Learn how the size of your air conditioning unit is essential to cooling off your living space. Find out why installing an attic fan is not recommended with central air, and how ridge and soffit vents will help to cool down your space.
LESLIE: Kevin in Georgia is having some difficulty with the air conditioning. What can we do for you?
KEVIN: I’ve got a unit  that runs just the third floor of my house and a separate unit that runs the first and second floor.
KEVIN: And it seems that the unit that runs the first and second floor will run; keep it cool at a desired temp – say 79 – and it runs on and maintains and kicks off relatively quickly. But the third floor, it just seems to run constantly for quite a bit of time – maybe 15, 20 minutes; constantly blowing if it’s really hot in the middle of the day. And I’m wondering if it’s just a matter of lack of insulation in the attic or where the thermometer is located in a little hallway or what might be the issue.
TOM: Or perhaps the third floor is the hottest room in the house.
KEVIN: Oh, yeah. Yeah. I mean physics are applying here, definitely.
TOM: It’s probably a factor of the size of the compressor. Now this is a central air conditioning system on the third floor?
TOM: So it’s a split system? You have an air handler and you have a compressor?
KEVIN: Correct. Yeah. And both are relatively new. The compressor is about two-and-a-half years old and the internal unit that’s in the attic is like a year old.
TOM: Well, I suspect that it may not be big enough.
TOM: Because air conditioning systems are fairly dumb and when you set that thermostat, it’s going to keep trying to meet that goal of whatever thermostat setting you choose. And so if it’s really hot upstairs, like it’s going to be in Georgia in the summer, what’s going to happen is it’s going to run and run and run and run until it reaches that particular temperature.
So I would question the size of the unit with the HVAC contractor that put it in. I’d also make sure that the system is serviced in that there are no refrigerant leaks because even though it may feel cool, it might have to run longer to do the same job. You mentioned insulation; absolutely. Insulation and ventilation play a big part in keeping your home cool in the summer, especially if you have a central air conditioning system.
Do you happen to have an attic fan as well?
KEVIN: No, that was my other question because the attic is …
TOM: Yeah, well no; don’t put it in. Don’t put it in, if that’s what you’re going to ask me, because that would just rob more air conditioned air. When you have central air conditioning, putting in an attic fan is not a good idea because it typically reaches down into the conditioned space of the house and sucks out the cool air.
LESLIE: That you’re paying for.
TOM: Yeah. Continuous ridge and soffit vents  are the best way to vent an attic. But I think that those are all the areas that you need to look into but I do suspect that the system is probably too small; that’s why it runs so long.
KEVIN: How long do you think it can run safely constantly before I’m looking at issues with the blower or the compressor?
TOM: Oh, it can run pretty much indefinitely. It’s going to run til it dies, you know? It just generally doesn’t run for 20 minutes at a time, you know? It’ll run for probably seven or eight minutes and then shut off and then come on back on again.
KEVIN: Right. Each room is like 10x12 and I’m pretty sure it’s like a 1.5 ton. Do you think it needs like a 2.5 ton or …?
TOM: It depends on a lot of factors. A contractor has to do something called a heat loss calculation. All I’m saying is that what you’re describing to me sounds a lot like an undersized system.
Kevin, thanks so much for calling us at 1-888-MONEY-PIT.