A word about Design Day and not-so-warm homes.

@hot_rod: "Natethehousewhisper.com put a link to a temperature database.

How Much of the Year Will a Heat Pump Work In My City?"

That's an unhelpful characterization. Basically he's taking a complicated subject and simplifying it in a way that's not useful.

First, it's just the wrong way to think about it to say when does a heat pump "work." It won't stop working until the refrigerant stops being able to boil, which is going to be around -40F. When does a heat pump stop being practical to run? That's a complicated question that depends upon your climate, the construction of the house, the choice of equipment and the local cost of electricity and competing fuels.

But Nate chooses to just say that heat pumps don't work below 25F.

As a first-order approximation, anyone who quotes a specific temperature as the point where heat pumps stop being practical, without bringing in all of the other factors, doesn't understand heat pumps.

doghouse design day = sweater+wool socks

Just to provide a real world counter point to Nate. I'm located with an outdoor design temp a couple of F warmer than Minneapolis.

Large part of the house is uninsulated double brick construction with original cast iron rads. No issues with keeping the place warm down to my design outdoor temps with an air to water heat pump without aux heat. Larger spell bellow design temp does use aux heat along with the heat pump but only because the higher temperatures the rads need.

as I mentioned I use his data for the temperature info, not a hp selector

I look at it as a just a guide, all systems need to be designed to match equipment with the specific applications. HPs are certainly less forgiving no doubt

Maybe that is why there was a HPWH with a gas condensing burner in the lower 1/3 at AHR😗