It's acid-test time. Getting rid of the gas furnace and installing a #heatpump in Colorado was a bit of leap of faith, even though the consensus was that they are good enough to work in this environment now.

We're midway through a stretch of sustained sub-zero weather (as in, below 0°F, not that wimpy 0° used in other parts of the world), so we are definitely putting it to the test.

So far, so good. We did put in a backup 10KW resistance strip, that that has only come on once for a few minutes as far as I can tell. Even so, it's good that we like a cooler house than many; I think it would be hard-put to sustain the sorts of temperatures that a lot of people like to keep in their houses around here.

It *has* certainly burned through a lot of electricity; our summertime surplus from the solar panels is dwindling rapidly. The hope of getting all the way through the winter on our banked electricity seems to be falling by the wayside.

Still, the goal of turning off the fossil-fuel feed to the house remains on track.

@corbet Similar results here, although we don't have the same low temperature range that you do. We put a lot of $$$ and effort into ensuring the house was extremely well-insulated and airtight (with an ERV for fresh air) before doing it, as ti was clear that the energy delivery capability of the heat pumps would be far lower than the natural gas boiler we had previously.

Of course all of that work would have reduced gas consumption too, but the boiler didn't give us cooler air in the summer and the heat pumps do 🙂

@corbet what's the type? Air-to-water? We've got an air-to-water one and it worked flawlessly in -15C (5F). And the service guy told us that they successfully tested it in -32C (-26F). But insulation is key. What also matters greatly is the temperature of heating water, the higher the lower the COP is.

@sesivany It's air-source... anything-to-water tends not to work well in Colorado, where water is scarce. We looked into ground-source, which would be a lot nicer to have, but that's a $100K drilling experience here.

@corbet @sesivany the problem with air source is that the energy requirements scale with delta t squared, meaning you have way more difficulty surviving extremes. With ground source they scale with delta t.

I did a ground source from air source conversion a couple of years ago and the energy savings are impressive (of course, so too were the federal tax credits at that time).

@corbet By air-to-water I meant air as the source and water as the heating medium. We have the same type then. It's sufficient here. We only have 2 weeks of such freezing weather a year. Our annual energy balance is 8.5 MWh produced and 9.6 MWh consumed. A deficit of 1.1 MWh is great for a house that is completely on electricity.

@corbet The unspoken downside is that many of the 'frigoriste" techs that repair these are scammy and that's if it was even installed properly (eg: proper vacuum on lines) so that it doesn't leak in five years. One service call and all your savings are lost.

@corbet What's the mechanism for banking electricity from summer to winter?

@soaproot It's called "net metering"; in essence, the electricity meter simply runs backward when the panels are generating more power than we are using. That generates a credit that we can draw on at other times.

Every state has its own arrangements, some are far better than others. Colorado is pretty good in this regard, fortunately.