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Garage Slab Radiant Heat Source

tg2012
tg2012 Member Posts: 6
I built a garage 30'x30' with 12' ceiling height. I poured a 4" slab with 3 - 300 foot - 1/2" pex ckts.

I'm looking into using an electric hot water heater to use for my heat source and need to know how to size it.

I have 2 8'x9' insulated steel garage doors on the north side, 2 4x3 windows on the east, 1 4x3 window on the west with a 3' door, on the south side a 6' double door. Walls have 6 inches of blown in insulation r29, and ceiling has r60

Comments

  • Ironman
    Ironman Member Posts: 6,970
    Heat Loss Calculation

    You need to do a heat loss calculation. You can find some simple programs online. Professionals know how to do them and have programs as well. However, this what we get paid to do so I doubt that anyone here is going to do it for gratis, especially with many of us being swamped with work right now. More info is also needed.



    An electric water heater is not the correct choice for heat source and it's not approved for such. The heat elements are only going to produce 15k btu's per hour regardless of the size of the tank. An electric boiler could be used, but electricity is usually the most expensive fuel source.



    Please understand also that a radiant slab is a high mass heat emitter and it can take several hours or even days to get it up to temp. Once you get it up , you have to keep it there. It's not something you can turn on and off or set back.



    It appears that you're a little thin on your loops also, but the load calc. would determine that. It should have been done before the floor since it's always the first step in designing.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • zacmobile
    zacmobile Member Posts: 211
    heatloss

    I just did a quick calculation as I am bored with nothing better to do ;) (it's a snow day here) without knowing your outdoor design temperature (where are you located?) I used -20C (-4F) and come up with approximately 37,000 BTUH or 10.84 KW. As Ironman mentioned the tubing won't cut it, you can only deliver about 23,000 BTU with that amount installed.
  • Zman
    Zman Member Posts: 7,403
    It is a garage

    Your heat loss is certainly not 41 btu/ft in a well insulated garage. It is probably 1/2 that. You should do a heat loss calc. As mentioned hot water heaters are not designed for radiant heat. That being said it does comply with code in many areas and it is only a garage. The watt rating on an appliance times 2.93 will give you BTU/hr. Most electric water heater have either 4500 or 5500 watt elements. They are factory wired to fire only one element at a time. Some allow you to rewire and energize both.

    Carl
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • zacmobile
    zacmobile Member Posts: 211
    estimate

    It depends on several as yet unknown factors though. In my neck of the woods this is a pretty common scenario: DIY tubing in detached garage connected to 40gal electric water heater. I've had 2 calls of under performing ones this year and they both had no perimeter insulation, even worse is the veritable conveyor belt of heat through the slab poking out from under the bay doors. They would run for days & never get above 14C.
  • tg2012
    tg2012 Member Posts: 6
    PHASE III PE110LP LP BOILER

    Actually the design and heat loss has been done by a reputable plumbing supplier here in CT.

    I have just read a lot about using a water heater for radiant on line lately and didn't know how to figure out how big and if it was feasible.

    The quote for a tankless LP boiler was more than I can do this winter and was looking into lower cost up front solutions.

    The quote was $4k and when I see water heaters for about $7-800, well you get the idea.

    I have also seen a few less expensive tankless boilers but I'm not sure of their quality.

    In my business, IT the supplier was one of my customers so they always gave me wholesale or close to wholsale pricing for materials purchased through them.

    The title of this reply is the model they quoted .
  • SWEI
    SWEI Member Posts: 7,356
    good boiler, but

    We don't discuss pricing here as a matter of policy.



    What did the 'reputable supplier' say your heat loss was?  What was the inside design temp?  Why a 110k boiler?  Do you need DHW as well as space heat?  Given that you appear to be looking at either LPG or electric, do you have any other heat source options (solar, wood, etc)?
  • tg2012
    tg2012 Member Posts: 6
    Garage Slab Radiant Heat Source

    Sorry about the pricing.

    Not sure of the heat loss number, I started the build last winter before I left for a 6 month stint with the military.

    As for options for heat source, my wish list would be waste oil since this is my collector car work/storage garage.

    But realistically propane, fuel oil(that's what we heat the house with) and electricity are all I have.

    The garage is in CT and it's not used all day except maybe on weekends, so radiant might not have been the best choice, but after working in buildings with radiant in the floor it was almost a necessity.

    But getting a good trust worthy answer of the internet is sometimes impossible. Also for some reason the area of CT I'm in there are not many if any plumbers doing radiant heat. Most of the ones I've contacted either haven't done it or don't believe in it and want me to do something else.

    I've been trying to get a plumber to install a radiant retro fit fort my kitchen for about 6 years.
  • SWEI
    SWEI Member Posts: 7,356
    hybrid system, waste oil

    LPG is expensive enough that you might want to consider a hybrid approach.  Use the in-floor radiant to keep your slab above freezing and temper the air to something like 50F, then fire up an overhead infrared gas unit when you need to use the space.



    AHS makes good waste oil boilers, and also some multi-fuel models.
  • tg2012
    tg2012 Member Posts: 6
    that's what I was thinking

    I was thinking of one of those self contained units like we had in Afghanistan. We called them chigos but Panasonic, LG and others make them. It would give me hot air heat and A/C in the summer.

    Sort of a quick fix for now and add the tankless boiler later
  • Ironman
    Ironman Member Posts: 6,970
    edited December 2012
    Tankless or Boiler

    First, I thank and salute you for serving our country.



    Please don't get caught in the Internet peddler's trap of thinking that a tankless water heater is a boiler or that it's suitable for space heating: it is neither. You'll greatly regret it if you try to use a tankless where a boiler is needed. There are several threads on here addressing the issue.



    I think your idea of a ductless heat pump is viable and then maybe look to a lower end mod/con boiler with L.P. Navien, Bosch, Triangle Tube, Lockinvar all make them. The T.T or Lockinvar WHN are higher end, but have the best heat exchanger around.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
This discussion has been closed.