According to the manual of my AWB ThermoMaster 2HR 28.02WT Condensing boiler, it is compatible with a pre-heating solar system for tap-water only. It even has extra facilities to ensure that the coolest water goes through the condenser part (HR part) of the kettle before being preheated by the solar system to get a higher efficiency.

So this is the supported scenario:

enter image description here

They claim that it is not possible to use the solar system for pre-heating the central heating circuit. What is the reason for this? It should be a lot easier since I'd only have to insert the solar system in the circuit on the cold side just before it goes back into the CV?

In my situation it makes a lot more sense to use it for central heating instead of tap-water. The heating is on all day (my wife and children are at home most of the day). The kitchen has its own boiler, so the hot tap-water would only be useful for the shower (normally only used after the sun has set).

The plan:

Not supported

My reasoning was first that the CV system might be checking the temperature of the returning water to determine how much extra power it should add for heating, however the system looks simple and my guess is that it only takes the room-thermometer as input for determining the needed heating power.

Could anyone explain why it is not supported and what other systems do differently to support it? There seem to be many systems on the market that do support it, but what is the big difference between what I already have and those?

  • This is very product specific - you really need to press the manufacturer for a satisfactory answer. Commented Sep 12, 2012 at 0:14
  • 3
    A central heating system would be on most of the day, losing heat into the house almost continuously. The water system is only on for a few minutes at a time, and as you say, it has all day to heat up the water. Perhaps the solar panel simply doesn't have enough capacity to heat an entire house.
    – Niall C.
    Commented Sep 12, 2012 at 4:09
  • 1
    as mentioned this is very product specific and a brief Google search for the manual was unsuccessful ( as i can't read German) but if you do go down that path which is possible with most systems you may want to look into a solar bypass value for the boiler so that if the solar gain is sufficient you just bypass the boiler entirely.
    – UNECS
    Commented Sep 12, 2012 at 7:22
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    As it sits, the question is pretty product specific, and possibly of topic. However, I'd suggest editing the question to a "Is is possible to ..." phrasing rather than a "Why Can't I..." Try rephrasing it to be a bit more general, while still mentioning the products involved. Commented Sep 12, 2012 at 13:03
  • Ok, I edited it. The more I think about it I guess Niall C. is right about the the capacity. Probably the temperature difference between the (still warm) returning water and the solar reservoir is not big enough to do any good (the returning water might even end up warming up the solar reservoir in worst case). @UNECS: Using a bypass would require an extra pump and probably an "intelligent" system that switches between the two. Still I'm wondering how those other systems work, they all seem to tell the same story as my 2nd diagram displays. I must be missing something here? Commented Sep 12, 2012 at 21:55

2 Answers 2


You can solar heat the primary loop with high temp evac tubes in principle but what happens is you mess up the control of the return temperature which is critical for the operation of the condensing boiler. Preheating the tap water is the better approach because you are heating the water before the boiler loop tops it off. This doesn't affect the existing control logic at all.


After quite some research I have come to the conclusion that almost all heating systems that work with solar heating only work well in combination with radiant floor heating. Radiant floor heating works with much lower temperatures than standard central heating systems with radiators in each room. Another difference is that floor heating does not modulate (switch on and off) as frequently as central heating does.

The proposed scenario would not work with my condensing boiler. The boiler does not only use the room thermometer but it also measures the heat of the returning water. The reason for this is that each radiator can have it's own Thermostat to regulate the temperature in each individual room separately. When these are all closed, the temperature of the returning water will still be hot and the boiler will simply keep pumping it around without spending energy on heating it up even more.

Using a solar system in the planned way would cause the boiler to heat up the solar reservoir until it reached quite a high temperature, waisting quite some gas in the process!

I was wondering why no one could answer my question while after finding the answer it seemed to be such a generic and widespread problem. I searched on HR Kettle and to my surprise I didn't see what I expected. Searching the Dutch term HR Ketel will give you an idea of the system. It can be found in almost every Dutch house on the planet :-) I'm kinda wondering how the rest of the world calls this system, or is it typically Dutch?

  • 2
    When trying to find the English equivalent of a technical term, I use Wikipedia's "Languages" sidebar. HR-ketel leads me to Condensing boiler. "HR" is a Dutch term, "hoog rendement", which you would translate as "high efficiency".
    – Jan Fabry
    Commented Sep 15, 2012 at 16:05
  • Thanks I'll edit the question and answer with the correct term Commented Sep 15, 2012 at 17:24
  • @LouisSomers many of the answers you see here are from things we have some experience with, rather than googling for the answer. Using a solar system to run the central heating is something few, if any, of us have done. Thanks for the self answer. +1
    – BMitch
    Commented Sep 15, 2012 at 21:19

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