Find out what causes toilet tanks to sweat, and how to fix the problem. Get tips on installing an anti-sweat valve--after you have turned off the water, of course.
LESLIE: Well, summertime is a great time of year for barbecues and pool parties. And it also causes you and your family and friends to sweat it out a little bit. But it also causes your toilet to do so, as well.
And if you’ve ever noticed water dripping off of your toilet tank in the summer, you know what we mean.
TOM: So why does a toilet sweat?
LESLIE: It’s getting a workout.
RICHARD: Hey, guys. Nice to be back.
TOM: And it sounds funny but a sweating toilet can actually cause quite a bit of water damage, can’t it?
RICHARD: Absolutely. You know, condensation is the problem. Any time you have too much of a temperature difference – you’ve got cold water on the inside of the tank and then you’ve got warm, moist air on the outside of the tank, it will actually sweat on the outside of that surface and drip down onto the floor. And so we see it – anywhere east of the Rockies, we’ve got this humidity issue. In the West, we don’t have a lot of that sort of thing.
TOM: And we’ve seen some crazy solutions for this, including toilet-tank insulator kits and things like that.
RICHARD: Right. And that – I don’t want to say they don’t work but they don’t always work, because it really depends on that humidity level  in the building and just how cold that water is. And so, really, the best solution we’ve seen is this anti-sweat valve that is designed to be installed down underneath the toilet.
And what it does is it sends cold water but it also mixes a little bit of hot water – not a lot – but just enough to raise the temperature of the water in the toilet tank to get below …
TOM: Oh, interesting. So it warms up.
RICHARD: Right. To get below the dew point that would cause that condensation to form.
LESLIE: That’s interesting. I mean is that something that a homeowner can tackle themselves or because we’re dealing with water and the toilet and waste, is that something we just shouldn’t even mess with?
RICHARD: Well, it really depends on your skill set. I think it does require soldering; it requires you to shut the water off and to do a little bit of plumbing.
LESLIE: Which people forget: turning the water off. People forget.
RICHARD: Yeah, it’s much better to do the work with the water shut off, yes. So it tends to be a professional solution and it’s really as a last resort. But we did one on Ask This Old House and this guy had lived with this sweating problem. He just – everything in his house was perfect and he just couldn’t stand that the water was dripping on the floor. And we finally fixed it and he was just – it was great joy in his house.
TOM: I bet. Now, does it use a lot of hot water? Does it cost energy?
RICHARD: It’s not a lot. It does cost some energy because you’re using some hot water.
TOM: Right. Right.
RICHARD: You’re not literally – you’re not making that hot water, that toilet tank, be filled with hot water; you’re only just trying to temper it a little bit just to get it down above that 45-degree or 50-degree temperature.
TOM: And so I imagine this is a problem that’s much more common in a home that does not have central air conditioning.
RICHARD: Yep, yep. Yeah. If you get drier air – if the air conditioner is on and you’ve dried the air out – there’s a good chance you’re not going to have this condition.
TOM: Now, Richard, is this a problem that’s common to older toilets with the really big tanks or does it also happen with the more modern ones that are more efficient?
RICHARD: Well, it’s really any conventional toilet tank that has water in it. And so, most of the models can have this issue in a high-humidity area.
There are some pressure-assisted toilets. Now, if you looked inside the toilet tank on a pressure-assisted, you wouldn’t see the tank fill with water; you’ll actually look inside and see this black plastic tank or chamber. And in that case, you’re not going to have that temperature difference and it won’t sweat.
LESLIE: Now, I imagine in some situations, you’re dealing with a leaky toilet. Maybe you’re getting phantom flushing or issues where the toilet is running. Is it important to make sure that that’s sort of fixed or maintained before you implement this system, to not waste hot water?
RICHARD: Absolutely. Just imagine if water came into a toilet tank and even if it was cold, over time it would warm up. And so then the sweating problem is gone. But if you keep on adding new 40- and 45-degree water, that sweating problem will be a real issue. So you want to make the repair on two fronts: one to stop wasting water and the other is to help stop that condensation .
TOM: That’s terrific. Now, you have a video on how to do just this on ThisOldHouse.com.
RICHARD: That’s right.
TOM: Yeah and it’s really interesting because you can see underneath the terrible damage that this toilet had been doing to this poor guy’s ceiling for years. And with a very small plumbing assembly, like you’ve just described, it all goes away.
RICHARD: That’s right. Like magic.
TOM: Very, very simple fix. Great idea. Richard Trethewey from TV’s This Old House, thanks so much for stopping by The Money Pit.
RICHARD: Glad to be here.
LESLIE: For more great home improvement advice, you can watch Richard and the entire This Old House team on This Old House and Ask This Old House on your local PBS station.
TOM: And This Old House is brought to you by Trane. Nothing stops a Trane.