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One of the most interesting aspects of renovating the new house was the “heat loss calculation”. We wanted to see if we could go all-electric with a heat pump with only floor heating to keep the house warm. The temperature of the water pumped into the heating tubes by a heat pump is much lower than the hot water produced by a gas burner that’s pumped into radiators — so there were some calculations to be done. Based on the extra insulation we wanted to add, we should be able to calculate the amount of heat that is lost to the environment. The standard temperature difference taken is 30 degrees Celcius, with the idea that you should be able to keep your house at 20 degrees when it’s -10 outside.

Once you know the amount of heat lost, you also know how much heat you have to put in under those circumstances to keep your house warm if it’s already warm. So you want your heat pump to be able to output more than this number. But you don’t want to over-dimension the heat pump either, because the efficiency is low when it’s running at a low capacity compared to it’s peak output… So we went back and forth quite a bit.

This morning, it was -9 here. The heat pump was working at full capacity, but the indoor temperature dropped to 18.5 — half a degree lower than the set 19 degrees. But it was also heating the tap water at that time, and once that was done, it quickly regained the 19 degrees. It’s not been a sustained ‘middle freeze’ but it looks good.

But it does take a lot of electricity: the heat pump is less effective when the outdoor temperatures are lower, because there’s less heat in the air to pump (duh) and there are ‘thawing cycles’ needed because otherwise the outdoor condenser will just freeze shut. Yesterday was a peak day with 40 kWh used, and we’re on track to something similar today.

And that seems like a lot, but it’s just that now we have an app that gives us this information. In our previous, gas-heated house, we’d just burn gas and pump hot water around and we had no idea on how much we used.

Crossposted from my blog. Comment here or at the original post.