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Community and Q&A

Radiant Floors and Basement Condensation

Mydamnwell | Posted in General Questions on

Hi all, first post here. I know ‘radiant cooling’ is a thing, and that it can create lots of humidity as well. I’m looking for a ‘middle ground’ solution

I own a custom built home with hydronic radiant on the first floor and basement, I have a Fujitsu Halcyon mini split for a/c and auxiliary heat though I rarely use it. Vast majority of the house is concrete slab and cinder block walls, its built similar to the Bachman-Wilson home that FLWright designed. I have about 2200ft of PEX running throughout the home and detatched workshop, most of it 3/4″ and set within the slab. All heat loops are tied to a pair of Embassay manifolds to create a basic closed loop system. This is run through a condensing boiler (cheap Navien unit) with one of those bypass/mixing manifolds right below it (seems water flow can bypass the boiler?).

The issue I have is with basement humidity. There are no windows in my basement, it is 100% below grade and surrounded by the first floor slab, the stairways are open however and air is free to flow up to the first and second floors through the two open staircases. Regardless, even on a mild day the humidity down there sits at 70-80% without a dehumidifier running. On warm summer days this causes the plumbing to sweat (well pump tank and filtration is down there). In contrast I have a large great room with vaulted ceilings, the slab sits on grade and there are 40+ windows in this room, many of which are south facing and let in tons of light. This makes the slab pretty warm on that side of the house, and this space alone has three radiant loops within it. The far side of the house stays very cool, as does the workshop.

So in my mind there is a solution… Swap warm water from the great room slab with cold water in the basement/ workshop slab, in theory reducing humidity downstairs and potentially raising it upstairs (which is not an issue). Can this be done efficiently? Perhaps I can simply run the circulator pump next to the manifold and let everything mix to an even temperature. The workshop slab stays chilly even in the summer (below grade, no windows), and the two workshop loops (300ft ea) run underground between the structures, these two lines alone could act as a form of heat pump in my mind.  I know there are many variables and I can provide much more info if it’s helpful, but I dont want to overload my original post. Let me know what you guys think. Thanks.

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  1. jameshowison | | #1

    Check out the articles on dewpoint. If I understand correctly (and that's a stretch :) you'd be cycling heat from the slab in the great room to try to keep the other slab above the dewpoint temperature. While you'd be making the great room slab cooler you wouldn't be changing humidity levels, in the sense that you wouldn't be moving moisture around. You'd be changing temperature which would change moisture capacity of the air (so relative humidity would change).

    So you'd have to figure out how much heat is available in the great room slab and how much you could bring to the workshop, compared to the cost of running the pumps. If it wasn't enough to raise the temp in the workshop high enough to exceed the dewpoint then it wouldn't work,.

    Probably easier just to run a dehumidifier in the workshop and reduce the amount of moisture in the air down there. Which of course raises the question of where the moisture is coming from? If it's inward drying of the basement walls and slab then controlling that moisture ingress is probably the key thing?

  2. Mydamnwell | | #2

    That's correct, I would like to adjust the relative humidity downstairs by warming the slab, drawing heat from the first floor. I used a dehumidifier last year and I have no problem maintaining 45% down there. It is noisy though, and a power hog compared to a single circulator pump. I will read up more on those dewpoint articles.

    My closed loop system utilizes two circulators, one just prior to the manifold and one just prior to the boiler. The layout of the pipe should in theory allow water to bypass the boiler and boiler pump, I dont know what effect that may have on components though. I could add in a bypass closer to the manifolds.

    The main source of moisture is the sump pit in the far corner of the basement. Admittedly I never considered that it may be a large contributor to humidity levels. The lid is a cheap plastic thing with a U shaped gap for pipes to fit through. Perhaps an air tight lid is a good place to start?

    1. dickrussell | | #3

      If the sump pit has a cover on it, even with those gaps for piping, then I doubt it is by itself the major source of moisture in the air. There wouldn't be much area for diffusion of water vapor from the air space within the pit out into the basement. But the presence of the sump pit suggests the presence of ground water below the slab much of the time. I would hope that the slab was installed according to recommendations on this site, well-insulated below and with a poly vapor barrier right below the concrete. But even so, if the ground water is high and the slab is damp, that right there likely is the source of your humidity. Then, too, the block walls might also be a source of inward diffusion of water.

      I suspect the best solution to the immediate problem is to run a dehumidifier. Long-term, it would be useful to know if damp slab and/or walls is significant. You could test this with some sections of polyethylene taped to the slab and walls to see if you get dampness under the sheet. Then application of something like UGL Drylok over the whole slab and wall surface area might drastically reduce the inward diffusion of water to a manageable level.

  3. charlie_sullivan | | #4

    This sounds like an interesting and harmless experiment. If you had numbers on the temperatures, we might be able to estimate how much it would help. You'd be better off if you could take one of those two zone off the manifold, so you'd be sending the warm water to the basement and vice versa rather than mixing the to an intermediate temperature. If you find it helpful and plan to keep doing it, you might upgrade to a super efficient ECM pump so you use only a few watts to run it on low, continuously. Even if you still have to run the dehumidifier, you should need to run it less, and it should run more efficiently.

    But as others have said, you might want to figure out the source of the moisture. If we knew the temperatures and humidities upstairs and down that would help us figure out whether there's another source of moisture or if it's more a result of it being cool down there.

  4. Mydamnwell | | #5

    Before I make a new post lets try resurrecting this one. Another heating season has come and gone... I really want to try this again. My moisture issue has been remedied, a better fitting cover on the sump pit was all it took.

    I have only a few concerns, and it seems internet search engines are more useless than ever.
    1) is it safe/advisable to wire a 3 prong plug on my Taco 007 pump so I can plug it directly into a switched AC outlet? Idea being it is isolated from the boiler, flip a light switch or just unplug it to shut the circulator off at night. When cool weather returns I would remove the AC plug and connect it back to the Navien.
    2) If I do this and run the pump 'manually' is there any issue with water potentially flowing through the boiler while it is off? I would shut the valves above the manifold but that isolates the DHW loop from my expansion tank which seems worse. Without the boiler pump running I assume the vast majority of water will flow straight through the mixing chamber anyway.

    Ideally there would be a setting on my NHB110 to allow the system pump to run on its own. There is a freeze protection feature that runs the pump in ten min cycles but the supply temp sensor has to read 50f or below. Maybe I could use a small resistor to trick the Navien? I'm tempted to put the sensor in ice water and see what happens.

    What I don't wanna do is burn out a pump or blow out seals or fry a circuit board. Thanks guys.

  5. Expert Member
    NICK KEENAN | | #6

    I think you are going to be disappointed. The reason is that you need a temperature gradient in order to get heat to flow. Let's say your basement is at 65F and your upstairs is at 75F. In the best case your water will leave the basement at 65F and then leave the upstairs at 75F. Which means that on both floors the radiant surface will have an average temperature of 70F, and have a temperature difference between the air and water of 5F. The amount of heat flow you're going to get with a temperature difference of 5F is very small.

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