Monday 20 October 2014

Solar hot water for combi boilers

Making use of solar hot water with a combi boiler is difficult because most combi boilers don't work with preheated water. However, Viridian Solar have a solution for this called the Solar Pod. It relies on the fact that most combi boilers will work with tepid (not hot) water up to about 30C: about 1/3 to 1/2 of the way to the temperature you are aiming for (assuming you want 45C-55C).

The system gives you a smallish tank (100 litres) dedicated to the solar panel. When you need hot water:

  • If the stored water is hot enough it is fed to the taps directly so your boiler does not switch on
  • If not, then the stored water is combined with enough cold so that it is cool enough to be fed to the boiler.

They cite some field trials showing that the system supplies around 1000 kWh over the year. This does of course depend on how much hot water you use as well as the size of the tank. The two test cases were a 4-person household (but with electric showers so with limited hot water demand) and a 3-person household. For comparison, my conventional solar thermal system is rated at 1152 kWh over the year, supplying two people.

For the Solar Pod system it is critical that the tank is the right size for the panels. If it is too large then the water may not get hot enough that you can bypass the boiler, so you have to mix it with cold and get less benefit. However, if the tank is too small then the tank will get hot quickly and you will have to reject heat a lot of the time.

I have a bit of a concern about legionnaires disease. To kill the bacteria you are supposed to heat your tank to 60C periodically. The problem would come if the tank was hot enough to bypass the boiler and feed the taps directly but not hot enough to kill the bacteria. However I should think this is unusual - at least not likely to continue for days at a time. If  you are worried then you could arrange for an immersion heater to heat the pod to 60C occasionally. Of course this would add to your electricity use.

Viridian also recommend their system for use with conventional boilers, because you don't have to change the tank for a twin-coil one, you can just add the extra tank. Also, having a tank dedicated to the solar heat means  you don't have to worry so much about the timing of when you run the boiler. However, this is easy enough for most households to sort out. (See Optimising your solar heating).

Viridian also have a version that works with PV panels, using the spare electricity  to heat the solar tank.  However, if you have a conventional boiler with a hot water tank there are other options (see Should I use electricity from my PV panels to heat my  hot water tank).

I heard about this at the Cambridge Carbon Footprint forum on Low Carbon Energy.


  1. I have been asked to explain why combi boilers can't take pre-heated water. This is not completely clear to me. In the past I have been told that it is because the boiler can't 'modulate down' enough to handle the little bit of extra heat needed. They either fail to turn on altogether or heat the water too much so it is scalding.

    To check, I asked Vaillant and am still baffled. The first answer they gave was 'Combination boilers are not designed to take pre-heated water so there has to be a maximum otherwise the boiler parts could be damaged. This is 25ยบ'.

    However, they also sent me an attachment with more details and this says 'Whilst all ecoTEC and ecoMAX/2 combination boilers can accept inlet water temperatures of up to 60°C...' . This implies that the boiler can take preheated water. Then it goes on to say '... there is currently no industry wide WRAS approved method of connecting such systems to a combination boiler.' WRAS is the Water Regulations Advisory scheme. It seems the concern is about the risk of Legionella bacteria growth. If that is the only problem, well there are ways around it as I mention in my post.

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  3. Anyone know the practice in 'more advanced' countries; e.g., Germany, Netherlands?


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