One of the biggest question marks when building the house in Vermont using passivehaus ideas was how it would handle truly cold weather. It is recognized that heat pumps cease working well at low temps because their main mechanism of action is to extract the heat from the outside air and turn it into hot air to heat your home. When the temperature is down where it was this week in Stowe, you need to start relying on other forms of heat to keep temperatures up, ironically when the outside is at its very coldest.

What follows is a very boring breakdown of temperature management in our house. I apologize in advance, but if you are reading this, I assume you’re at least somewhat curious how this experiment is working out so… here we go.

This morning we woke up to -30F outside and this is the coldest in a string of very cold days which have been dipping into the -20s. To prepare for the dip in temperature, I put both heat pumps on and ran a small space heater I bought as an experiment this week. The question it is meant to answer is whether or not we want another electric wall mounted heater installed in the main room. It was suggested by our architect last week and the electrician, when prepping and wiring the house, put electrical leads to several walls where we thought added heat might be required so it would be a simple matter to install them. But my wife and I resisted because these are things that would only be used, at most, for one month of the year and only a handful of days during that month (January obv), but we’d have to look at the heating element all year long. So the space heater was a stand-in to answer the question – would another electric heater even help or would the heat it generates rise up to the 20’ ceilings and be useless to us anyway.

The results were somewhat inconclusive but useful nonetheless. The problem we are honing in on is this – on days when it is bitterly cold it is generally also very sunny, so the time in question that we are very concerned with keeping up temperatures is during the night and morning hours until the sun gets high enough to come into the house. The solar gain in our concrete slab, we have seen this week, acts as a battery storing heat throughout the day and doling it out during the night. If you string together a few sunny days (which seems increasinly rare lately) the house temperature stays very warm, almost too warm by the second and third day despite temperatures ranging from -10F to -14F. Below that and you start seeing other issues emerge.

After our -30F night, the house temperature was a chilly 64F this morning. We keep the house set at 68F so it wasn’t a huge drop but you could feel it. I had the small space heater running in the spot where we imagine the wall-mounted unit to be and the nearly-useless heat pumps were running on both floors. We started a fire in the fireplace around 8:30. About an hour later, the temperature was back up to 67F and rising, the sun is blasting through the south wall and it seems on target to be very toasty in the house soon, despite the temperature outside remaining a stubborn -20F.

Performance-wise, we had a final blower door test this week and got a .6 which is the target for a passivehaus. This comes despite having exterior penetrations for a range hood, dryer vent and fireplace. That said, those were the three leakiest areas during their testing with the fireplace taking the prize for being least helpful as far as preventing heat loss. The folks who came out from Efficiency Vermont seemed impressed with the result and our builder was able to seal a few leaky conduits in the mechanical room which appears to have helped quite a bit. Also helping the mechanical room temperature is the fact that I set our heat pump water heater into full electric mode until it warms up a bit. Since the heat pump water heater was making that room cold, it was competing with the heat pumps trying to keep the house warm. The electric bill will be slightly higher but I think we will get some efficiency as a result.

When the temperature outside hit -20F we starting seeing condensation and even ice in some spots on the doors. On the front door, one hinge had some condensation and below that some ice droplets had formed. On the lift-and-slide door, the seam had ice along its length and condensation along the bottom. As the sun hit these, the black metal frames warmed up and dried the condensation and felt warm to the touch.

Despite the extreme angle of the roof, the solar panels are mostly covered with ice and snow. Usually the sun will bring the temperature up to the low 30’s clear the ice and snow and we start the process over again but the outside temperature hasn’t risen above 15F for about two weeks or so which makes the ice and snow from the last big storm linger. It looks like we might hit 32F on Tuesday so, with a little luck, we may shed some of the obstructions and get back to generating electricity. I’m not worried about it but it is interesting to see how this plays out over the course of the season.

Overall the house is standing up to these temperatures well. I think -30F is about the coldest we’ll see for a while. The average in Stowe for January is 17F for the highs and 2F for lows. The coldest day on record in Burlington is -30F and that was in the 1950’s. I think we are well outside of what we expect for “normal winter weather” right now. If we can make it through the next five weeks with no problems and a good sense of how to handle temperatures like this, I think we’ll be in great shape for the future.