Immersion heater only heats top of cylinder

[immerse]

I have just got my system going with a home-built controller to use 'excess' PV generated electricity to heat my indirect hot water cylinder. This has a top entry 3kW Steeple 80027BT immersion heater (which I imagine is 27 inches long, I cannot find any technical data) and a Diamond WT18 thermostat, which I believe is 18 inches long and appropriate to the immersion heater. I use a step down transformer to deliver 1.5 kW into the immersion heater when there is sufficient 'spare' PV power available.

This all works as expected except that at the end of a long sunny day the water temperature is highly stratified, very hot at the top and only mildly warm half way down the 90 cm x 45 cm cylinder, where I would have expected the thermostat sensor to be positioned. There is plenty of energy available, about 12 kWh, enough to raise the whole cylinder from room temperature to boiling point. Conceivably this could arise because the immersion heater is not being driven to its design operating power - can anyone comment on this, please? (A number of other people, using either transformers or Triac load switching, comment that they get a full cylinder of hot water).

Alternatively, there could be some sort of fault with the heater or thermostat (although the max. temp. of the water is about the 70 deg C I set on the thermostat). Or maybe this arises because the heater is a long thin thing delivering heat over its whole length? If so, why do others not experience the same problem? Is it possible to purchase a 27 inch immersion heater wherein the whole power is delivered at or near the bottom end rather than along the whole length? This would lead no doubt to better convective stirring and less stratification.

Any advice would be gratefully received!

 

[Worcester]

Best thing to do is to add a pump to circulate the water - a de-stratification pump - it's part of the stanadard installation proposals that we recommend, and using the Immersun controller has the facility to control that built in. The Immersun monitors the immersion heater, when it swithes off, starts up the de-strat pump, then a few minutes later switches the immersion back on. Voila - a ful tank of hot water All from "free" leccy!

pm me for more details.

 

[immerse]

Thanks for this advice. It seems to me that quite a feeble pump would suffice, since a cubic meter/hour should be plenty at what must be a very low pressure differential. What sort of pumps are used? I daresay canned-rotor glandless circulation pumps are a favourite, but there must be smaller/lighter/cheaper solutions?

Have had a look at Immersun. These PV immersion heater proportional controllers are almost at a reasonable price now!

 

[zeljuga]

At 2.5 kw/24h Heat loss for top insulated cylinders(older ones up to 10kw/24h heat loss) all storey with dumping excess PV output in to it is highly questionable. As converting energy of photons thee time over accrues in conversion loss up to 95% roughly, not counting loss in cable resistance and hot water pipes/taps.
Storing Hot water at close to boiling point is another waste of energy as human can hardly recognise temperature difference between 45 C and 50 C. Where 45 C is ideal temp, for comfort of use and reduced heat loss in cylinder.On top of that you may incur scalding problem ( if storing at more then 75C) at point of hot water delivery which needs to be addressed separately.
Overall can of worms. But as good as it gets solution for now.

 

[immerse]

Do you mean kWh, not kW?

 

[Worcester]

Yep you don't need a big high powered pump, the cheapest option would be a low powered central heating circulation pump set on a low flow setting
circulation pump domestic | eBay
or
Salus MP100 Central Heating Domestic Circulating Pump Replaces Grundfos 15-50 | eBay

PM me for the circuit diagrams

 

[Dave-geo]

Yep you don't need a big high powered pump, the cheapest option would be a low powered central heating circulation pump set on a low flow setting

Think you might need a bronze pump as this will be installed on the domestic hot water.

bronze pump circulating | eBay

 

[snowhead]

You definitely need a bronze or stainless pump.

 

[sambotc]

Yes you need a bronze pump, can't see a standard pump lasting long in this setting.

Never thought of the issues you've mentioned using this idea before now but it makes sense, top entry immersions are designed more as a back up. The thermostat being at the top of the cylinder is the issue (not just for this kind of set up)

These immersion switches could turn out to cost to much to install unless a simple product is found as a solution, bronze pumps are about £200

 

[solarsavings]

The last stainless 15/60 pump I bought from here Euro Boiler Parts cost about £60.

 

[mountainman]

Could it ne that someone has stuck an 11" immersion into the top of the cylinder?

 

[Gavin A]

Could it ne that someone has stuck an 11" immersion into the top of the cylinder?

that'd be my assumption.

 

[sambotc]

Out of interest what is the cylinders size/dimension? just in case you didn't know, you can remove the thermostat from the immersion element with out having to drain down.

 

[immerse]

To the responders of the last 3 posts: I give the tank size and immersion heater/thermostat specification in the first post of this thread. Although I can find no info about Steeple, the 27 in the model number suggests a 27 inch length, and the thermostat is definitely 18 inches long. If you think about the effect of convective heat transfer stratification is inevitable.

 

[immerse]

How about a plastic body to avoid the high cost of stainless or bronze? I see Grundfos make a model PU.120 for example which is plastic bodied for potable water applications. Unfortunately I cannot find it on the Grundfos website, the info is on the Euro Boiler Parts website. I'll bet someone will come back that this is only for cold water use.

Washing machine pumps are plastic bodied and obviously tolerate very high water temperatures, so plastic should be OK as long as the right material is chosen, with no leaching of plasticiser for instance. Does anyone know what washing machine pumps are made of? The material is white and looks like acetal or polyamide. Of course the building regs could outlaw such a solution on 'elf and safety grounds.

 

[solarsavings]

It's for boiler use and POTABLE water so you're in.

 

[Gavin A]

To the responders of the last 3 posts: I give the tank size and immersion heater/thermostat specification in the first post of this thread. Although I can find no info about Steeple, the 27 in the model number suggests a 27 inch length, and the thermostat is definitely 18 inches long. If you think about the effect of convective heat transfer stratification is inevitable.

alternatively it could be 27cm / 11inches....

but yes, if the thermostat is 18 inches long, then that would suggest the immersion is at least that length.

btw you didn't exactly give the specifications in the op, you used the words - 'I imagine' and 'I believe', which leaves the distinct impression that your assumptions could be wrong.

If that's not it, then it's likely to be something to do with the way the thermostat works, where effectively it takes the average temperature across the entire thermostat length, instead of the temperature at the bottom of the thermostat. So if the tank is highly stratified, then the thermostat will measure an average of 70 if the top part is significantly hotter than this, even if the lower part is barely luke warm. A destratification pump would probably be needed to solve, or an external thermostat on the lower portion of the tank, but that'd be a fair amount of electrical jerry rigging to get set up.

 

[sambotc]

Is that PU.120 listed on euroboiler spares just the body though for £53? Still works out a lot cheaper than a bronze pump even if it is

 

[mountainman]

Im clutching at straws a little in this thought but here goes.
I assume that the immersion heater is rated at 3kw 230v.

If that is the case then i would be inclined to connect it directly to your household circuit, leave it on for an hour and see if more of the cylinder is hot than it was before.

This should eliminate one line of enquiry imo.

 

[sambotc]

Been having a think about this and using a thermostatic mixing valve you could preheat the cold feed, might even get some gravity circulation too.



Not 100% it will work as I imagine, but cheap to install and worth a try. If anyone does I would be interested to hear how it went.

 

[Gavin A]

I use a step down transformer to deliver 1.5 kW into the immersion heater when there is sufficient 'spare' PV power available.

Do you know what voltage the step down transformer is outputting at?

I'm just wondering if it's possible there's been some misunderstanding, and it's actually outputting at 110V, which counter intuitively would only give you 660W of power into the immersion, as explained in this calc I nicked via google.

Ohms Law V=IR and I=P/V
An immersion heater rated at 3kW on a 230V supply will use approx 13A of current. (I=3000/230).

So, using Ohms Law the resistance of the element would be 230/13= approx 18 ohms.

Using the same element on a reduced voltage using Ohms Law again 110/18=approx 6A

P=IV

6 x 110=660 watts. So if you reduce the voltage to 110V the power is reduced to approx 20%, not 50%. So the answer is 0.66kW not 1.5kW.

[credit to unphased]

I'd suspect that at only 110V, and with only 660W of input that the majority of the heat output is then confined to the upper part of the immersion heater, though I'm not entirely sure on that.

If I'm right, then the same would also apply but to a lesser extent if it was stepped down to a voltage that did give 1500W output - ie the heat would be more concentrated in the upper section. I could easily be wrong about that though.

 

[The Solar King]

Grundfos also do a range of circulation pumps suitable for potable water Grundfos Comfort | Grundfos

A couple of caveats to all of this. Please be aware of the L8 legionella regulations, and their impact on hot water storage.
If you are displacing gas as the energy source, you will have to save at least 2200kWhrs of gas for every £100.00 of cost to break even on what you are doing. (based on 4.5p per kWhr)

 

[Worcester]

We're on oil.... Only thing the massive Rayburn does during the summer is drain our oil tank at a rate of knotts to heat the water, we'll make our return in saved oil in under two years. Last summer we turned the oil off and used the paid for leccy at 9.5p (plus VAT) / unit, so ignoring the oil, I'm saving 9.975p/kWh, so need to save 1003 kWh to save £100.

The Solar King, you're right, if you are using a natrual gas condensing boiler to heat your hot water at present, then the savings could take a long time in coming - It's not a panacea for everyone.

http://www.hse.gov.uk/pubns/priced/l8.pdf

 

[The Solar King]

Clause 158 is the one you need.

This also means you need to have a TMV fitted to the tank outlet to avoid scald risk. On a vented tank this can restrict flow and fitting TMVs at point of use may be an alternative. Best practice would be the latter and would allow compliance with TMV2 which will come in to force for domestic properties in the not too distant future.

There is debate as to the efficacy of clause 158. If sized properly and used regularly, the tank is not a standing body of water in which bacteriological growth can occur for long enough to create a hazard. You can only be infected by inhaling contaminated aerosols. Infection in domestic premises is most likely to occur from a shower that hasn't been used for an extended period. (water sat in the hose loop). If you are not meeting the guidelines of clause 158, always give your tank a hot blast and run your showers before use on return from holiday .

Best advice if fitting the kind of gear discussed here for a customer is to ensure the boiler brings the tank up to temperature every day as per clause 158. If the tank is already at 60deg C, the boiler will not fire. This is also the guidance in CE131, the solar thermal equivalent or DTi 2nd edition. The last thing you want is to be blamed for a Legionella outbreak.

There may be new guidance over tank disinfection in the revised BS5918 (Solar heating systems for domestic hot water – Code of practice) when it is published. It has just been out to consultation.

 

[Gavin A]

Grundfos also do a range of circulation pumps suitable for potable water Grundfos Comfort | Grundfos

A couple of caveats to all of this. Please be aware of the L8 legionella regulations, and their impact on hot water storage.
If you are displacing gas as the energy source, you will have to save at least 2200kWhrs of gas for every £100.00 of cost to break even on what you are doing. (based on 4.5p per kWhr)

why do they need to break even in a single years usage on equipment that should be in place for 10-20 years plus?

L8 - well, sort of. L8 regs only fully apply to units in a workplace environment, and are mainly (but not exclusively) intended for hotels, hospitals etc with very long pipe runs, rather than domestic set ups. Not that I'm saying that legionella risk can be ignored, and if you do apply L8 regulations to domestic set ups then they'll obviously be protected, but it's overkill in most circumstances and not required by the regulations.

As confirmed by this paragraph in the overview of the latest changes to L8 from HSE

Domestic systems can present a risk, depending on the circumstances of use, but the ACoP will only apply to systems from which the risk arises in relation to any work activity.

[hse]

Interesting to note that they've actually removed the 300l limit that previously applied to L8 though, I wasn't aware of that until I just read this document. That potentially brings a whole array of hot water systems within the L8 scope that previously weren't.

 

[The Solar King]

Problem is L8 is referenced in standards relating to domestic installation. You have exactly the same situation of under heating a tank due to insufficient input using PV as you do in a normal ST system. If there wasn't a risk the guidance for ST would not be given. Whilst the risk may be small it is the same in both circumstances. I would consider it imprudent not to take this approach until there is clarification in BS5918 which will replace CE131.

There are other advantages in storing at higher temperature. You do not need as large a tank to meet daily hot water requirements at normal usage temperatures. To get the full benefit of any kind of solar input, ideally you only want auxiliary heating input once a day. On days with good solar gain, solar input will bring the tank up to 60deg C without auxiliary input (at least it will with ST) giving a greater energy input than with lower storage temperatures. Sized correctly, the auxiliary energy input is the same for a smaller tank heated to 60deg as it is for a larger tank heated to 45deg.

I appreciate that all nuances of system design cannot be taken into account when dumping excess PV energy into a tank, but feel it rash to ignore what is already established as best practice.

 

[immerse]

Immersion heaters are constructed from resistance wire running their whole length, so regardless of the input power, they deposit heat uniformly over their whole length.

I have a 170V custom transformer which deposits 1.5kW into the heater.

Note that the UK mains is 240Vrms, irrespective of what anyone tells you. Put a voltmeter on if you do not believe me.

 

[sambotc]

What about trying a 36" immersion? could just about get it in

 

[immerse]

Label on tank states 'maximum length of immersion heater 27 inches'

 

[sambotc]

Don't really see what difference it will makes myself as long as its not touching the bottom/sides/cylinder itself, happy to be proved otherwise though

 

[immerse]

I imagine the manufacturers have put the label there because a 36 inch heater would touch either the sides, bottom, or more likely the indirect coil.