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Old 04-23-22, 05:09 PM   #2
Helper EcoRenovator
Join Date: Oct 2020
Location: Cambridge, UK
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Hi, great project!

From your description, I understand your gas boiler has an internal pump - hence you need another for the heatpump. You intend to convert the A2A unit to a split A2W unit I think, with presumably a freon-water BPHE indoors on your heating circuit?

Perhaps you could measure your present system, in order to understand the performance expected? Turn all TRVs(Temperature Regulation Valves) to full, boiler to full, pump to full. What we need is the steady state flow and return temperatures, and also the gas power used - these will indicate how effective the radiators are at dissipating heat into your house. Your boiler modulates down though, so this is only useful if you can tell the power.

Best performance is likely to be had with a large area (many plate) BPHE. If there are many TRVs or other thermostats that can restrict the heatloss(bad), then a 2 pipe buffer is useful in preventing short cycling, but if your system is simple I wouldn’t bother with a buffer.

The bphe is likely to have a thermal transfer of around 2000W/degC/m^2. So for example a 40 plate 70mm*180mm unit would have about 0.5m^2 surface area, and at 5kW have a 5degC temperature drop across it. Bigger is of course always better, but around that size is what I would get. I bought some units a bit like that from aliexpress. Nb: Get one that has a high enough working pressure, many are lower pressure than the A/C gas operating pressure.
As this is going into an existing system, I would add a magnetic filter if you haven’t one before the bphe, to keep radiator rust out of your bphe.

As to the pump; more power will always give better heat transfer, eventually the COP will reduce if the pump power is also considered. It’s usual for internal circulator pumps to be 30-60W or so, and the better electronic ones are generally speed controllable like the one you suggested. My 2.5kW gshp uses 40W(wilo pico) on the external pump, 20W(grundfos alpha2) on the internal. It would be a little more efficient to run the internal pump faster, but I like it silent.

That toshiba unit uses R410a gas, with a gwp of 2000. How do you deal with the gas in it now- it shoudn’t be vented, but carefully captured etc.

Good luck!
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