I tried an experiment this week during our cold snap. We’ve kept the door closed to the first floor ell (bedroom and bath) and let it run cold, because the Fujitsu wasn’t sized to heat that space too. I opened the door early in the cold snap, and let the heat pump go, leaving it set on 70°F. What I found was that overnight the main space went to 66°F, and the upstairs and back bedroom were 3° to 4°F lower.
My calculated heat loss in these conditions is about 24,000 BTU/hour, and the heat pump is rated at about 17,000 BTU/hour at about 10°F. You’d think it would not be able to keep up.
My heat loss number may well be too high, and the rated output of the unit may be quite conservative.
A variable-speed minisplit tapers its output as it nears the setpoint
One other thing I wonder about is that even though the room was not at the setpoint it seemed that the unit didn’t run on full output much.
My system has the temperature sensing built into the wall cassette, so it may be sensing a higher temperature than out in the room. It may make sense in severe weather to set the thermostat up to 72°F instead of our normal 70°F.
Unlike a boiler, these variable-speed units taper off the output as the space approaches the setpoint instead of always running at full bore, so that may be a disadvantage of a smart unit — it’s trying to stay at a more efficient operating point instead of making me as comfortable as possible.
Anyway, comfort trumped further experimentation and we closed the door again to the ell.
Modem and router energy usage
On a totally unrelated topic: lately I’ve tried to identify the electrical loads in our house that use power around the clock.
I am reluctant to have anything in the house that uses energy 24/7. With 8,760 hours per year, small usages add up. One such is the exhaust fan on the composting toilet — at about 20 watts it uses 175 kWh/year, or 4% to 5% of our energy usage.
Another is the cable modem for internet access we have from Comcast, and the Apple Airport Express wireless router. I’ve been measuring both. The router uses just over 3 1/2 watts, while the modem uses 6 1/2 watts. Together they use 88 kWh/year.
I imagine that if we were off-grid, we’d unplug them in the winter when solar power is short. Being grid-tied, we’re just lazy about it.
What’s using power 24/7 in your house?
Marc Rosenbaum is director of engineering at South Mountain Company on the island of Martha’s Vineyard in Massachusetts. He writes a blog called Thriving on Low Carbon. Marc teaches a 10-week online Zero Net Energy Home Design course as part of NESEA’s Building Energy Master Series. You can test drive his class for free.
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