combi boiler with no hot water tank / immersion heater. I can see that I could install a pressurised tank, Immersion fed from solar surplus, Combisol Mixer valve (to either send hot water to taps or preheat boiler water feed) but can not see an economic feedback - I hate to see me "wasting" solar PV but can anyone persuade me that my thoughts are wrong?
TIA
Andy

I have had someone in your situation install an unvented tank with dual immersions and then feed it to the combi boiler. It's not cheap £300 plus but the saving per year is over £100 so pay back is around three years

 

[pauldreed]

Hobbo2006 - I got your PM re Solar Power Manager, but could not reply as your inbox is full and you can't receive any more messages.

Good luck with your project, and if you get stuck, please post here in the forum, and we will try and assist.

 

[Hobbo2006]

thanks Paul, not sure what's going on as the inbox is empty. Could it be that I'm still restricted as I'm a newbie?


*cue flood of posts*

 

[Hobbo2006]

what I'm looking for, and doesn't seem to exist just at the moment. Is a device that we measure both PV generation and electricity usage with those clip on sensors. It would then need to act as a switch to turn on a 1kW immersion element when there was sufficient surplus.

There seem to be lots of homemade ideas floating which require getting on the roof or being able to put circuits together, but none that the average Joe could install.


A couple of those sensors into a transmitter.... wirelessly connected to a box, which would link to the 1kW element would be ideal...

 

[echase]

There seem to be lots of homemade ideas floating which require getting on the roof or being able to put circuits together, but none that the average Joe could install.

That will be solved when my unit comes commercially available soon. And don’t need the expense of a 1kW element. But it is still slightly cheaper to make your own from design ideas here if you have the skills.

A couple of those sensors into a transmitter.... wirelessly connected to a box, which would link to the 1kW element would be ideal...

Nice in theory but in practice quite difficult to design hence all the good ideas here. Don’t recall any of them needing a climb on the roof.

 

[Plotty]

what I'm looking for, and doesn't seem to exist just at the moment. Is a device that we measure both PV generation and electricity usage ...

Hi Hobbo, That's what we're all looking for here! :6:If you go through the posts, some contributors - Inie Meanie, echase, pmcalli, pauldread, etc. -have designed gizmo's that appear to do this, some of which are for sale: contact them to find out. There are also a number of commercial systems on the market, or just off it, like EMMA and IntellyPower. The industry's been slow to react to customer demand, but things are looking up now. Meethinks that by the end of this year, there will be several systems available with reasonable payback periods.

 

[Hobbo2006]

If the parts are modular and I have instructions then I can do that but that's as far as it goes. Would be willing to pay some money for the right device to be made for me. If it means I don't have to change the immersion element that that's even better.

 

[Whip1971]

Hi hobbo, I would PM pmcalli. He's been extreamly helpfull to me. I'm an electrician, not an electronics technition. But with his help Ive just started building his circuit. And he answers to his emails. Good luck

 

[JimJo]

Hi hobbo, I would PM pmcalli. He's been extreamly helpfull to me. I'm an electrician, not an electronics technition. But with his help Ive just started building his circuit. And he answers to his emails. Good luck

Hi Hobbo
I've just found and joined this forum because your requirements for a system to switch an immerssion heater on around noon to use generated current instead of exporting it is just the system I want - I am also willing to purchase such a system as it should have a reasonably quick payback. Is anyone out there that can design, manufacture and sell such a system - if so let us Know?

 

[flyman]

while you're waiting for the ultimate, with the summer months coming this may be a cheap solution that those with a small DIY ability might consider. Water Heating with Surplus Solar PV - Solar

At under 32 quid, payback should be quick.

 

[echase]

At under 32 quid, payback should be quick.

Not at all. If you use it with a 3kW element you may well import a lot to heat your water. And with a 1kW element there is the added cost of element plus not a lot of actual PV generation transferred to that element. See all the posts on page 51. These switched type systems, particularly ones that do nto actually measure the amount of house export, have an unproven benefit and many of us think they won’t pay back any time soon, if at all. They are just creating an illusion of diverting enough PV to the immersion to be saleable. You need one that tracks export and puts variable power into the immersion.

 

[flyman]

Not at all. If you use it with a 3kW element you may well import a lot to heat your water. And with a 1kW element there is the added cost of element plus not a lot of actual PV generation transferred to that element. See all the posts on page 51. These switched type systems, particularly ones that do nto actually measure the amount of house export, have an unproven benefit and many of us think they won’t pay back any time soon, if at all. They are just creating an illusion of diverting enough PV to the immersion to be saleable. You need one that tracks export and puts variable power into the immersion.

Echase,
Possibly so but there seem to be a number of new enquiries from people with limited skills wanting something that doesn't yet exist in a commercial form. Whilst some may be competent in producing something that meets the requirement others might be looking for something that is available now even if just an an interim solution.
Naturally circumstances vary for each case dependant on size of system and usual source of water heating. Personally with the small system I have (1.5kwh) and being out most of the day I'm happy to explore the options of utilising a low power immersion to offset the cost of water heating with the gas boiler. I have no problem identifying the yield of my particular installation as I am familiar with PIC, PICAXE and Arduino microprocessors. I've been datalogging on an Arduino with both CT's and pulse counting directly from my Elster A100C export meter and switching loads remotely for months. During the last few days the excess yield over 1kwh has been prolonged or consumption to the point where I'll start to look at the economics of a new immersion.

I appreciate that not everyone has the time or inclination to look at a DIY solution but there are several option available or due to be available. Payback time will depend an many factors and in many cases will be impossible to calculate. I wonder sometimes if the desire to wring out every watt available isn't overtaken by the financial benefit.

 

[echase]

I wonder sometimes if the desire to wring out every watt available isn't overtaken by the financial benefit.

I accept the principle that paying for a Ford rather than a Rolls Royce is fine if you are prepared to accept some performance compromise in return for a lower price. But what you don’t want is a Ford that never actually moves forwards, but only goes in reverse. Some of these stepped controllers, particularly if not set up properly (and who has the patience or necessary test equipment to do this?) could actually use more grid electricity than the amount of export they will save.

A well designed controller will deliver the right amount to the immersion with no setup or special knowledge needed by the user. With that £32 one you have to choose a PV generation level to set it at. Those of a worrying disposition may be constantly wondering whether they go it right as it’s very subjective and likely to need altering summer to winter or indeed several times a year. Unlike altering your house thermostat, where heat can be sensed by one’s body, electricity is not something easy to measure/visualize so it’s easy to make a mistake in the setting.

 

[legoman]

Hi,
New to this forum. Got an immersion question.. anyone know if there is a problem fitting an incoloy 1kw immersion to a stainless steel cylinder. I have been told that if the 1kw immersion has a copper stat pocket there may be a reaction with the stainless steel.Has any one done this/had problems? I have an old copper cylinder now and would like to renew it.
Stainless cylinders are cheaper than copper ones and also have a 10 year warranty. I would like to fit a 27inch immersion in the top of my new cylinder, also an 11inch one in the bottom through the 1/2 in drain.
I cannot find either with a incoloy stat pocket.

 

[markthorpe999]

Didn't know that by trying to help people, I would be told I need to know the laws of physics. Quite a rude and kurt response to a new member?.

 

[pauldreed]

Didn't know that by trying to help people, I would be told I need to know the laws of physics. Quite a rude and kurt response to a new member?.

I don't think that the response you got from pmcalli was rude and kurt?
He gave you an accurate and comprehensive reply, and yes, the rules of physics need to be understood otherwise you could be wasting money on a heating element that would not fulfil your goal.

 

[David Bowen]

Hey guys,

I've taken a look at how much money I could save using solar PV to power my immersion heater:

How much water can I heat with spare Solar Power?

Notably, I've also run simulations for these device types:

* Switch that triggers based on solar output
* Switch that triggers when there's sufficient spare power
* Controller which ramps up the immersion in proportion to the spare power

 

[suntrap]

Very impressive article - well done.

 

[echase]

Hey guys,

I've taken a look at how much money I could save using solar PV to power my immersion heater:

Very comprehensive but needs more work. I have not checked through it all carefully but it may be about right for stepped controllers. But there is something wrong with your maths for proportional ones as even if you only put ¼ of your total generation into water heating you will displace around 600KWh pa. At an electricity cost of 13p that is £80. ¼ is conservative as it could be up to ½. So the benefit is much higher than you say.

I think there is a serious error in your “Ultimately, with just 3kWh capacity in the hot water tank we can only capture 1.1MWh a year even if the sun shone every day. That's 110L oil which is 72GBP” as 1.1MWh of electricity costs £140,000! Your oil cost of 6.6p per kWh is too cheap as it will cost significantly more than that per kW actually put into the water due to boiler and pipe losses particularly in summer when the lost heat has no value in home heating.

EMMA claim £454 benefit pa based on 13p electricity. http://www.coolpowerproducts.com/documents/EMMA-ROI-calculator-120216-web.xls I think that is over optimistic (in particular I’d challenge the 3375 W, 30% and 65% figures in that table) but it is based on a comprehensive calculation and 10% energy price inflation. Interestingly they claim a 20% improvement as they claim that heating water slower with a low power (or dimmed 3kW) element reduced churn in the tank and so reduces losses. (Rather contradicts the idea of having a pump to churn it for you).

Fitting some of these devices is so simple that I don’t think it’s essential to employ an electrician unless you are altering meter tails. Under Part P regs I think you will find that an electrician is not mandatory although clearly desirable. As this is the Electricians Forum lots of you out there will know better than me; I am only an electronics engineer.

 

[David Bowen]

Very comprehensive

Thanks

but needs more work.

Yep, I'm happy for folks to enlighten me, it's my first attempt.

But there is something wrong with your maths for proportional ones as even if you only put ¼ of your total generation into water heating you will displace around 600KWh pa. At an electricity cost of 13p that is £80. ¼ is conservative as it could be up to ½. So the benefit is much higher than you say.

I think there is a serious error in your “Ultimately, with just 3kWh capacity in the hot water tank we can only capture 1.1MWh a year even if the sun shone every day. That's 110L oil which is 72GBP” as 1.1MWh of electricity costs £140,000! Your oil cost of 6.6p per kWh is too cheap as it will cost significantly more than that per kW actually put into the water due to boiler and pipe losses particularly in summer when the lost heat has no value in home heating.

I think my setup would have an absolute maximum of 3kWh x 365 days = 1,095kWh for the year. I'd abbreviated that to 1.1MWh - Wolfram|Alpha. I should generate 3,116kWh per year according to the SAP figures for my PV so it would be a maximum of 1,095/3,116 = 35% I could capture.

You're right about oil not being 100% efficient but I don't have a sensible multiplier to use here - I'm open to suggestions.

At an electricity cost of 13p that is £80

Yep, if I was displacing electricity for heating water it would certiainly make the economics more compelling.

EMMA claim £454 benefit pa based on 13p electricity...

I believe you nice chaps will come up with a much cheaper device before EMMA breaks even.

in particular I’d challenge the 3375 W, 30% and 65% figures in that table

For reference, for the 21 December to 21 March period my solar generation was 397kWh. If I had a perfect proportional controller it could have used 93kWh so that's 23% of the generated power. We used 270kWh of the solar to power devices, so even if my immersion store was unlimited, we'd only have had 127kWh spare power for heating = 32%. Clearly this could be higher in the Summer, but with my limited immersion capacity I'm still stuck with the 35% upper bound.

A back-of-the-smoke-packet calculation: For September 21->March 21 I can capture 2 * 93kWh and then an upper bound of 183 days * 3kWh in the sunny months = 735kWh per year = 24% of my generation.

If I've got this wrong please come back to me as I'd love for this to be more lucrative.

 

[echase]

an absolute maximum of 3kWh x 365 days = 1,095kWh for the year. I'd abbreviated that to 1.1MWh - Wolfram|Alpha. I should generate 3,116kWh per year according to the SAP figures for my PV so it would be a maximum of 1,095/3,116 = 35% I could capture.

Why only 3kW per day? Most people use more hot water than that, maybe 6kWh. Are you very abstemious? If hot water is effectively free with a controller it makes sense to convert dishwasher and washing machine to hot fill to limit the time their elements need to come on at times there is no/limited sun. That would increase hot water usage whilst reducing electricity import.

You're right about oil not being 100% efficient but I don't have a sensible multiplier to use here - I'm open to suggestions.

Based on gas I’d say 50 -80%. Particularly bad if you run a pilot light all summer just for water heating, like I would have to if I left boiler on.

For reference, for the 21 December to 21 March period my solar generation was 397kWh. If I had a perfect proportional controller it could have used 93kWh so that's 23% of the generated power.

Don’t understand where the 93kWh comes from. I get much more than 32% in summer.

It’s a bit dangerous extrapolating winter figs to all year around. In absence of a full year’s figs better to take the benchmark figs from SAP calculation and use that. My 18 months of stats show I exceed the SAP fig by 10 - 20% in Hampshire.


Sorry my “1.1MWh of electricity costs £140,000” was wrong. Take off 3 zeros.

 

[David Bowen]

Why only 3kW per day?

Our immersion thermostat kicks in after we've consumed about 3kWh. Maybe we have an 11inch element? If the sun ever shines again I'm going to use my heat-sensor to see how much of the tank it's heating.

convert dishwasher and washing machine to hot fill to limit the time their elements need to come on at times there is no/limited sun. That would increase hot water usage whilst reducing electricity import.

At the moment we're using the solar electricity directly to power them and they heat up their water themselves. You're right though, it would be nice to buffer the solar power into the hot water tank as you describe if they had hot water inlets.

Based on gas I’d say 50 -80%. Particularly bad if you run a pilot light all summer just for water heating, like I would have to if I left boiler on.

Yes, as I mentioned in my article, the vagaries of local setups make the economics quite a local thing. Luckily for me I don't have a pilot light.

Don’t understand where the 93kWh comes from.

That's the amount I would have used in the three months from December with the best possible proportional controller, a 3kW immersion element and a 3kWh hot water capacity.

It’s a bit dangerous extrapolating winter figs to all year around.

I'm not. As in my article, I'm saying that September->December is the reflection of December->March in terms of solar input so I took that 93kWh and doubled it. Then I said that I was going to use the optimal case for the Summer, that is 3kWh per day.

Sorry my “1.1MWh of electricity costs £140,000” was wrong. Take off 3 zeros.

You're right - if I was displacing electrical heating and I got full solar power every day of the year I could save 140GBP. Of course I'm not actually going to achieve that figure. Also, that isn't a very high bar if I have to pay a few hundred pounds for a device, especially something like EMMA.

Thanks for your suggestions, keep them coming.

 

[suntrap]

If hot water is effectively free with a controller it makes sense to convert dishwasher and washing machine to hot fill to limit the time their elements need to come on at times there is no/limited sun.

I'm not sure that's such a good idea. I think washing machines work better with the water starting cold and then being heated - mine, for instance, churns around for quite some time before the element is activated. And if you plonk hot water straight into a dishwasher I think it tends to bake certain foods on.

 

[inie meanie]

Just to add to the calculation, I have just fitted a small storage heater in the lounge, so that when my water is up to temp it switches over to this. Now I know when it, s very dark etc it won't be so usefull but at the moment we don't need to switch the heating on in the evening because the lounge is warm enough, thus adding extra savings as heating on in the evenings can soon clock up a pound or two! Oh yeah heater was off ebay for just a few pounds. :- ) ( I do have 8kw on the roof so I have the capacity )

 

[echase]

if you plonk hot water straight into a dishwasher I think it tends to bake certain foods on.

I once had a dishwasher that I put on hot fill only and it worked very well. Speeds up the cycle.

Am still getting lots of private messages asking for kits. Please can I repeat that I am no longer doing kits as my design will be available as a commercially built unit. Can’t give a date yet.

 

[David Bowen]

Just to add to the calculation, I have just fitted a small storage heater in the lounge, so that when my water is up to temp it switches over to this. Now I know when it, s very dark etc it won't be so usefull but at the moment we don't need to switch the heating on in the evening because the lounge is warm enough, thus adding extra savings as heating on in the evenings can soon clock up a pound or two! Oh yeah heater was off ebay for just a few pounds. :- ) ( I do have 8kw on the roof so I have the capacity )

Good stuff. I have a temperature record for the whole of last year, but my solar didn't go in 'til October so I can't run a simulation of using the power when the temp is below some threshold. I'll have a ponder.

I also got a device so I could avoid using the boiler. It's called a termowentylator which makes me smile every time I say it. It's Polish for fan heater

 

[daveUK]

Hey guys,

I've taken a look at how much money I could save using solar PV to power my immersion heater:

How much water can I heat with spare Solar Power?

Notably, I've also run simulations for these device types:

* Switch that triggers based on solar output
* Switch that triggers when there's sufficient spare power
* Controller which ramps up the immersion in proportion to the spare power

Very interesting article which I think a lot of people on here could usefully read.

A couple of minor points.

I agree with echase that you've ignored the efficiency of fossil-fuel fired hot water heating. Before implementing my system for using free PV energy to heat water, I measured the efficiency of my gas-fired system (modern boiler but with fairly long poorly?-lagged run to hot water tank) at only 55%, which surprised me somewhat. At the marginal cost of my gas in summer (£0.081/kWh), this means water heating in summer using gas costs me about £0.15/kWh (cf. the figure you've used of £0.066/kWh). (This means at the moment it's actually cheaper for me to use paid-for electricity for hot water heating in summer at my marginal rate of £0.11/kWh) Obviously these figures are only pertinent to me (as your £0.066/kWh is to you) but illustrate the other end of the range of hot water heating costs.

Many of your comments refer to your assumed maximum usage of 3kWh hot water per day, which echase seems to dispute. I suspect it's a fairly appropriate figure for our 2-person household (but I do only wash when necessary!)

I'm happy to see that your figures suggest a simple single-element switched immersion system is optimal (in winter/spring) for a 1kW heater since I've gone down the cheap & cheerful 3kW-immersion-powered-by-site-transformer route which creates a 700W heater.


Thanks again.

 

[echase]

I measured the efficiency of my gas-fired system (modern boiler but with fairly long poorly?-lagged run to hot water tank) at only 55%, which surprised me somewhat. At the marginal cost of my gas in summer (£0.081/kWh), this means water heating in summer using gas costs me about £0.15/kWh (cf. the figure you've used of £0.066/kWh). (This means at the moment it's actually cheaper for me to use paid-for electricity for hot water heating in summer at my marginal rate of £0.11/kWh) QUOTE]

I have not measured mine but suspect it’s even less than 55% as I have a pilot light and very long pipes. I was using full price electricity instead of the boiler all summer. How do you get £0.081/kWh? Is that because in summer you fall below the usage required to get into the cheaper tariff? Not sure how they decide this tariff, is it based on a quarter by quarter usage or averaged over a year?