Learn about mini-duct A/C systems, which is ideal for any older home or anywhere you want to install A/C with minimal construction. Find out how they work, whether they are energy efficient and how much they cost.
LESLIE: Well, one of the biggest projects that we undertook at my old house was installing central air-conditioning . Believe me, it was well worth it but it took quite a bit of construction to get all of the ducts to fit.
TOM: And that’s pretty typical when you need to retrofit a home that was built before air conditioning was common. There is, however, a way to install A/C into an existing, older home or really any home where you’d like to minimize the construction necessary to get those ducts where they have to go. These systems are known as "mini-ducts" and they can actually be run through the wall with very little disturbance.
RICHARD: Hi there.
TOM: Now, when Leslie’s system was installed, the mess actually released enough lead paint into the air, which put her family at risk. Minimal disturbance is really a hallmark of these mini-duct systems, though, isn’t it?
RICHARD: Right. In a typical, conventional, ducted system, you need to have large ducts. You need a separate supply to every room and a return from every room. With a high-velocity system, it has one 2-inch flexible supply for every 8x10 or 10x10 area. So most rooms would have just two of them. And you’d have one common return, so it’s minimally invasive to put into the building.
TOM: Now, you just mentioned high velocity. I think that’s an important point to make. Existing systems are bigger in volume but they’re low-velocity; these are smaller in volume but they’re high-velocity, so you can move enough air to actually do the job.
RICHARD: Right. It actually works to your advantage. If you go conventional, low-velocity system, wherever the register is is where you’re going to have heating and cooling . Then the air becomes a prisoner of the temperature that’s leaving the register. If it’s in heating mode, most of the air wants to stay at the ceiling. If it’s in cooling mode, it wants to drop right below that register and it’s going to be pulled over to wherever the return area is in the room, if it’s there.
With high-velocity, it’s a stream of air and it makes the air in the room blend together, so you have no more than 2-degree temperature difference side to side or top to bottom.
LESLIE: And then it’s sort of sucked back in through the return duct and then cooled again through the air handler, which is maybe in your attic.
RICHARD: That’s right. And the cycle repeats. It just puts the right amount of air in every room, comes back through one central return that has a filter on it. The air goes back and gets either reheated or recooled.
LESLIE: Which system tends to be more efficient?
RICHARD: A high-velocity system will dehumidify. In areas of high humidity, the small-duct system will outperform on efficiency standpoint, because it takes out 30 percent more humidity because you have less air across a very, very cold air-conditioning coil.
TOM: Now, is there anything different about the return-duct setup with a high-velocity system?
RICHARD: No, only that there’s only one. I mean the properly-done conventional system should have a return in every room and it should actually have two returns: one for installation of a cooling system  and one for a heating system.
RICHARD: You should close one off in the appropriate season.
TOM: But you almost never see that anymore.
RICHARD: No. No one does it and so that’s why you have so many complaints from conventional systems of being dirty, drafty, dusty, unbalanced, some cool – cold 70. "The thermostat just has shut off and I feel uncomfortable." So, it’s mostly because of improper installation.
LESLIE: Mm-hmm. Now, in the high-velocity systems, because they’re moving so much air so much more quickly, is there any noise associated with that? I would imagine you’d get a whistle.
RICHARD: Well, properly installed, it’ll be as quiet as any system. So just like anything, if it’s not done properly – and the other thing that was introduced in the past year is a thing called an ECM motor. This is really like cruise control for a blower fan, so it feels how much resistance and just brings on the fan at the right speed. And that’s really made a big difference, because it tries to overcome the realities of poor installation.
TOM: Now, that’s got to have an efficiency benefit, as well, because you’re only really moving as much air as you absolutely need to.
RICHARD: Yes. That blower will be about 10 to 15 percent more efficient than the conventional blower.
TOM: How about the cost of a high-velocity system compared to a standard? You have less construction disturbance.
TOM: Is the equipment cost a little bit higher? Does it all balance out?
RICHARD: Absolutely. The material is higher, the installation is almost always lower. What happens is people – installing contractors will say, "I’ll try one of these," but they only usually try it on the hard job where they couldn’t do conventional.
RICHARD: And then they become hooked. They say, "Boy, I could do this a lot of different places."
TOM: Good advice. Richard Trethewey, the plumbing and heating contractor from TV’s This Old House, thanks so much for stopping by The Money Pit.
RICHARD: Glad to be here.
LESLIE: Alright. You can catch the current season of This Old House and Ask This Old House on PBS. For your local listings and a step-by-step video on installing a mini-duct system and even other projects, visit ThisOldHouse.com.
TOM: And This Old House is brought to you by State Farm Insurance. Like a good neighbor, State Farm is there.