I’m often asked where to put heat for energy-efficient homes, and the question quickly turns to heat pumps, and where to put the indoor components, especially those wall-mounted jobbies, what people are calling “heads.” That’s a new term. Somebody in the last few years came up with the bright idea of calling them “heads”, but I don’t know quite why. I’ve spent some time staring at them, I don’t see eyes or ears or nose or smile or anything else head-like, but I’m the last one who’ll argue with a good fad, so head it is. (Back where I come from, we call them “fan coils”, but some marketing whiz clearly thought that sounds too nerdy or techie or something.) So, the question becomes, where should I put my head? Or, more specifically, heads.
The origin of the question is understandable. Each of these heads is not cheap, and if we put one in every room in a house, that will run us up a good bill. So, we have LOTS of people coming and asking “How about we just put one on each floor?” And this is quickly followed by, “Maybe we just put one on the landing upstairs, outside the bedrooms.”
I get worried not putting heat in every room in a house, especially not in bedrooms. We’re quickly setting ourselves up for cold rooms. That head in the hallway is going to do a great job of …. heating the hallway. Assuming we don’t overheat the hallway, the hallway should be heated to a comfortable 70 degrees. If there’s no heat in the bedrooms, the bedrooms MUST be less than 70 degrees. I hate to say it. But it’s physics.
The temperature in the unheated room comes to an equilibrium temperature between the hallway temperature and the outdoor temperature. And, yes, because there’s insulation in the outdoor wall, and there’s no insulation in the interior wall, that equilibrium is closer to the hallway temperature than it is to the outdoor temperature. But it still can get cold. I did a back of the envelope calculation, and with 30 degrees outdoors, and 70 degrees in the hallway, it’s not hard for the bedroom to be colder than 60 degrees.
So the next question that we often get is “How about putting in a transfer fan, to move warm air from the hallway into the bedroom?” Well, that will move a teeny-tiny bit more heat, but it still can never get the bedroom up to the temperature of the hallway. And you need a LOT of air just to even get it CLOSE to the hallway temperature. Like hundreds of CFM. Compare this situation to a typical old forced-air furnace. That furnace is sending air that’s like over 120 degrees into the bedroom (older systems send even hotter air). For a typical airflow of let’s say 100 CFM per room, the heat the furnace is delivering is like about 1.1 * 100 * (120-70) = 5500 Btu/hr. Now with a pretty good transfer fan, let’s also call it 100 CFM (as big as a typical kitchen exhaust fan), and let’s say we’re pushing 70 degree air into the bedroom, and the bedroom is at 60 degrees, we can only deliver 1.1 * 100 * (70-60) = 1100 Btu/hr. That’s 80% less than the furnace provides! Puny. And the bedroom is still pretty darn cold. OK, so we get smart and push air from the ceiling of the hallway, at maybe 75 degrees. The bedroom warms up a little, so now it’s 65 degrees instead of 60. And these examples are for a typical winter outdoor temperature of 30 degrees. As things get nippy in midwinter, like down into the 20’s or teens or colder, as is often the case here in Ithaca, that bedroom is going to get pretty nippy itself, like below 60 degrees.
To get the bedroom temperature back up into the mid-60’s or warmer, we need to put in a transfer fan that’s like 500 CFM. That’s a howler. And it will use a good bit of electricity. It just does not make sense.
Now, I have some hearty friends who do just fine with bedroom temperature below 60 degrees. But we can’t design houses and hope that all their occupants are experienced in winter camping. So I’m not a fan of “just heat the hallway”. And I’m also not a fan of … transfer fans. Or, at the very least, if you’re going to use transfer fans, please do calculations for the fan size, fan power use, and target bedroom air temperature, with conservative assumptions for the hallway’s air temperature. The calculation is not hard. Send me a note if you need a hand with it.
Now if you’ve been reading some of my recent Building Evidence posts, you might recall that there is an upstairs/downstairs thing going on with heating, that works in your favor for upstairs bedrooms, at least where there’s an open stairway. For sure that helps. But I STILL think we need heat in all rooms. And I still don’t like transfer fans. I’m not saying we need a head in every room. Maybe a small stretch of electric resistance heat. Or the heat pump could use ducted indoor units to distribute heat. And there are many other options.
But leave heat out of rooms at your own peril, is my bottom line.