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...