Page 1 of 2 12 LastLast
Results 1 to 13 of 15
  1. #1
    Join Date
    Jul 2004
    Location
    MN
    Posts
    17

    How to Dehumidify Slab-Heated Garage in Minnesota Winter?

    Here in Minnesota, a common heating option for garages in homes with hydronic heat in the basement is to also add in-slab heat to the garage. It works great as far as keeping the space warm and making the snow melt off the cars, but I worry about where all that moisture is going. It is not uncommon to see the windows in such a garage iced over and water literally dripping from the steel garage door (even if insulated).

    The warm floor really makes this more of a problem since it directly drives the evaporation of the large puddles on the floor.

    What would be the best strategy for drying this space in the winter? I am thinking an HRV dedicated to just the garage with a humidistat, but am concerned since the manufactureres don't really talk about this application.

    Will running an HRV in a normal setup pull out the tens of gallons of water per day dumped into a garage in Minnesota winter? Any thoughts on how to size the unit?

    One thought I had was to run the HRV not in a ventilation setup, but swap the connections to short-circuit it to be just an outdoor-air-cooled dehumidifying machine. But I worry this will just turn the unit into a block of ice, even with its defrost cycle.

    - Rick

  2. #2
    Join Date
    Jun 2001
    Location
    Louisville, KY
    Posts
    12,189
    Floor drain?

    Got one in my garage. Of course, it's not quite as cold in my slab.
    Perhaps you should have read the instructions before calling.

  3. #3
    Join Date
    Jul 2004
    Location
    MN
    Posts
    17

    Drainage

    I do agree that simply draining as much water as possible is the first thing to ensure. Usually, the floor is pitched to slope out under doors, but sometimes the concrete work is not always perfect or has settled and drainage might not be optimal. Also, slush and road sand can interfere with drainage, so in general, there will always be puddles.

    I think floor drains in garages are generally precluded by code in Minnesota because they collect oil and encourage dumping of waste oil.

    - Rick

  4. #4
    Join Date
    Nov 2002
    Location
    minnesota
    Posts
    200
    run a small fart fan and keep the slab temp around 40 degrees ---this way the snow will melt slowly on the floor instead of cooking off and the fan will exhaust what vapors are produced

  5. #5
    Join Date
    Oct 2005
    Location
    MN
    Posts
    133
    I've got a floor drain in my garage here in the twin cities and it wasn't a problem as far as code. I don't run my garage heat unless I'm working on something in the garage, but unless it's really cold (below 0*) it doesn't freeze anyway, and the stuff that drops off the car runs down the drain just fine.

    I usually turn the heat on and wash the cars about once a week and I make sure I flush the floor drain with lots of water when I finish. No problems after 18 years of use.

    I don't recall ever seeing any frost buildup on my doors (insulated) or anything else. It gets pretty steamy when washing the cars because I have hot water to the garage, but it doesn't condense and freeze on my garage doors.

    I'm supprised you get that much moisture....my garage doors have rubber seals around them but still are not all that tight, plus they get opened several times a day and that alone ventalates the crap out of the garage.

  6. #6
    Join Date
    Jul 2004
    Location
    MN
    Posts
    17
    Its not a problem when the heat is off - only when the slab is running at 70 degrees or so to keep the space above 50 degrees

    The ones I have heard are the most problematic are the well-insulated tuck-unders where the walls have sheetrock (and I presume vapor barrier). Sealed up just like finished, conditioned spaces in the house. Which is probably good because I would hate to think of all that water going into the wall cavities.

  7. #7
    Join Date
    Oct 2005
    Location
    MN
    Posts
    133
    Do you have a Floor Drain? Is the floor sealed or painted so water runs to the drain instead of soaking into the concrete.

  8. #8
    Join Date
    Jul 2004
    Location
    MN
    Posts
    17
    Well, the puddles never dry out for the whole season, so I do imagine the concrete is getting a bit soaked.

    I turned the heat down from 55 to 45. I put a humidity meter in there and it reads 55% at 46 deg F temp. Exterior temps are running zero to 20F these days.

    I would love for it to be bone dry in there so the meter reads off the bottom of the scale "L%".

    - Rick

  9. #9
    Join Date
    Oct 2005
    Location
    MN
    Posts
    133
    So you DON'T have a floor drain? Plus it sounds like there is not enough pitch to your floor to allow water to run toward the drain or out the door. If you can't get rid of the standing water I can see why you have a problem. Who ever did your floor, didn't do you any favors. Every floor that we have ever installed has 2 to 3 inches of pitch toward the doors or drain. My floor is usually dry in just a couple of hours after a car wash.

    If it's as wet as you say try leaving the door open for 15 or 20 minutes, I'll bet she dries right up with the outdoor temps like they are right now.

  10. #10
    Join Date
    Apr 2002
    Posts
    11,808
    I had a similar problem with a volunteer fire station, except it was over head radiant tube heaters.

    I used an HRV. Dehumidifiers are not made to dry out winter air they will freeze up.

    A simple exhaust fan would work also, controlled by a dehumidistat
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

    http://www.ductstrap.com/

  11. #11
    Join Date
    Jul 2004
    Location
    MN
    Posts
    17
    I had an old 8" duct boost fan, so I stuck it through a piece of plywood and nailed it over an open window. I cracked one of the other windows open about 2" to get some incoming flow.

    Ran for 12 hours today. Humidity meter has gone from 55% down to 35%. Seems to be working pretty good - the puddles have almost all dried up, the windows have defrosted, and the steel doors are dry. When the last of the water on the floor is gone, I expect the humidity to drop a bit more.

    I have no idea what the CFM of the fan is, though. Can't be much more than 100, though, right?

    Permanent solution would be to put the fan through the wall with a vent with a flapper, but still I am trying to decide if the payback on an HRV would be worth it, and what size would be appropriate.

    - Rick

  12. #12
    Join Date
    Aug 2007
    Location
    MN
    Posts
    2,677
    We have used HRV's and Humidity sensing bath fans, the HRV will freeze up unless you keep the garage temp at 60 or better. The best one yet was a 100 cfm broan fan with built in humidistat, with a 6" fresh air intake on the opposite side, both in the ceiling. Just make sure the 6" flex for intake is trapped in the attic or it will just spill cold air in all the time.
    You can't fix stupid

  13. #13
    Join Date
    Apr 2002
    Posts
    11,808
    If you get a big puddle every day maybe an HRV.

    If you get the puddles just after a snow fall, do like you have been doing.

    24x24 garage 9 foot ceiling is a volume of 5184 cubic feet.

    To change that air once an hour is 5184 cubic feet/60 minutes =86.4 CFM.

    100 CFM is plenty.
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

    http://www.ductstrap.com/

Page 1 of 2 12 LastLast

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event