Derek Cole explains why single HVAC units struggle in two-story homes: heat rises, and different thermal loads between attic and crawl space create temperature swings. He recommends zoning with separate thermostats for each floor instead of buying a second unit, though you'll need an existing duct chase to make it work.
The problem with one unit covering both is that you have different demands upstairs than you have downstairs. So when you look at a house and think about it, heat rises obviously, right? So when heat rises upstairs typically will be warmer than downstairs, right? So chances are, I've seen this before, the thermostat is downstairs. You have a unit that runs the whole house up and down. So to get upstairs comfortable, you then have to make downstairs freezing. When I first got married, we lived in an apartment that was 2 stories but had 1 unit. So I mean, when you went downstairs at nighttime, it was like you better put on like a big coat and everything because it can happen. Because upstairs you have more of a load than you have downstairs. Downstairs, heat rises. You also have the attic upstairs, so you have these forces you're going against. You have the attic, you have windows, you have upstairs heat rises and downstairs, of course, cold air falls and you don't have an attic downstairs. So you do in— and the crawl space is not that much of a heat load compared to an attic. So it's different demand. So how you solve that problem is either 2 units or what we've done quite a few times, which is zoning. You make downstairs one zone, you make upstairs one zone. It's very simple. If you have a unit that covers up and downstairs right now, you can very simply make it a two-zone system. That way you have a thermostat downstairs, you have the thermostat upstairs. You can keep it that temperature you want, you keep it that temperature you want, and they can make it work that way. So, uh, the challenge with some houses is that there is not that duct chase in the house for the duct to be run to have one unit for the whole home. But if you already have that, obviously that's available, but you just need to make it a zoning situation. I wouldn't get two units, I would just zone it upstairs, downstairs, and that will solve the problem. But until you do that, it is what it is. I mean, the thermostat downstairs, upstairs is warmer, and In the summertime, it's going to be— so you're just going to have to freeze it out downstairs and get upstairs as comfortable as possible.
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