In floor radiant heat BTU

[skamin20]

I know the right answer is "get a heat loss calculation", but I am doing this myself. I have used a few online calculators and seem to be all over the map. Any way I have a 40x40x14 pole barn. No attic so the peak is probably 20ft. But I hear radiant heat doesn't travel up since it is heating objects and not the air.
I had it spray foam insulated (closed cell) last winter, foam insulation in concrete, 2 small windows, walk in door and 16x10 new overhead door.

I live in SW Iowa. Last year was a cold winter and when it was zero or below my building stayed around 42 degrees. I plan on setting the temp around 60 and just leaving. I have been looking at a Laars FT boiler with a 130K BTU/hr output. Just curious to opinions is you think this is overkill or not enough. Based on a few detached garages around and their size that is what I have come up with. It would just be 1 zone, 6 run. This would be ran off of propane.

Thanks for any feedback.

 

[Bert_]

130kbtu is crazy big. I'd guess more like 50,000 btu

 

[Chris705]

I have a 40x44x14 with a walk-up attic. It is well insulated with spray foam. I calculated my heat loss in NY at about 35k btu. I honestly think it is about that or less. I also have a Larrs wall hung boiler rated at 150k. I use it currently with the in floor heat, fin tube on second level and indirect water heater. I plan eventually to have a small snow melt driveway and sidewalk. If I didn’t have the snowmelt planned I would be very comfortable with a 35k boiler with how well the foam spray insulation insulates and air seals.

 

[skamin20]

I have a 40x44x14 with a walk-up attic. It is well insulated with spray foam. I calculated my heat loss in NY at about 35k btu. I honestly think it is about that or less. I also have a Larrs wall hung boiler rated at 150k. I use it currently with the in floor heat, fin tube on second level and indirect water heater. I plan eventually to have a small snow melt driveway and sidewalk. If I didn’t have the snowmelt planned I would be very comfortable with a 35k boiler with how well the foam spray insulation insulates and air seals.

so you are saying if you were just going to heat your bottom floor you would be ok with just a 35K boiler? How thick is your spray foam, mine is 2-3 inches but seems to seal and insulate real good. My inside never got below 42 degrees all winter which i thought was good

 

[skamin20]

130kbtu is crazy big. I'd guess more like 50,000 btu

thanks for the input, i was starting to wonder if 130K was over kill

 

[yeldogt]

My 1600 sf studio in PA can be heated with 15k BTU's ...... It has a 32k net BTU unit in it. My new weekend place is over 4000sf and is heated with a net of around 60k boiler -- that heats the domestic water by way of indirect.

I'm sure the smallest boiler you can buy will work well

 

[86turbodsl]

2500 ft in michigan, and my load calcs were ~ 65K btu. With radiant it's fine.

 

[engineer2]

25 BTU per sq-ft is a common number you hear. More if you are in a cold climate and/or not much insulation. If in doubt, talk to the boiler supplier.
The floor will radiate heat into the air, but not as quickly as directly heating it with a hot air furnace.
Ceiling fans will help even out temps. A toasty floor is very comfortable to work on.

 

[yeldogt]

25 BTU per sq-ft is a common number you hear. More if you are in a cold climate and/or not much insulation. If in doubt, talk to the boiler supplier.
The floor will radiate heat into the air, but not as quickly as directly heating it with a hot air furnace.
Ceiling fans will help even out temps. A toasty floor is very comfortable to work on.

PEX in a slab: Once you get to 30BTU per ft of output -- that's going to be hot on your feet.

This is why you need to do a load on an older building to see if the radiant will work -- on a new building or a garage you will not need that output.

Other types of radiant -- Warmboard, lightweight gypsum, plates .... and especially staple up will be less. Staple up and the thinner plates need very hot water and may never get you even 16btu per foot ... that's often not enough for retrofits in old buildings.

 

[skamin20]

2500 ft in michigan, and my load calcs were ~ 65K btu. With radiant it's fine.

thanks for your feed back

 

[Firebrick43]

So you say there is foam in the concrete(which I think you meant under) but I ask how much and what type?

I have a friend with a 30x60 insulated like yours, maybe a little better but nothing in the slab. It will stay in the mid 40s all winter even in sub zero weather.

On the other hand my ICF garage with 3" foam under the floor and 6" in the walls would freeze the first year before I got the heat on. The ground heat was blocked due to the insulation under the slab. If you have little insulation the heat loss to the ground is going to be an issue. I hope you didn't use the radiant bubble under the slab.

 

[skamin20]

Yes it was 2 inch styrofoam board layed before the concrete was poured, similar to the attached picture. Did it 7 years ago and just now getting to the boiler part.

So why did your garage freeze with that much insulation? Are you saying the insulation under the floor didn't allow the ground heat to come up?

 

[Firebrick43]

Yes it was 2 inch styrofoam board layed before the concrete was poured, similar to the attached picture. Did it 7 years ago and just now getting to the boiler part.

So why did your garage freeze with that much insulation? Are you saying the insulation under the floor didn't allow the ground heat to come up?

Yes, no ground heat. 2" is pretty good, I have 3 but there may of been other issues

 

[skamin20]

Yes it was 2 inch styrofoam board layed before the concrete was poured, similar to the attached picture. Did it 7 years ago and just now getting to the boiler part.

So why did your garage freeze with that much insulation? Are you saying the insulation under the floor didn't allow the ground heat to come up?

so in your opinion do you agree 130K is way too big, and 60K or so would do just fine. Crazy thing is going from 60K to 100K is only a couple hundred bucks.

 

[haveissues]

I did a load calc on my 30x50x14 and came up with around 30k btu in NY so probably similar winters to you. I think a 60k would be plenty but look at the turn down. I don't see a laars 60k listed but the 130 is 10:1 so it should be able to run at as little as 13k btu while their 93k is 5:1 so its minimum will be 18k something btu.

 

[yeldogt]

so in your opinion do you agree 130K is way too big, and 60K or so would do just fine. Crazy thing is going from 60K to 100K is only a couple hundred bucks.

Yes 130 is way too big ... if my same size building can be heated with 15k BTU in PA to 65 when it's 5 degrees out .... you are only looking for a 18 degree delta. 130 is too big by a factor. Like I said .. .my whole radiant house is only 60k. for 4000+ sf of living space.

Turn down is fine ..... but, you don't want it always running at 10%

 

[Firebrick43]

so in your opinion do you agree 130K is way too big, and 60K or so would do just fine. Crazy thing is going from 60K to 100K is only a couple hundred bucks.

I also agree that 130K is to big. I have a modulating condensing water heater(boiler with reservior) at 75k and its set on its low firing rate which I believe is 36K, and it doesn't run enough with 2300 square feet. The 60 gallons of water helps to get longer run times and not cycle so often. Do not use a regular water heater as they can not take the condensing

 

[Bert_]

so in your opinion do you agree 130K is way too big, and 60K or so would do just fine. Crazy thing is going from 60K to 100K is only a couple hundred bucks.

Getting the right size isn't about cost. Could cost the exact same and you still don't want to oversize it. Right size is more comfortable and the equipment should last longer.

 

[skamin20]

I did a load calc on my 30x50x14 and came up with around 30k btu in NY so probably similar winters to you. I think a 60k would be plenty but look at the turn down. I don't see a laars 60k listed but the 130 is 10:1 so it should be able to run at as little as 13k btu while their 93k is 5:1 so its minimum will be 18k something btu.

great info, yeah the 93K is their smallest, i kind of forgot about the turndown

 

[skamin20]

Yes 130 is way too big ... if my same size building can be heated with 15k BTU in PA to 65 when it's 5 degrees out .... you are only looking for a 18 degree delta. 130 is too big by a factor. Like I said .. .my whole radiant house is only 60k. for 4000+ sf of living space.

Turn down is fine ..... but, you don't want it always running at 10%

understood, thanks for your help and comments.

 

[skamin20]

Getting the right size isn't about cost. Could cost the exact same and you still don't want to oversize it. Right size is more comfortable and the equipment should last longer.

thanks for your help.

 

[Chris705]

so you are saying if you were just going to heat your bottom floor you would be ok with just a 35K boiler? How thick is your spray foam, mine is 2-3 inches but seems to seal and insulate real good. My inside never got below 42 degrees all winter which i thought was good

Yes, I am certain a 35k boiler would be fine heating my garage. I used open cell foam and filled my stud cavity and rafter cavity. 5.5 & 11.25 inches respectively. 3” rigid under slab and at edges. I think your closed cell foam is probably equivalent to my foam. IF I was to only heat my garage I would be considering a tankless water heater….. a buddy of mine has two separate garages heated for about 10 years using tankless. Cost savings on the initial cost over a boiler is worth looking into…..

 

[jlv03]

I'm on the SE side of the state, 24x28 attached garage with open cell on both the outside walls and ceiling separating from 2nd floor primary bedroom. My floor heat is a 6kW electric boiler - has no problem keeping it in the 55-60 degree range. Not cheap to run, though.

 

[86turbodsl]

Don't forget about the outdoor reset. The concrete is such a large thermal mass, on days that are swinging temps quite a bit, if you don't have it, you're going to be sweating or freezing more often. At least thats what i found. I have a well insulated building also.