Tuesday, October 29, 2013

Air source, Ground Exchange, GSHP, MVHR discussion

29 Oct 2013: Continuing the conversation, David R writes: "With the DHSS and some iterations of the Viking House solar/passive slab the earth store is insulated from the fabric of the building but is continuous at its base to the surrounding earth which would hopefully mitigate most of the effects you describe.
      Of greater concern to me is the size needed for effective storage; our build is a super insulated triple-glazed SIPS structure which should have relatively low space heating requirements with MVHR. On the Viking House site they indicate that some of their stores are sized sufficiently to cover the required number of heating days for the thermal load of the building, I was just not sure how accurate that would be?
   Thanks for the info re the boreholes, four shallow ones would certainly seem to be a way around the problem. Do you know how GSHP compares to ASHP in terms of power requirement? That seems to be one of the issues we have for our SAP calculations in that we need a load of PV to offset the heat-pump usage.
     As an aside re ASHP, for our MVHR unit we are installing a Rehau EcoAir Ground to air heat exchanger to pre-heat (or cool) supply fresh air to the MVHR. As ASHP seem to have a significant power penalty and COP loss due to de-icing during winter why do none of them allow for such a simple pre-heat fix as a ground buried supply pipe?"

David N-C replied same day: I don't have a comparable consumption figure for the ASHP although there is data on the web, e.g., the Mitsubishi Ecodan brochure shows useful information. We have one in one of the ecohouses on the campus here, but i enquired and nobody is keeping metering records for it…. so what is the point. Its lived in by PhD students who could be keeping records!!
     If the store under your Viking house is uninsulated downwards, I would merely have about a metre down drop of perimeter insulation, and let the underside of the store reach down to the infinite everywhere below. Perimeter losses are very seasonal and wasteful. Is the footprint of the house sufficient? Mine is two storey, with a footprint of only 9.6x7.2m2…. If the pipes below are unjointed and made of ground loop or borehole rugged plastic, then there is better security against fracture. They should still be buried as deep as you are willing to go. If you are actively solar charging the store, you can protect against a frost heave, and if the occupant (you and successors) have a temperature sensor in the store, you can turn the solar charging off if you have an extremely hot summer and are worried about over heating.
     Ground to air exchange is a good passive method for pre tempering your MVHR air and Rehau are the best experts in it, including the means of dealing with health risks etc. That will be around the perimeter? I have an undercroft below this house, and should have thought of using that as an air intake if we had a whole house MVHR fitted.
     An ASHP is a closed box in that it is not designed to be adaptable. A GSHP is really a general purpose device that could be adapted by being linked to solar panels, air to air exchanger or ground boreholes. the one we have included the water tank, so is inside the house, to shorten pipe runs.
     The reason that ASHP doesn't use ground air exchange is that the airflow rate is many times that of MVHR. The flow rate would be so fast that there wouldn't be time for a heat transfer, whereas the MVHR only requires 0.6 ac/hr. Actually, I have just done a calculation and that seems quite a lot of air! For my house of 600 cubic metres, that means 360 cubic metres of change, which is 6000 litres/minute or 100 litres per second…. which has to be reheated by the MVHR.
    An ASHP is flowing 50-100 cubic metres/minute of air through its fan depending on size. Taking 50 for a house with excellent insulation, that would be 833 litres per second. A larger model would use twice that amount. It needs to draw the air from the infinite everywhere and not be hampered by the frictional losses of a pipe with an air intake big enough for children and animals to get lost in!

PS, to most people DHSS is the Department of Health and Social Security... only a few thousand google hits before finding the one you mean!

David R replies: "Thanks for all your input... We have a bigger footprint (around 115m2) but I can still see this being marginal for an effective heat store unless the pipes are trenched in at some depth, which has its own health and safety issues. I think I will try to get some comparative costs for what would be needed as a sub-foundation store compared to a borehole scenario.
    With respect to using ground air heating to pre-heat an ASHP, I had suspected the flow rates would be the issue and your reply certainly puts that into perspective.... I think the badgers have a surfeit of available habitat around here without adding more artificially!
     I will let you know how things pan out, thanks again for your consideration!"

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