Do I Need a Return Vent For a Whole-House Dehumidifier?

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  • Transcript

    LESLIE: Let’s say hi to Katie in Massachusetts who has a whole-house dehumidifier question. What’s going on?

    KATIE: I purchased a WAVE ventilation system. It was advertised on one of the radio stations that I listen to. Company is based in Canada. I checked them out, Better Business Bureau, they’re all fine. But my electrician had a question for me that he’s really not sure that I should keep it, because you hook it up in the basement away from the …

    TOM: Heating system.

    KATIE: The heating system, thank you. And then you open up a vent to the top of the house, which we have. We did that up the basement stairs into the mudroom. So there’s a vent there that will feed this system but there’s no return.

    And my electrician said, “Gosh, if you don’t have a vent from the outside feeding in air to circulate …” – he just doesn’t see how the whole system will really work efficiently if there is not something feeding this flow.

    TOM: I think you’ve got a great electrician there and a guy who really understands building science. Because I’ve got to tell you, I hear the advertisements for these systems all the time and they leave me scratching my head. Because what they claim to do is to dehumidify your basement. And the way they do that is they simply take the basement air and they pump it upstairs, which is not exactly a dehumidifier.

    LESLIE: Yeah. But then what do you do with all that moist air in the rest of the house?

    TOM: Well, upstairs, you don’t notice it as much because it doesn’t collect and sit. And the temperatures are warmer so it gets absorbed into the air. And of course, that means that the basement is going to be less humid because that moisture is being pumped upstairs. But if you pump too much airflow upstairs, you’re going to depressurize the basement. And the reason it has to be that far away from the furnace is because if you depressurize the basement, guess what’s going to happen to all of the fumes that are generated by your heating system? It’s going to – the draft is going to reverse and you’ll start filling your house with that combustion gas, including carbon monoxide.

    Now, in a typical ventilation system – let’s say you have a really high-efficiency house. Like my cousin is building a house right now that’s an ENERGY STAR-rated house. He’s using foam insulation. You know, it’s going to be a really tight house. And I was explaining to him the other day that because it’s so tight, you might need to bring in fresh air to this house.

    And typically, the average house, we don’t worry about bringing in fresh air because our homes are naturally drafty. But when you build a tight house, you have to bring in fresh air to exhaust stale air. And the way they do that is basically by pulling in cold air from the outside and exhausting it with stale air from the inside. But they trap the heat so you’re not exactly just filling your house up with cold air. You’re going to able to sort of transfer; there’s a mechanical way to do that.

    These ventilation systems that you’re describing are only one-half of that. They’re basically just sucking the moisture out of the basement and pumping it upstairs. So, to me, it just seems like somewhat of a pointless exercise that potentially could go horribly wrong if the basement was depressurized. Have you noticed that the basement is less humid as a result of running this thing?

    KATIE: Actually, we haven’t even put in yet because we’re a month away from moving in. But I purchased it. But this WAVE ventilation system, the system itself is – it’s ducted to the outside. So what it does is it sucks the air in from the bottom – from, obviously the basement but it draws from the top half of the house and it expels it, so – but I don’t know how it’s replaced. It just doesn’t make any sense and that’s what our electrician said. So, the air that is circulated through this system is not pumped back upstairs; it’s actually expelled through the house. But what replaces it?

    TOM: And also, the other issue here is if you’re going to take all the moisture, all the air from inside the house and pump it outside, then you’re going to depressurize. And again, you may have to – you may drive up the heating cost as a result or the cooling cost as a result. Listen, I honestly don’t think they’re necessary.

    KATIE: OK.

    TOM: I would never put one in my house and if you’ve not – if you can cancel the contract, I’d recommend you do that.

    KATIE: Really? OK. So what do we do to keep the basement dry?

    TOM: Alright. So let’s talk about that. So, there’s a bunch of things that you can do. Keeping your basement as dry as possible starts at the foundation perimeter outside your house. You want to make sure that the soil slopes away from the wall. You want to have it drop about 6 inches over 4 feet, well-tamped down and then covered with stone or mulch or grass. But you always want to have that sort of slight slope away from the foundation perimeter.

    In addition to that, you also want to make sure that the gutters are clean and free-flowing and that the downspouts are extended 4 to 6 feet away from the house. That can help move the water away from that critical area of the foundation perimeter and stop it from building up in the soil right against the foundation walls, where it will get into the house. Those two things alone will make a huge difference in how much moisture gets down there.

    Now, is the basement finishable?

    KATIE: Yes. And it’s beautiful. It’s all rock. The original owner who was previous to us built this home and it’s a fortress.

    TOM: OK. So if you were to ultimately finish the basement and heat it, that is also going to dry it out, too, because warm air is going to absorb any moist air – any moisture that’s in the air.

    The other thing that you can do is you could paint the interior walls with a damp-proofing paint that stops just the normal soil moisture from evaporating into the house itself. And if it does ever get damp, I would put a dehumidifier down there before I put one of these big ventilation systems. I’d just make sure that I drain it outside. And you can do that through something called a “condensate pump.”

    KATIE: Sure. OK. Thank you very much.

    TOM: I hope that makes sense. Thanks so much for calling us at 888-MONEY-PIT.

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