In Floor Heat for Shop

mopar_mudder

I am building a shop in Northern Wisconsin and I am going to put in floor heat in it. I am trying to decide the best route to go with tube size and spacing. My current garage is 30x54 and has 1/2" tubing @ 12" O.C. and heats just fine. But doing some research it seems it is recommended to go more like 9" O.C. with 1/2" tube.

New shop is going to be 56x88 and I am going to do it in two zones that are 56x44 each.

If I do 1/2" tube @ 9" O.C. with a max loop of 300 feet that is 12 loops per zone, seems like a lot.

What about going 5/8" @ 12" O.C. with a max loop of 400 feet that is 7 loops, would this be as good?

Or through all that out and tell me the best way to heat this big beast.....

Thanks for any help.

SWEI

It all begins with a heat loss calculation. Will the shop be regularly or intermittently occupied?

hot_rod

Also, what type of boiler? If you are considering a condensing boiler, all things being equal more tube= lower fluid temperatures which increase boiler efficiencies.

Here are a couple pages from Idronics 12 that helps explain how tube spacing changes the floor output.

Tighter tube will recover the temperature faster is what Kurt is thinking, as you have more heat emitter transfer. So if temperature setback will be used, more tube will speed recovery.

There is also some comfort difference, with wide spacing there are cool areas between the tube runs. That may not be a concern in a shop.

Tightening up tube spacing can only be done once, it is usually worth the extra

$$

mopar_mudder

I can run a heat calc on Monday. Can't tell you what kind of boiler because I'm not even sure that I will ever heat it, but I might want to some day. It is going to be storage for now but could turn into a shop down the road.

Firecontrol933

Unless you plan on walking around in bare feet in the winter time in it 12" spacing is fine. I personally would go with the 5/8" tubing and keep the loops shorter than the max recommended (400') only because you can get better flow both in the floor and at the boiler. Shorter loops also are easier to get less temperature drop across them which translates into a more even temperature across the slab, again this being a shop that might not be a high priority.

hot_rod

Many of the shops like that here in Missouri turn into living space, at least part of them. Consider that with loop layout, maybe an office space in part of it?

If I had one do-over in my radiant career , it would be to have all my past shop installers with more tube, especially some of the 18" OC.

That additional tube may allow solar or heat pump input someday down the road. If or when energy prices go up again, that extra could be worth the front end cost difference. As you can see in the example above, required supply temperature drops considerably with more tube.

Underslab insulation is another place to pay attention, years ago 1" was considered adequate, now 2" or at least 1-1/2 is common.

SWEI

hot rod said:

Underslab insulation is another place to pay attention, years ago 1" was considered adequate, now 2" or at least 1-1/2 is common.

In Northern Wisconsin I would suggest four inches.

mopar_mudder

Will be 2-1/2 under the slab, 2" is the norm here. Will never be living space, got the house for that, just space to store cars and camper. We also restore cars which I do in my attached garage , might move that out to this shop down the road though.

SWEI

High mass radiant runs best on constant circulation with outdoor reset control. For intermittently-occupied spaces, you can set that up so it maintains a constant low indoor temp (think 45-50°F) paired with either a unit heater (Modine, etc.) or gas-fired overhead radiant to bring the space up to temp quickly when needed. The radiant floor heat keeps the building mass from going arctic, which keeps the equipment happy and makes working on it far less of a challenge.

Zman

You posed an interesting question so I ran a quick comparison using Siggy's software.

The 1/2" tubing scenario would require 3.5 to 5 degrees colder water than the 5/8" scenario depending on the temps.

With most condensing boilers the 1/2" option would give you an efficiency increase of about 1% at most.

The supply/return delta t on both options is are very similar using identical circulators. There should be minimal comfort difference.

As long as your heat loss is not more than 200,00 btu/hr (you should be able to get yours under 100k) for the whole building, a condensing boiler should work well for either option.

I started out thinking 1/2" would be better, now I think 5/8" would make more sense.

mopar_mudder

I ran the heat calc and get 104,538 btu/hr

SWEI

That's a lot of BTUs. What indoor and outdoor temps was that based on?

hot_rod

SWEI said:

That's a lot of BTUs. What indoor and outdoor temps was that based on?

Just under 5000 sq ft in northern WI, a tad over 21 btu/sq ft. for a shop building may be about right?

Ironman

SWEI said:

hot rod said:

Underslab insulation is another place to pay attention, years ago 1" was considered adequate, now 2" or at least 1-1/2 is common.

In Northern Wisconsin I would suggest four inches.

The federal energy code, which is superimposed upon the building code, requires 3" in the mid Atlantic area. It may require 4" in WI.

SWEI

hot rod said:

SWEI said:

That's a lot of BTUs. What indoor and outdoor temps was that based on?

Just under 5000 sq ft in northern WI, a tad over 21 btu/sq ft. for a shop building may be about right?

Might be, though it really depends on the assumptions. Most calculators default to an interior temp of 65° - 70°F.

hot_rod

SWEI said:

hot rod said:

SWEI said:

That's a lot of BTUs. What indoor and outdoor temps was that based on?

Just under 5000 sq ft in northern WI, a tad over 21 btu/sq ft. for a shop building may be about right?

Might be, though it really depends on the assumptions. Most calculators default to an interior temp of 65° - 70°F.

Overhead doors can be a factor, even insulated versions have what, 1-1/2" of insulation, maybe R-7 and a high infiltration potential.

mopar_mudder

I use a program called ResCheck that puts the temps in for you. I know outside is -25, don't know what it uses for indoor. Funny thing is it gives me the same number basically even if I don't put the door in. About 50,000 each for Conductive and Infiltration.

As far as the slab goes all that is required is an R-5 to a depth of 4'. 2" under the entire slab is the norm here.

Garage door is 2" thick R-17, but you loose a lot around the perimeter, no good way to seal them up.

I think what I am going to do it 5/8" tubing with an average length of 270'. Will take 18 runs to do the entire thing.

SWEI

mopar_mudder said:

I use a program called ResCheck that puts the temps in for you. I know outside is -25, don't know what it uses for indoor.

Probably 68 or 70°F.

Funny thing is it gives me the same number basically even if I don't put the door in. About 50,000 each for Conductive and Infiltration.

That doesn't seem right. If the door is small it might not represent that much of the total load, but the infiltration should be noticeable.

As far as the slab goes all that is required is an R-5 to a depth of 4'. 2" under the entire slab is the norm here.

The norm is rarely right. Re-run the heat loss calc using three and four inch and see what you get. Try a radiant heat loss calc like LoopCAD -- there's a free trial.

FranklinD

I'm in Douglas County (Superior area), and designing our new garage/shop to be built next year. I planned for 5" under the slab and 6" around the perimeter and down around the footings. Our design temp here is -20, so about the same as yours.

I can't speculate on the heat loss or tubing loop lengths yet, as I haven't yet decided on 2x4 vs 2x6 walls, Fiberglass batt vs spray foam, and several other variables. But in our climate, the more insulation the better. Don't follow the minimums...I have a good friend who built a new shop out in the county a ways and he under-insulated his slab. It's amazing to watch the frost creep in from a few spots along the edge...I'm waiting for the frost heaves/cracks to start.

mopar_mudder

All I can say is that I am in the building industry. I design and sell everything from garages to million dollar homes. You get to a point of diminishing returns on insulation. Just because company push all these rules through the building code to make more money doesn't mean they are going to save you anything.

Wisconsin code calls for R10 under slab and R15 for the perimeter. BUT if you run it through a program like ResCheck and it passes you don't have to follow those codes.

Bottom line is that I feel anything over a R10 and you are not going to get your money back on the investment. But it certainly can't hurt to put in more.

SWEI

Insulation under a radiant heated slab is a different beast, especially with those design temps. When you look at the total budget, the incremental cost of going from 2" to 4" sheets of EPS/XPS is not significant -- and the labor difference is negligible.

mopar_mudder

When I run it through LoopCad using the same settings it comes up with 67,000btu

mopar_mudder

What are thoughts on slab sensors? I would like to have them so that if the doors in open the sudden cold air doesn't call for the heat to kick on. But I am not sure of a way to get them in toward the center of the slab where they should be. A 56x88 building with no interior walls doesn't leave much for options.

ratio

1/2" PVC conduit. Just throw a run into the slab, maybe let it raise up in the center. You can use a fish tape to shove the sensor out, or run the pipe all the way across & just pull it halfway in.

Tinman

You'll have some scrap pex on the job. Just use that with a pex plug.

mopar_mudder

Is their any limitation on how far you can run the wires? Something like 100+ feet to the thermostat.

ratio

It's possible that the sensor'll need calibrated, but I run that far frequently without issue. If you want to check, attach the wire to the sensor leaving the joints exposed. Let it sit for a while to acclimate, then measure the resistance at the sensor & again at the end of the wire. There should agree pretty closely. The paperworks for the sensor should have a chart showing temp vs resistance. if they're more than one or two degrees off, calibrate the sensor in the control.

SWEI

3/8" Type L soft copper tube is a loose fit for a 1/4" thermistor probe.
1/2" Type L soft copper tube is a loose fit for a 3/8" thermistor probe.

If you need a truly accurate reading, wire four thermistors in series-parallel.

hot_rod

I use pex tube with a caped copper stub at the end. You could fill that copper stub with some heat transfer grease also. Fasten it to the mesh or rebar, mid pour thickness if possible.

I like the concept of fastening the sensor to an old fish tape and leave it in the tube, in case a sensor needs to be replace.

When ever I would see the electricians leave behind an exploded fish tape I would grab it for cutting off sensor fishes.

Uponor and tekmar both have nice dual stats for floor sensing and ambient, you can program a not to exceed or a low point for the floor sensor. Nice for bathroom floors also.