LESLIE: Well, if you live in an older home that wasn’t built originally with central AC or maybe you’ve got a system that’s past its life expectancy, now might be a really good time to invest in a new AC.
TOM: With improvements in efficiency, there’s no reason a new AC system can’t leave you cool and your budget comfortable. Here to talk us through the options is This Old House plumbing-and-heating contractor Richard Trethewey.
RICHARD: Hello, people. How are you?
TOM: We are well. Looking to keep cool this summer. And first and foremost, central AC is much more energy-efficient than room air conditioners, right?
RICHARD: Well, it generally is if it’s done right. You know, not only are they more efficient than a room air conditioner but central air-conditioning systems are much more efficient than they were just 10 short years ago.
TOM: So let’s start by talking about the basic types of air-conditioning systems, because there’s some new systems that are out there and combinations of systems that can be confusing.
RICHARD: Well, the standard system that was always available to us was to have a cooling coil installed on the top of a gas or an oil furnace. You’ve all seen them. Down in your basement or in that garage is the furnace and that has a burner and a blower and that pushes air out through the ductwork.
Now, at the very top of it, there’d be a coil that had refrigerant running through it. As the air went across it, it was cooled. It connected by refrigerant lines [that’s to outside] (ph). And that was the standard for many, many years.
And so what they’ve done the last 10 years is they have insisted on higher efficiencies. It used to be that you could get away with a 10 SEER and now you need 13 SEER or now you need 15 SEER in different places. And that is a Seasonal Energy Efficiency Ratio.
TOM: So that’s a way to kind of compare units to units and efficiency to efficiency.
RICHARD: Right. Sort of like the energy ratings on appliances or the mileage for cars.
TOM: Alright. So that’s the core system. Now, there’s another system out now called a “mini split-ductless.” How does that …?
RICHARD: Right. And this really comes to us from Asia. And really, the rest of the planet does it this way. The first system I described was called “unitary.” It’s where you have one device and that sends heated or cooled air through the building.
These splits are very easily zoned systems where you could have – in each room or a group of rooms, you could have the thing called a “high-wall cassette.” You’d see it on the wall and it has a way to have heating and cooling come out of these ducts. You’ve seen them all …
LESLIE: It’s about 3 feet wide by 18 inches tall.
RICHARD: That’s right.
LESLIE: They’re white. They mount high on the wall. There’s little vents on it.
RICHARD: That’s right. Right.
LESLIE: I mean they’re attractive and they kind of go away.
RICHARD: And they’re thermostatic and they’re quiet.
And so now you’ve got zonability (ph) where normally, with that unitary system, you had one thermostat, generally. It brought on the whole house for cooling and then you turn it back off again. This gives you the chance, as the sun tracks around the building – now there’s more load on the south side. That unit on the south side can come on and keep up with it, because you have multiple units inside the building.
LESLIE: Now, there’s one unit on the interior, which is that split system.
RICHARD: That’s right.
LESLIE: And then on the outside, you’ve got your condensing unit.
RICHARD: Condenser. Right. Well, it used to be that you’d have to have one indoor unit matched up to one outdoor unit.
LESLIE: To one outdoor.
RICHARD: And in the old days, the outside units used to be so big. And they still are.
RICHARD: Now, with these splits, the units are much smaller and they can stack. They’re almost a small rectangle against the building. They can even hang on the wall brackets.
LESLIE: And you can have more than one interior unit to one outside condensing unit.
RICHARD: That’s right. But that’s the evolution. It’s just changing now. It used to be that it was always one to one.
LESLIE: One to one.
RICHARD: Now you can have one magic box, so to speak, outside and you can connect to three, four, five or six units inside.
Now, you’d think that was enough but no, the next thing that has just shown up is a variation of this where you can have a single box outside. That box has a thing inside it called an “inverter.” And the inverter will not only allow you to have cooling to four, five, six different units inside the building but it actually is so efficient it can reverse itself. And in the cold, cold weather, down to about 5 degrees outside, it can find enough heat in the outside air to still heat the building.
That’s really what people just can’t believe. “How do you get heat out of cold air in the winter?”
TOM: That’s interesting. So that’s kind of like a heat-pump tech now.
RICHARD: That’s right. It’s a heat pump that works. Heat pumps never did much once you got above the Mason-Dixon Line. But these units have really got some excitement. They actually are so efficient because they’re not just cycling on and off; they’re actually on all the time a little bit, just grabbing a little bit of heat all the time and putting it back in the building.
TOM: That’s really cool.
RICHARD: The other thing that’s great is now you’ve got some other choices. It used to be that you could only have that high-wall cassette inside the building. Now some of these units actually accept ductwork off the units. So you could still have, hidden away inside the building – comfortable. Another one is a picture frame so that a picture frame acts as the cooling and heating unit in the space.
LESLIE: Oh, I’ve seen that one.
RICHARD: It’s a – yeah, they’re really pretty cool. A lot of choices now and it’s an exciting time to be in this game.
TOM: So, if you’re thinking about upgrading your existing system, maybe your outdoor compressor fails and you need to make a decision, what kinds of things are important to know before you actually do that work? Can you always go sort of part for part? Is it going to fit? Can you put a better, more efficient air conditioner, perhaps where you had one that was less efficient, and still have it work?
RICHARD: Well, Tom and Leslie, this is really a minefield now because of regulation. You might have your inside unit and your outside unit. And the outside condenser, which is exposed to the elements, fails. Now you just want to get a new outdoor condensing unit. Well, the rules have changed about how efficient you have to put that unit is. And the rules have also changed about what type of refrigerant that you can use inside those.
So, now, you might say, “I just want to replace the outdoor condensing unit.” Now, by mandate, you’re going to have to not only change the outdoor unit, you’re going to have to change the indoor unit to match it.
LESLIE: So they speak to each other.
RICHARD: And you’re going to have to change the refrigerant that goes through it. And that involves evacuating all the refrigerant out of the lines and it’s not a small deal anymore.
TOM: There’s nothing much that you can save in this process. You’re pretty much going to be replacing everything.
RICHARD: That’s right.
LESLIE: Now, when you close your air conditioning up for the season – if that’s something that you do; you do a turn-on and a turn-off – are they draining the lines at that point? Or it always has refrigerant in it?
RICHARD: It should always have – the refrigerant you put in, if you don’t have a leak, it should be in there 25 years from now.
RICHARD: The only thing I’ll tell you about winterizing is if you’re in a place where you’ve got some really dirty tree over that condenser, you should cover it so that all those pine needles or leaves don’t get down inside. Because what’s inside that condenser you don’t see are relatively delicate fins – aluminum fins – on a refrigerant coil. And if that is filled with all sorts of foreign objects, it’ll work really hard and it’ll ultimately fail because of it.
TOM: Yeah. Richard, in terms of the cover, you’re not talking about sealing the unit as much as keeping tree debris out, are you?
RICHARD: Well, you cannot confine the air inside of a condenser for it to operate. And so during the season, when it’s got to run, you’ve got to make sure the air can pass through and up and out of the condenser.
RICHARD: But in the winter, in a place where you’ve got dirty trees, you might want to seal it up tight to keep those pine needles and leaves out. And be sure to pull it off before you start it in the season.
TOM: Makes sense.
Now, I want to finish up with one question about sizing. People think that bigger is always better when it comes to AC systems. But it’s really not about getting the biggest system; it’s about getting one that’s designed to work properly with your house, right?
RICHARD: Yeah. That’s the biggest mistake we all make in both the cooling and the heating world. We think bigger is better.
Now, in the example of cooling, if I put in two times too big of a cooling unit, thermostat comes on, it now quickly tries to make the air cold and then it shuts off. It means that you haven’t run that air conditioner long enough to actually take any humidity out of the building. So now you end up with a cold, clammy space.
LESLIE: And it feels colder.
RICHARD: Absolutely. If we’ve done our job as heating-and-cooling professionals, that air-conditioning system, on the worst day of the year, would never shut off; it would just be on all the time. The fact is that’s never the case; it’s on, it’s off, it’s on and off because they are oversized. The same thing is in heating in reverse. If we have too big of a furnace or too big of a boiler, it’s going to cycle.
Think about the example. If I had an automobile where I turned it on and off every two minutes, the engine would be harder pressed to run clean. The same thing with an air conditioner. If you cycle that compressor a million times an hour because it’s too big, it’s going to be short-lived.
TOM: And because of all the power it takes to get it on, initially, you’re probably using more electricity.
RICHARD: Absolutely. That’s right. It’s the take-offs and the landings that use all the fuel.
TOM: That’s the important part.
RICHARD: That’s right.
TOM: Richard Trethewey, the plumbing-and-heating contractor on TV’s This Old House, thank you so much. I’m sure you’re making a lot of us more cool and comfortable this summer.
RICHARD: Stay cool, you guys.
LESLIE: Alright. You can catch the current season of This Old House and Ask This Old House on PBS. For local listings and step-by-step videos of many common home improvement projects, visit ThisOldHouse.com.
TOM: And This Old House is brought to you on PBS by Lumber Liquidators. Hardwood floors for less.