LESLIE: Well, it never fails: if your A/C is going to go on the fritz, it’s going to happen on the hottest day of the year.
TOM: So what causes the most common central air-conditioning unit failures and 
how do you figure out what’s happening? Here to tell us is This Old House plumbing-and-heating contractor Richard Trethewey.
Welcome, Richard.
RICHARD: Hi, guys. How are you doing today?
TOM: Here’s a guy that always keeps his cool and that’s really important. Because when my A/C fails on the hottest day of the year, I’m certainly not going to be very comfortable.
RICHARD: No one is normal when their A/C goes out.
LESLIE: No.
RICHARD: Those calls come in and they just – “One of you please help me with this. The whole world is ending.”
TOM: Oh, absolute panic. So what are the common causes of failure of the A/C systems?
RICHARD: Well, an A/C system is a pretty straightforward and simple device. It has two ends of it. There’s an outdoor condenser and an indoor evaporator. And then between it, there are copper lines that are filled with refrigerant. The number-one issue we have is a leak on that refrigerant. So the slightest, little leak will now take that perfectly delicate system and take it out of balance and make it no longer work. So you’ve got to find out where that leak is; it’s generally done by a professional. You know, putting more refrigerant in and not finding the leak is not the solution.
LESLIE: Right.
RICHARD: And many people do that. They do it with their automobiles when you have a leak. They just keep on putting a little more in.
TOM: Yeah. But slowly…
RICHARD: Right, right. And the other thing is I should warn you that this refrigerant is a dangerous hydrofluorocarbon. It’s a dangerous thing to release to the atmosphere. It’s illegal to release it. So if you have a leak and you don’t fix it, by implication you’re illegal to do that. So you really want to make sure the refrigerant that goes in there stays there, both legally and also so you have an A/C system that works.
LESLIE: And you really also need to maintain these pieces. Not only would you find a leak as it was happening, before it became a problem, but you would sort of prohibit other things happening because you’re allowing the machine to operate so efficiently.
RICHARD: That’s right. You’ll know your system. You’ll turn on the thermostat, it usually gets X cold in this amount of time. If you now put that unit on and it runs for 40 minutes and it hasn’t gone down, you know or you can at least suggest that you have a change in the refrigerant inside the system. Because the basic process – I need to make it clear what happens.
Heat always goes to cold, so inside your central furnace, where the air goes across, it is a coil. That’s like an automobile radiant that is wicked cold. Cold, cold, cold. And now the warm air goes across it, heat has no choice but to go to cold. And so it gets absorbed into the refrigerant that’s in that coil and now that heat travels to outside.
Now, the outdoor condensing unit, using a compressor, now dumps that heat to outside, making the refrigerant brutally cold again as it comes back into the building. So you – what you do with an air conditioner is you’re not making cold. You’re extracting heat and the absence of heat is cold. Absolutely.
TOM: It’s cold. Yeah, yeah. That’s interesting.
So, what about the controls themselves? Electrical controls, the sensor controls. Do they often fail? Is that common?
RICHARD: They don’t fail as much as you’d think. And they – many of these devices now are so smart that they’ll tell you with fault codes. And so it’s a lot – it’s actually – in many ways, it’s a lot easier. It’s more daunting to people to see this fancy spaceship of a modern furnace or air conditioner. But all in all, an air-conditioning system works really, really hard and doesn’t fail all that often.
LESLIE: What about drainage? I mean you’re taking all this moisture out of the air because you’re removing the heat and making it cooler. Where does all that moisture go?
RICHARD: Well, every single air conditioner has to have a condensate drain.
TOM: Right.
RICHARD: If you have a unit that’s up in an attic, there’s an additional thing you should always think about. One is to not only have a drain for the air-conditioning condensate but you have – should have an additional safety pan underneath the air conditioner.
LESLIE: Underneath it.
RICHARD: You only have to pay for one ceiling or have one ceiling come down to realize how brilliant this is.
Just like it’s nice to have a safety pan underneath an upstairs washer, same so to on any air-conditioner unit that’s up in there. Because when you are removing heat, one of the byproducts of removing heat and leaving cold is that the moisture, the humidity that was in that air has to come out of the airborne solution and now becomes what they call “condensate.” Now, you have to make sure that all that water leaves there. And if that little condensate – you’ve often seen these little, 1-inch PVC pipes that carry that water away.
LESLIE: Dripping away outside on a hot summer day.
RICHARD: Dripping away. Drip, drip, drip.
And if you have – what happens over time, sometimes the slime that builds up there, now it backs up a little bit. Now, if it backs up, it’s going to come out through the overflow and come down through your ceiling.
TOM: Richard, sometimes we see compressors actually freeze up in the summertime. What’s going on in that situation?
RICHARD: Well, that delicate balance I talked about – you’ve got the refrigerant moving between inside and outside. And what’s critical on that process to work is you have to have plenty of warm air going across that cold coil inside the house. And you have to have plenty of ways to release the air to outside on the outdoor condenser. If either one of those things are blocked -let’s say your filter is clogged on the inside air handler – now that warm air doesn’t come across it. Now, the ice – that coil inside freezes like a block of ice.
And what the symptom many people have is, “I had cooling and all of a sudden, it stopped. And then I let it sit and I turned it off for a long time and I turned it back on, it seemed to work again for a little while.” What happens is each time you do it, you let it melt and thaw and then when you do it again …
LESLIE: Freezing it again.
RICHARD: What it usually means is a lack of airflow across the most important component, which is the inside unit.
TOM: Huh. So that means something that’s blocking that – dirt, dust, whatever – has got in there.
RICHARD: Right. When you break it down to its simplest form, it’s pretty straightforward. I just need the refrigerant to go back and forth between inside and outside. And I just need air to be moving through the inside unit so I can have that really warm, moist air be converted into nice, cool, dry air inside the building.
TOM: Well, it sounds like this is not any kind of a do-it-yourself repair possibility. But it is nice to know what’s going on inside our system so that we know what really needs to be done once it starts to act up a bit.
RICHARD: Right. And even when you’re hot, you know that sooner or later you’ll be cool again.
TOM: Good advice. Richard Trethewey, the plumbing-and-heating contractor on TV’s This Old House, thank you so much for stopping by The Money Pit.
RICHARD: Great to be here.
LESLIE: Alright. 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 State Farm. Like a good neighbor, State Farm is there. Up next, a cleaner, better-looking deck is in easy reach. We’ve got tips on a hot, new product that combines scrubbing, rinsing and disinfecting into one easy tool.
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