wood boiler for garage heat

[DonNH]

Hi all,

Currently heating my 24x36' 2-story garage with a small wood stove. However, I'd like to take advantage of the tubing I put in the floor when I buit it, so I'm looking at wood boilers.
I've found a couple used candidates, but am trying to determine a good size.

The floor is 6" thick and the 1/2" PEX is in the middle of the slab on 12" spacing.

It'll see intermittent use, so I'd like to be able to go in when it's cold - say 30 - 50 degrees, and fire it up, pushing as much heat as possible in a short time into the slab, which would hopefully keep the building reasonably warm for quite a while.
So, my main question is how much heat can the slab accept (btu/hr)? This would help determine how big a boiler I can use.

Thanx
Don

 

[jebatty]

The purpose and function of radiant slab heat is constant, low temp water feed, not a rush of heat and then off. The slab takes a long time to heat and a long time to cool. To get what you want, your wood stove setup probably is the best if wood is to be your fuel source.

 

[Medman]

I agree - it takes me almost four days to get my slab on grade house up to temp. Last weekend we went away for four days, temps in the low teens, and in that time the slab dropped from 72-73* to 60*. Since Sunday night I have been burning almost constantly given the 0*F temps and the slab finally reached 72* last night. So it takes a lot of heat to get the slab warm, but once it is warm the slab radiates heat for a long time. Not the best solution for intermittent heating.
However, you might look into a storage tank to use with your boiler - that way you could fire the boiler and heat the tank, then draw heat off the tank slowly. This would keep the floor at a low but even temp, even if firing intermittently. Remember that radiant floor heat can use constant temps that are much lower than other types of radiators.

 

[DonNH]

This is exactly why I'm trying to determine at what rate the slab can accept heat. My immediate concern is deciding between a couple used boilers that are available, before they end up sold to someone else.

One scenario would be that I could fire up the boiler and feed it to a Modine style radiator to quickly heat the air for immediate use, then as long as I'm there keep feeding the heat to the slab to attempt to keep the building at a moderate temp (or at least above freezing) when I'm not there. An oversized boiler's only advantage would be how much wood it can hold. In a perfect situation I could keep the boiler running at maximum whenever I'm available to keep adding wood. This would maximize efficiency & minimize creosote buildup.

Don

 

[Duetech]

Just going by square footage then I would think something the size of an EKO 25 (around 35kw) would do. What that would mean in regards to the other boilers you are looking at I don't know and I'm not sure what btu that is but I use an EKO40 for my home and it is rated for a place bigger than my home. Your square footage is very lose to mine.

 

[Medman]

Eko 25 is 85,000 BTU max, but realistically somewhere around 70-75,000. For the slab, you can do a heat loss calculation on the building. There are a lot of factors here - type of construction, your lowest outside temp, insulation, etc. The calculators are online for free, but I can't remember where I found them. If you search here, there should be a link. What you are looking for is your worst-case BTU loss on the coldest day. For a garage you could go smaller because you are not as concerned about keeping the other residents happy.

My garage is 24x36 with one end (13x24) insulated and vb with the eko 25 installed. The temp in the boiler room is about 80* and the temp in the uninsulated car bay never goes below freezing. This morning the thermometer in the car bay was just at 32* with the outside temp around -25*. This is just from the heat radiating off the pipes and the unit itself.