[Greenbuilding] Ductless Heat Pump Performance - November 2011

nick pine nick at early.com
Sat Dec 3 14:06:13 PST 2011

Paul Eldridge <paul.eldridge at ns.sympatico.ca> writes:

>... virtually all of our space heating needs are satisfied by two ductless 
>heat pumps and our DHW is heated electrically... our consumption for
November can be viewed at:


Your heat pumps made 964.4 kWh of space heat with 13.1x30d = 393 kWh at a 
2.45 COP?

>Our goal this year is to get our total household consumption below the 
>10,000 kWh mark...

Less than 10K kWh in an average year seems easy with the help of some solar 
siding :-) Energy Plus weather data stats 
say Shearwater NS has a 4.8 C average temp and a 6.8 max on an average 
November day. December has -1.9 with a -0.2 high. PVWATTS 
says a Shearwater south wall receives 2.75 and 2.33 kWh/m^2 of sun on 
average November and December days.

So 1 ft^2 of US R2 twinwall polycarbonate south solar siding with 80% solar 
transmission and a 20 year lifetime and a materials cost of $1.30 would 
transmit 0.8x2.33x317 = 591 Btu on an average December day. With average 80 
F air inside and  a 90 F outlet temp and a 30 F daytime temp over 6 hours, 
it would lose 6h(80-30)1ft^2/R2 = 150 Btu, for a net gain of 441 Btu/day. If 
your house needs 964.4x3412/30d =  109684 Btu/day when it's 20 C indoors and 
4.8 outdoors, its effective thermal conductance is 
109684/24h/(20-4.8)x1.8F/C = 541 Btu/h-F (a lot), so it would need 
24h(20-(-1.9))1.8x541 = 512K Btu on an average December day. This might come 
from 512K/541 = 946 ft^2 of solar siding... 473 ft^2 might provide half the 
house heat in December,  at a materials cost of  $615.

Perhaps you have too much direct gain solar heat, ie too many windows losing 
heat at night and on cloudy days?


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