Heat pump COP vs. capacity
A while back Marc Rosenbaum mentioned tuning his heat pump to more closely track the needed capacity in heating mode, and reported significant efficiency improvement. I dug into the documentation for the Daikin here and while it doesn’t really have truly *variable* capacity in heating mode — it tends to just run full-blast because there’s no issue about latent load — it does have a “demand” setting which simply reduces the output to a configurable percentage. This is either a fixed setting or can be externally switched in and out via wired control from, say, a building-management system. The default reduction is to 70% of nominal capacity, so I chose that to play around with. In this case that brings an 18,000 btu/hr down to around a ton, which is approximately the design load for this house anyways.
Last night in our [rather rare!] subzero temps I saw the first
instance of this system not quite keeping up with the load —
overnight the house didn’t get above 64F, with a 68F setpoint.
It ran continuously with the inevitable pauses for defrost,
and at those ambients there’s a slight derating curve so I’m
guessing that it was doing about 10,000 btu/hr on average.
[I was undoubtedly also getting bit by the “polyiso problem”
*and* heavy night-sky radiation at the same time.]
The problem is that there’s no real way to measure the present
running COP of the system. So here’s the question: how likely
is it that deliberate capacity reduction in a heat pump, in
heating mode, will raise [or lower?] the effective COP? The
major observable change seems to be that the delta between
evaporation and condensation temperatures shrinks a little bit.
Doing my standard meter-read calculations seems to so far return
a lower BTU/hr/degF figure [that assumes a COP of 3] in this reduced
configuration, which might be indicative but there may be a couple
of other factors in play from minor insulation improvements hereabouts.
It’s really hard to tell with these things, but if someone can offer
a little theory on whether reduced compressor RPM might affect
performance that’s sort of what I’m fishing for.
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