Heat Pump: Ducts Open or Closed?

LESLIE: Cinda in Missouri has a venting question. What can we do for you?

CINDA: Yes. We recently purchased a home and it has a heat pump.

TOM: OK.

CINDA: We are – someone told us – actually, the owner told us to make sure to keep the vents open to – from the outside of the house to make sure it gets enough air. Is that something we really need to be concerned of? We’ve got tile floors and it’s cold on the – seems to keep the floors cold.

TOM: So, you say vents open on the outside of the house. You mean the supply registers on the exterior walls?

CINDA: I believe that would be true. It’s, yes, down near the foundation of the home.

TOM: Right. OK, well, look. The way a heat pump works is – this is an electric heat pump, I’m going to presume. A standard, fossil-fueled furnace like a gas or oil furnace is going to heat up your air to somewhere around 130 degrees or 140 degrees so when you put your hand in front of it, it really feels warm coming out of the register.

A heat pump is only going to heat it up to like around 100, 110 degrees. So the difference is that when you put your hand in front of it, it doesn’t really feel warm because, in fact, what’s going on is that wet air is blowing across your skin and the force of the moisture evaporating off your skin makes your skin feel cooler than it really is. Even though it heats the house, it does so differently and very gradually.

So closing those off is not going to help you. You close those off, it’s going to take even longer to heat up the house. The best way to use a heat pump is to set it and forget it. Choose a temperature that you like; leave it at that temperature. Come hell or high water, it’s always at that temperature.

Now, you can use a clock setback thermostat with a heat pump but you have to have a special one that moves the heat up very, very slowly because the way a heat pump works is it only maintains a two degree differentiation between what it’s set at and what the temperature is in the room. So if you set it at, say, 74 and the temperature falls to 73, heat pump comes on; 72, the heat pump is still on; 71, the heat pump says, “Whoa, I can’t keep up with this because it’s more than two degrees between what the temperature is in the room and what the thermostat says it should be.” And then it brings on the electric resistance heat – which is a backup to the heat pump; it’s built into it – but that costs about two to two-and-a-half times as much to run, so you end up heating your house all winter with straight resistance heat.

So, with a heat pump, it’s never going to be as warm as a gas or oil system – if that’s what you’re used to – but you’re best to just sort of set it and leave it alone. Don’t close those vents; it’ll take a lot longer for the house to heat up.