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Green Building Bible, Fourth Edition
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  1.  
    We have a large solar thermal system and are planning on installing a wood burning stove with a back boiler. I know these should have a radiator or similar to act as a great dump in case of overheating. Is it possible to use the solar thermal? There is an option to circulate the heat back to the tubes of the water in the store gets too hot. Presumably though the tubes are not very good at losing heat compared to a radiator?
    • CommentAuthorowlman
    • CommentTimeSep 13th 2017
     
    The overheating issue in back boiler systems is usually taken care of with a feed and expansion tank in the case of an open system, or an expansion vessel in the case of a sealed system. Although the former is the usually more preferred one.
    What is the main reason for having a back boiler? Are you heating a domestic your hot water cylinder? Do you intend connecting it to radiators, or both?
    From what you say your Solar Thermal has water in the system, That's new to me, all the ones I've seen plus my own have a Glycol solution ( antifreeze), in their veins. I assume by "Tubes" you mean the ones on the roof.
    Or have I totally misunderstood your question? Apologies if so.
    • CommentAuthortony
    • CommentTimeSep 13th 2017
     
    The answer is no, not fail safe.
    • CommentAuthorGreenfish
    • CommentTimeSep 13th 2017
     
    We have WBS with back boiler, fed into a thermal store via a loading pump. We also have solar thermal but that is irrelevent. The thermal store provides both UFH (no radiators) and DHW (via heat exchange coil). The WBS has a separate cooling water supply that is turned on in the case of overheating, and a small copper outlet pipe to the outside to release this coolant. It was tested during commissioning (I think by accident because the loading pump was connected the wrong way), but has not been needed since.

    What is useful to have is a display of thermal store temps somewhere by the WBS - if the store is already hot throughout then don't add any more logs! While we were still learning , and didn't have any remote display, we were back and forth to the plant room checking temps just in case. Now we have an EmonPi, and can see what is happening on a tablet, but you could also wire in something with a bit of advanced planning.

    Water in solar thermal rather than glycol mixed "thermal fluid" sounds unusual to me too.
    • CommentAuthorowlman
    • CommentTimeSep 13th 2017
     
    Ah, maybe the penny's dropped.
    What is the principle recipient of the back boiler heat? and in the event of that reaching it's max. then do you propose to deliberately overheat the Solar Thermal water store and then if that is max'd out use the heat transfer solution (glycol), circuit, by using it in a kind of reverse mode? I think you've answered your own question I can't see the solar collector tubes radiating enough heat away from the cylinder quickly enough, they are after all designed to capture heat not dissipate it.
    That seems complex and wouldn't it be better to put a diverter valve to a heat sink radiator e.g. bathroom towel rad, or drying room, and leave the solar cylinder alone.
  2.  
    @owlman, yes, our solar thermal has water in it, not glycol, but obviously separate from the thermal store water. I think you're probably right - as the tubes are so efficient, they won't lose heat anywhere like a radiator would.
  3.  
    <blockquote><cite>Posted By: Greenfish</cite>
    Water in solar thermal rather than glycol mixed "thermal fluid" sounds unusual to me too.</blockquote>

    Yes, it is unusual. If it gets too cold, the pump goes into reverse and circulates some of the water via the thermal store to prevent freezing.
    •  
      CommentAuthordjh
    • CommentTime7 days ago
     
    There are two types of tubes. In straight through tubes, the water flows all the way through each tube. These would dissipate some heat. The other type are often called heat pipe tubes, also 'thermal diodes'. As the name suggests these only pass heat in one direction and won't dissipate much heat from the insulated manifold either.
    • CommentAuthortony
    • CommentTime7 days ago
     
    Electronic frost protection, mine works down to -8 outside, unfortunately I have suffered -11 but recon the glycol mix would have frozen at that temperature too.
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