It would seem that the biggest problem with R290 (55C) is that its condensing temp is much lower than R410A (R71C) in a DX system running with a Te of -10C. That's is not bad for space heating but doesn't leave much room for DHW. This coolpack run was without a SLHX.
With a SLHX, the condensing temp on R290 increases to 65C and for R410 it goes up to 85C but the pressure is quite high. Under the same conditions the COP of R290 seems to be a bit higher.
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