Setting up the Immersun Vs Central Heating - Help!

[MicraShed]

The Immersun is reading "Water Hot" unless I have the thermostat on the Immersion element turned up to maximum.

Currently water coming out of the taps is at about 68 degrees - far too hot for comfort. But if I set the immersion element lower than this, the boiler thermostat calls for heat and the boiler kicks in.
I believe the problem is to do with the position of the thermostats - this diagram explains it a bit better.

I have a Primatic self priming water tank which also feeds the central heating system. The yellow diagonal line is the position of the Immersion element ( a 30-odd year old Redring 3kW) with the thermostat for this being right at the top of the hot water cylinder.

The orange square is the position of the boiler thermostat.

For now, I have turned the boiler thermostat down to about 45 degrees in the hope that this will prevent the boiler firing up to deliver heat to the cylinder - in winter this isnt an issue as the system is a gravity fed (non pumped) hot water set up so whilst the boiler is on for the heating water will be fed into the cylinder heating the hot water regardless of the cylinder stat status.
In summer the chances are that the Immersun will produce more than enough heat output for the cylinder so I can probably shut down the whole heating system. But its Spring & Autumn where I wouldnt normally have my heating on for the house but there may not be enough PV output to heat the tank - ideally I would rather set it up as a fit and forget, if the PV hasnt heated the water enough, the boiler kicks in automatically but at the moment I just cant get it right.

Three thoughts here - the old immersion element might be better replaced with a newer element (which also features a thermal cut out too for safety) possibly a shorter 2kW type - the thermostat on the immersion might then not cut out quite so soon. However I suspect its because the position of the thermostat is right at the top of the tank that it cuts out before the water lower down the tank is heated.

Two: Move the boiler thermostat further up the tank in the hope that the water is hotter further up the tank and that the boiler doesnt kick in unless needed - I need to draw off only one bath and one dishwasher worth of water a day, so really the tank is probably far bigger than we need.

Three: Fit a de-strat device (though I not sure I can to the primatic tank) to mix the hot and cold water at the top and bottom of the tank evenly.

Ultimatly I know Im going to have to fit some TMV's to the taps - probably the bath and the bathroom sinks. Looking at the tank take off types they dont seem suited to my plumbing set up (LP gravity hot water, and mains fed cold water at all taps).

So, any advice or thoughts?

 

[Gavin A]

is the boiler on a timer?

I'd think the best thing to do is to set the boiler on a timer to come on either just in the evening, or first thing in the morning, or possibly both, that way the immersion gets to heat the tank as much as it can through the day, and your heating system tops that up to ensure you always have how water still.

This really is what these systems are designed for, similarly to solar water heating, where they're designed to reduce the boiler usage, not replace the boiler entirely.

If you don't have a timer on the boiler, then I'd think that fitting one is likely to be the best method of solving this issue. Oddly enough we actually do have a couple of basic timers that could probably be used for this purpose if you came back an asked your installer, I'm sure we could assist

To be honest, this is a situation we've just picked up on, as I'd thought it was covered in the manual from Immersun, but I realised last week that it wasn't. It's now on my list to write instructions for pairing the immersun up with the existing system properly, to prevent this being an issue for people - I'm not sure why Immersun haven't included this in their manual.

 

[Gavin A]

trv - you'd have to fit a whole tank trv fed from the cold supply to the tank, as you need a balanced pressure. We've got one fitted to our gravity fed solar hot water system that works fine even when the tank's up at 85 in summer.

 

[zeljuga]

Ideal temp. set-up for cylinder is 45C, as body don`t recognise difference between 40 C and 50C And anything above is complete waste of energy as well as reducing performance of solar thermal system. Apart once daily bust to 60C to prevent Legionela. And TRV are mast with solar thermal to prevent from possible scalding on taps.

 

[yellowvanman]

Just thinking aloud.

Couldn't you fit a 2nd tank stat on the side and wire the immersion through that (rather than the top stat) but still wire in the overheat stat. Means you'll have another cable to wire from the immersion at the top to the stat on the side of the tank.

If you put this stat slightly below the boiler stat and set at the same temperature should stop the boiler kicking in too often.

 

[Gavin A]

I wouldn't do that, as the immersion will heat the top of the tank the most, so a stat on the side could well read 10 deg or more lower than the water at the very top of the tank, giving a significant risk of scalding.

 

[The Solar King]

The position of the immersion element is not ideal. Without destratification you can not heat the lower section of the tank. what is the capacity of the tank? How many people live in the house? ie what is your daily hot water requirement? This should be estimated at a usage temperature of no more than 45degC. If you store water at 60degC, you then need less storage capacity. Reason for mentioning this is if the volume of your tank above the boiler coil is sufficient for your daily requirement, there is no benefit in destratification. The position of the thermostat means that during boiler operation, the boiler will switch off when the tank has reached temperature where the thermostat is located, ie the volume above it has reached temperature. If it is a strap-on type stat, there are less accurate than the pocket type.

Gavin is right in saying correctly timed auxiliary backup from the boiler is the key to this. If you have sufficient tank volume, once a day in the evening is optimum as it gives the maximum opportunity for gain via the immersun.

I have seen a number of properties where the hot tank is small and obviously designed to be heated twice a day. This really limits the potential for the Immersun. If this is the case with your tank I would consider replacing it with a modern one, which will be far better insulated (less standing loss) and could be specified with an immersion element near the bottom making it far more efficient for use with the immersun as there would be no issues over the need for destratification.

 

[Dave-geo]

I think you have several options.

You could turn off the boiler completely during the summer and set the Immersun to boost the water automatically using the immersion, this could be set for 30 mins in the morning and an hour in the evening (vary the duration of the on times to suit your particular demand)

You could fit a new immersion of a smaller length but you will only heat hot water to the length of the immersion so you would reduce the amount of hot water heated by the immersion/Immersun.

Why not use the relay function of the Immersun to boost the hot water via your existing boiler?

Fit a destrat pump. This is by far the best option as you get a full tank of water on sunny days. Your Primatic cylinder could be a problem but I don’t think it would be as the air separation in the Primatic section of the cylinder should be a neutral point so you shouldn’t lose the air bubble. I doubt if this has been tested so you could be a Guinea Pig! You could always ask an opinion over on the plumbing forum.

Fitting a TMV on the outlet of the cylinder is a good idea, take a separate cold cistern fed feed to the TMV and you are good to go. This would allow you to increase the temp of the immersion stat and therefore increase the useful amount of hot water available on sunny days.

I have had an Immersun fitted for exactly 1 month. 3.45Kw system has generated 296Kw in the month and sent 105Kw to the immersion. The house has a fairly high daytime load of about 700W so I am quite pleased with this. We use oil so the boiler is now turned off during the summer. I use the Immersun to boost the water via the immersion for 1 hour in the morning and 2 hours in the evening, the immersion only runs until the stat is satisfied and this works well. I have a 210L cylinder which has an inefficient solar thermal system as well. Before the Immersun the solar thermal would only warm the water on good days. I have a complicated setup as I also have secondary circulation pump on the domestic hot water and have utilised this pump to act as a destrat pump via a motorised valve. It works great and now I have 65 degrees at the top of the cylinder and 60 degrees at the very bottom on good days.

Speaking with my plumbers hat on your Primatic cylinder is well past its sell by date and you should consider a new boiler and cylinder sometime soon!

 

[The Solar King]

Sorry, not awake enough to take in it is an IMI primatic cylinder. Dave-geo is right, long past its sell by date! You could hopefully split tank and boiler replacement if money is an issue. Also worth checking if the Energy Savings Trust are running a boiler scrappage scheme in your area. If you are on gas and your boiler is the same age as your tank, a new high efficiency system boiler will make the savings of the Immersun small in comparison. Completely replacing our heating system in conjunction with solar thermal has knocked over 40% off our heating bill.

 

[MicraShed]

Speaking with my plumbers hat on your Primatic cylinder is well past its sell by date and you should consider a new boiler and cylinder sometime soon!

Replace my boiler and cylinder??

But...But...its only 17 years old! I replaced both ancient leaky Vulcan Continental and leaky cylinder with nice shiny new Baxi Solo and new (insulated) Primatic tank in 1995. Its still new in my eyes!

Having thought about this, possibly the easiest and cheapest option at the moment is the use of a tank fitted whole house TMV. The boiler, whilst still being called for for heat isnt being used as much ( though todays solar PV production was quite dire and didnt throw anything into the Immersun). I can run the tank hot but not be troubled by hot water at the taps.
Changing the heating system means quite a bit more plumbing as presently I only have one loft tank for feeding both hot water tank and heating system, or if I move to a combi I lose the hot water tank which I use for the Immersun.

 

[The Solar King]

Go for an un-vented cylinder and ditch all the cold water storage. You can still have the benefits of the Immersun plus all the benefits of decent pressure on the hot water. Also makes shower mixers work properly. You are less likely to get problems with a TMV on the tank outlet as well. I have seen cases where hot pressure is low and flow has gone to a dribble due to the mis-match with the mains pressure.

A cased cylinder has better insulation than an uncased one. In general they are the same spec under Part L for both retrofit and new build due to nature of insulation. Uncased tanks have double the thickness of insulation for new build as they do for retrofit. The theory is you cannot get a tank with all this extra insulation in the existing cupboard, hence the difference. To meet Part L, the cased cylinders simply give a heat loss figure in kWhrs/day.

Only thing you need to watch with an un-vented cylinder in consequential flow rates to existing showers. You may need flow restrictors or something like an Ecocamel shower head.

Have fun.

 

[Dave-geo]

Replace my boiler and cylinder??

Why did you install a new Primatic in 1995??? That would have been the time to go for a fully pumped system. Although an ingenious invention the Primatic is well out of date now and was old hat back in 1995 as well.

 

[Dave-geo]

Go for an un-vented cylinder and ditch all the cold water storage.

2nd that but your gravity hot water will not work with the unvented system.

Is this the plumbing or electrical forum?

 

[The Solar King]

Some of us do PV and ST.

If it is an un-vented cylinder, what gravity feed hot water? You will have got rid of it as hot cylinder is fed at 3 Bar through Pressure reducing valve directly from mains. Cold feed to taps taken from same point so equal pressure on hot and cold. Whole thing configured as S-Plan on boiler and away you go.

 

[MicraShed]

Why did you install a new Primatic in 1995???

Is this the plumbing or electrical forum?

Cost ( I had no money having just spent it all on buying the house) and ease of exchange - simply unbolted the old cylinder and bolted the new one in place. Nothing else needed to be altered.

I agree, this is getting sidetracked now from setting up the Immersun to replacing the central heating! I'll wander off and see whats on the Plumbing forum...

 

[Dave-geo]

If it is an un-vented cylinder, what gravity feed hot water?

Primatic is gravity flow and return from boiler so would need to be changed to fully pumped for unvented to work (need motorised valve for safety and higher resistance on heating coil)

simply unbolted the old cylinder and bolted the new one in place. Nothing else needed to be altered.

Sorry just having a joke, I may have done the same if I was you. Primatic cylinders do cost a fortune though!

 

[Gavin A]

can I just point out again that the correct use of a timer function on the boiler controls will resolve this problem for no cost at all, or very small cost if there is no timer and one needs to be fitted.

little point replacing a cylinder until it's actually borked IMO, expecially if it involves entirely replumbing much of the house, and new boiler as the suggestions above would involve.

 

[Johnuk]

mmmmmm did u know there is a cold water tap near the hot water one lol

 

[MicraShed]

Yes, there currently isnt a timer - I'll photograph and post up the control unit in a little while. But basically the boiler runs solely off two thermostats - one room stat and one cylinder stat.

 

[Gavin A]

stick a timer in the circuit to the cylinder stat, jobs a goodun. (I think, but I'm not entirely familiar with this set up - is the heating circuit pumped or gravity fed as well?).

 

[MicraShed]

stick a timer in the circuit to the cylinder stat, jobs a goodun. (I think, but I'm not entirely familiar with this set up - is the heating circuit pumped or gravity fed as well?).

Its a home brew affair. The hot water is gravity fed, only the heating is pumped.
The cylinder stat is a strap on type:-

The control box is basically a power relay which switches depending on whether there is a "call" from either room stat or cylinder stat.
This one is the original prototype - the one I have on the boiler now is neatened up a little and in a plastic case but is essentially the same:

Power is sent to the boiler controls for the water / heating depending on whether there is a call from the thermostats. A neon on the top of the unit indicates if its the water & heating or just the water that is active (ie if the pump is running or not).

No doubt people will exclaim WTF? But at the time this was necessary to ensure I had hot water when i needed it and heating when I needed it as I work erratic shifts (I might have mentioned Im a nurse these days?) So the house (quite well insulated) remains at a background temp of 17 degrees, when I get up I simply put the heating up to 20, when i go to bed or go out I shut it back down to 17.

Sounds horrific? Well by maintaining a background heat it takes less time to get the house up to a comfortable temperature. Gas use for a four bed semi over 2011 averaged between 13kWh/day (Summer) to 33kWh day (winter).

 

[Gavin A]

that's a lot in summer.

the problem I'd suspect is that the stat probably has a 2 deg hysterisis setting, so will be cycling the boiler fairly frequently every time the cylinder drops 2 deg, which won't take that long with only 25mm insulation (if that). Each time it cycles it has to heat up all the fluid in the presumably 22mm or higher pipework, plus the pipes, plus the boiler etc not just the water in the tank, so the efficiency of this operation is fairly low.

I think this is a question we really need to be asking people, as it'd be a simple process to put even a basic inline timer into the system on the same circuit as the switched live for the stat, plus extra jacket to keep the heat in (I know, sorry it's somehow still in our warehouse and didn't make it to your cylinder).

 

[MicraShed]

I think this is a question we really need to be asking people, as it'd be a simple process to put even a basic inline timer into the system on the same circuit as the switched live for the stat, plus extra jacket to keep the heat in (I know, sorry it's somehow still in our warehouse and didn't make it to your cylinder).

Meh! Dont worry about the jacket, its not the end of the world.

Some of that gas use is cooking, but not much - you are of course bang on with the cycling of the gas boiler. So an inline timer is possibly the simplest option at this point, though I have to say having read up a bit about unvented systems I wonder if ultimately that may be the way forward in the future. Not this year though as Im still paying for the PV install. And whilst I have the width Im not sure about the height in the existing cylinder cupboard (though thats a fairly simple matter of removing the ceiling and utilising some of the loft space above.

 

[Worcester]

Just as a comment, you can get programmers just for the cylinder stat. We are oild fired and the hot water and heating are linked to the same time so if we have heating on in the am, then it kicks in the hot water adding a progarmmable cylinder stat timer to just the cylionder stat circuit lets us programme a completely different regime for the hot water (within the bands of heating on / off) and allows us to alter the temperature as well, so it's not just when it's on / off, it's also what temperature we want it at. That allows us to give the ImmerSUN the 'priority'.
Have a look at this as the 'ultimate' - less expensive options that do the same thing are available.

WP75-RF
http://danfoss-randall.co.uk/PCMPDF/34248v06.pdf

 

[Worcester]

p.s. we had a custom built unvented dual cylinder made espcially for us as a one off due to height restrictions, so it's short and fat ... it actually ended up costing less than an off the shelf unvented dual cylinder.

 

[hawgraham]

I had an ImmerSun unit installed 2 days ago. I have now had a chance to analyse how it works. Unfortunately there is a problem in that the unit outputs bursts of 3kw sine waves. In otherwords part of the power being put to the immersion heater is coming from the mains not your PV panels. Over a period of time the unit does output the average power quoted, but because of the way it works it would not be able to run off grid. If you think about i,t if your immersion heater is 3kw then that is the power it will take unless the voltage applied to it is reduced (like when you run it from an 110v transformer). This unit does not reduce the voltage it just outputs 3kw pulses, if you have 500w to spare from your panels the unit will still output 3kw pulses with a gap inbetween so that over a period of time the average power output is 500w. Most of the time you are still putting 500w into the grid. Your household meter does not run backwards so when the 3kw pulses are used 2.5kw of it is coming from the grid (in this case) so over a period your meter will clock up power used.

If you want to see what I mean just stick a current clamp on the cable and stick an oscilloscope on the end.

 

[BruceB]

.............. Your household meter does not run backwards so when the 3kw pulses are used 2.5kw of it is coming from the grid (in this case) so over a period your meter will clock up power used.

If this is the case then the people at Immersun have made a fairly basic mistake which would surprise me. I suggest you ask them before being critical of the product. If the burst firing of the triac is within the averaging period of the meter electronics then there should not be a problem.

 

[Worcester]

@hawgraham,

You're both right and wrong here.

An understanding of the various power control methods and how meters work is important.

Yes the Immersun uses burst fire so 20% would mean in very simple terms that 1 cycle in 5 is passed through at full power and the other 4 at zero, (so the system exports during those cycles). The alternative to burst fire is phase angle control.

Lets assume that the voltage is 230 volts that the immersion is a 3 kW unit (13.04 Amps) and that there is 720W of power available
So in say a 1 second period, over 50 cycles, the current flow MAY look like this: (see attached AMPS)
and the cumulative energy consumed from the grid is therfore: (Cumulative Watt hours) - i.e NET consumption = 0 (Zero) Wh

The little light that flashes on your meter is recording 1/1000 of a kWh, i.e 1 Wh or 1 Watt Hour (1 cycle passes 0.0166r Wh @3000w)

The meters actaully measure AVERAGE energy consumed over a short period of time, and hence they take into account both the forward and reverse flow during that period.

The system was configured to use burst fire after extensive discussions with the meter manaufacturers and a large number of tests with different meters, and the manufacturers of the ImmeSUN unit have yet to come across a meter that works any differently. If you can prove otherwise I am sure that they will help you out.

So, all you ImmerSUN customers can rest at ease.

 

[Graeme Harrold]

I was listed last week as an installer on their site, and have quite a few in the pipeline already. Previously I was fitting the Parsons Switch, unit cost is comparable, but Immersun is easier to fit as you dont need to change the immersion. The first Parsons I fitted ended up with a new tank as the copper ripped as the old immersion was being removed.....Immersun all the way now......

 

[MicraShed]

Does the "pulsing" explain why my immersion switch neon looks like its at a 1990's illegal warehouse rave?
(Click on pic for video)

 

[Gavin A]

yes.

I'll have to add that to our web page - now with add rave effects in your airing cupboard.

 

[Worcester]

You guys obviously don't remember the 70's party trick of shoving a flourescent starter in series with a normal (coloured) 60W incandescsent bulb and creating great random flashing lights for home parties. -- But then no self respecting spark would ever admit to knowing about that trick!

 

[Gavin A]

we had lasers and video projectors at our parties....


well actually I do remember one party where the lighting effects somehow ended up involving a torch and a christmas baubble being spun by hand.

There was also one party that involved some electrical work on the fly using tin can fuses and plugs being ripped off the decks and them being wired direct into the socket before I erm sorry my friend had to switch to that deck before the record ended on the other deck.... which probably wouldn't pass muster on the old risk assessment. Or maybe I imagined it.

 

[MicraShed]

I was running round in short pants in the 70's but have been guilty of occasional iffy electrical lighting effects for the Venture Scout disco in the 80's.
Then I moved to Manc in the 90's and the less said about the electrics in the nurses digs the better really.