Immersion Heater inline with central heating?

Wondering if anyone has come across this idea before, of putting an immersion heater inline with the central heating system.
So usually the hot water tank would be used to provide hot tap water, however our biggest gas usage is still the central heating.
On sunny days we are producing plenty of excess solar, so the idea would be to store that excess as heat in the hot water tank, to reduce the amount of gas needed to run the central heating.
Not yet ready to completely replace our combi boiler, as gas is still so cheap, but trying to think of a system to reduce the gas usage when we have excess solar.

Thanks in advance

Have a look at the sunamp heat batteries,they have a different name now but a quick Google will find them. They store heat in a phase change material rather than water but can be configured to store any excess solar energy then they preheat water going into the combi boiler ( both hot water and central heating return). The boiler then doesn’t need to fire because the water is hot enough already.

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Good shout, but at ÂŁ1800 for the smallest one:

I feel like a hot water tank and immersion heater would be a cheaper installation… But if the sunamp can be put inline, no reason a hot water tank couldn’t be?

I think a hot water tank used in that way is generally called a “thermal store”. They work a little differently from a standard hot water tank as you don’t use the water in the store directly, you run it through a heat exchanger into the heating / hot water system instead. I don’t really know anything more about how they are set up or controlled.

The problem with the tank being inline rather than running through a heat exchanger would be that when you don’t have excess solar your boiler would have to heat all the water in the tank as well as all the water in your heating system.

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Could you have a water tank in series with your hot feed going to the radiators?
Sort of like a conventional system in reverse. So the immersion heater heats the tank, then the coil (which would normally heat the water within the tank) would be connected to the radiators, therefore reducing the time required to heat the water in the system.

Our heating system is a closed loop I think, so as Matthew mentioned if there isn’t any excess solar, the gas boiler would need to heat the tank as well… unless it is put in parallel with some clever valves?

Not sure if I understand your thinking on this. I divert excess solar to my hot water tank. Unless the day is really exceptional this soaks it all up. That’s energy that doesn’t need to come from the boiler. There are lots of diverters out there. I use Myenergi’s eddi because I also have their EV charger. Excellent UK-designed product.

Hi Paul, currently I don’t have a hot water tank, just a combi boiler. Looking to try and preheat water with excess solar, to reduce the boiler usage or negate it entirely, but without the need to heat up with gas if there hasn’t been any excess solar. Otherwise I don’t want the hot water tank to turn on

With the combi boiler this is an odd one. What may work is to put a small mains flow hot water tank in the pipe work loop of the central heating. This would be heated with an immersion heater with its own thermostat. So when CH is off the water tank is heated by PV, and would turn off when water at max temperature. When the CH is ON the boiler will operate normally.
This is the tricky bit, if the water tank is on the outward side of the boiler then when the boiler starts it is pumping all the cold water in the CH system before it gets boiler gets up to temperature. During that time the hot water tank will not cool down, and the boiler will always see the cold return water first.
If the hot water tank was on the return side of the boiler then very quickly the boiler will be seeing hot water so the boiler turns off. Eventually the hot water tank will get colder and the boiler will see colder return water and start automatically.
If the solar PV was not enough to power the CH then the boiler will switch on normally
Unfortunately it doesn’t work in real life. The water tank temperature will not go below the CH return temperature, which at the end of a central heating period will be fairly warm. The insulation is so good that only a small PV heating top up is needed. Nice idea starting from cold but not daily.
A traditional hot water tank is at low pressure, but a mains flow hot water tank Like Santon would work.

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I see what you mean, once the CH is run once, it will heat up the tank and that will be that due to the insulation. Trying to imagine a situation where the tank is in parallel with some special valves that only open when the tank water is hot, but all seems a little complicated for what it is now. Maybe the solution to this is going to be the Sunamp heat battery mentioned before, or individual instant hot water boilers in key areas, with electricity stored in a conventional battery… :thinking:

I’ve seen some articles from a few years ago about a product called “the pod” from viridian. Looks like it did exactly what you are talking about. Maybe you can get in touch with them, see if they still sell it. It seems like it shouldn’t be impossible but it’s the diverter valves / controllers etc that I think needs a more professional touch.

Brilliant, thanks Matthew, if this is still available, seems like a product many people with PV should have to reduce gas usage!

The pod looks good but with a combi boiler there is a problem if hot water replaces the cold feed. This article explains it and gives suggestion how to use the POD. I would add a safetym thermal switch on the hot water feed to the combi boiler to be certain that it is not too hot.


Hi Paul, I am considering solar for my hot water and possibly a thermal store.
I’m just wondering aprox how much solar you generate. We have a 10 panel system - 3200 - 3800 kWh per year. I believe 2kwp - would this be enough for hot water for the heating too - or just domestic hot water.

Hi Carla,

In the summer we generate plenty for hot water and spill some excess to the grid. We have 3kW peak but a 10 year old system that rarely gets to 2kW.

However, in winter we use a lot more in the heat pump to keep the house up to 21deg. We have a cheap night-time tariff and small battery but still use a lot of grid power.

Hope this helps.


Our solar PV is 10no. 330W panels, so 3.3kWp.
We typically generate 2590kWh/yr (the roof does get some shading), and
the panels reach ~2.4kWp in midday sun.
They can briefly reach 3kWp immediately after a cloud goes over then house, followed by a burst of summer sun. This is because cooler solar panel produce more power but the panels get hot in the sun, so when you have had some cloud followed by bright sun that is when you get most instantaneous power output. Clearly though longer term output is better with continuous sun (hope I’ve explained that OK).
I am attaching a graph which shows how much solar PV get consumed at our house as a % of what is generated and as a % of what the house actually uses. You can see that although we are now using nearly 100% of what the PV can generate in winter (thanks to PV divert into Sunamp from Sep-19 onwards) the actual % of our household needs in the winter is quite small (around 7-10% of our electricity consumption in Dec-Jan).


Hi Dom,
thanks for sharing this and your very clear explanation. One further question though -
How have you been able to generate this data from your solar?
Do you have an additional monitoring system?
:pray:Thanks again

Thanks Paul :pray:t3:
Do you mind if I ask a couple of questions for me to compare and estimate house sizes, and energy I might need to run a similar sytem.
How many bedrooms and inhabitants are there in your home.?
Aprox how many kilowatt hours do you pay for in your winter bill ?
Also about your heat pump - what size is it and s it air to air or air to water?

Thankyou in advance

4 beds. Anywhere from 2-6 people
Air to Water 16kW - probably 50% too high.
Cannot yet separate usage but have now installed a measurement system so next year I’ll know.

My big message is chose suppliers more carefully than I did and get a survey before choosing installers.

For comparison, I am hopefully getting an air to water heat pump soon. :crossed_fingers:t2:
3/4 bedroom semi detached, solid uninsulated walls, loft & under floor insulation, treble glazing.
Solar thermal & 4kWp PV
6kW heat pump.

This is to replace the 17 year old boiler under the BUS grant scheme.