I'm finally building a new garage/woodshop. slab-radiant is a must!

glandjr

Hello all,
The new building is planned to be 650sq ft. 9' 2x4 walls R25. rafters or trusses cathedral framed and insulated R30-R50? I was thinking 4"XPS foam R20 under slab and sides. I received estimate from Radiantec for their kit. they ran a heat loss calc. I believe. My question is should I go with their proposed NG condensing boiler, Noritz NRC661? makes sense for wall hanging/space saving. or is their an equivalent WH tank that would be another option? I believe I need around 19,500BTU I also want to use rebar to suspend the pex close to center of 6" slab. Thanks for any insight anyone wants to add. This is a long time coming!

Hot_water_fan

What they proposed is a domestic water heater, NOT a boiler. You can use a water heater for space heating (many do!) but you'll get a lot of pushback here because it's a bad solution. It's easier and can be cheaper to just do it right, first. It's just the wrong tool - the head loss is too high for one. A tank type water heater (some boilers are also tanks) can be as efficient as a tankless heater and would have many advantages over this Noritz.

Second, that's too big. I'm not convinced a boiler is the right solution for your project in general, but for certain a boiler with a minimum fire of 15kBtu in a space this size is the wrong choice.

Third, I think that heat loss is too high. Don't let Radiantec do your heat loss - you can do a spreadsheet calc just as well, if not better. You could EASILY have a heat loss close to 5kbtu with those insulation options.

Last, I think this space is so small and your heat loss so low, some new options open. An electric boiler would be cheap to install but probably more expensive to run. Since the heat loss is low, that might not be a big deal and upgrading to gas might never pay back.

I think you'll have a more successful project if you read some of the Caleffi Idronic journals.

hot_rod

2" of foam is typical, 4" if you have the $$. Use a board rated for ground contact 15- 25 psi rated.
I'd go with 1/2" tube 6" on center. Four 300' loops would be close. This allows the lowest possible SWT and quick response.
Excellent idea raising the tube into the slab, it makes a difference in SWT also.

Consider the 6X6 mesh instead of rebar. I used the heavier wire version, in 20X8' sheets. I think it was a 6X6 #6 gauge wire. It is much easier to get a nice even tube layout with mesh compared to rebar grid.

For slab strength the fiber mesh is a good idea
A good compaction of the sub grade helps keep a slab from cracking also.

Are you putting heavy equipment inside? If not a 4" or 5" is plenty. Less concrete, faster response up and down.

You can get continuous strip bar chairs to keep the mesh up, or I raised it up with small chunks of the 2" foam.

I'm heating my shop today with 85° supply!

Stay away from tankless. A small boiler like this is a much better option, has outdoor reset and many more heating specific adjustments. 10- 1 turn down also.

Hot_water_fan

@glandjr I think "Tankless" is too ambiguous a term, what @hot_rod means is avoid a "tankless domestic water heater." A wall-mounted boiler is also tankless, but built for central heating. They look pretty similar and are often made by some of the same brands, but there's a difference.

KC_Jones

glandjr said:

Hello all,
The new building is planned to be 650sq ft. 9' 2x4 walls R25. rafters or trusses cathedral framed and insulated R30-R50? I was thinking 4"XPS foam R20 under slab and sides. I received estimate from Radiantec for their kit. they ran a heat loss calc. I believe. My question is should I go with their proposed NG condensing boiler, Noritz NRC661? makes sense for wall hanging/space saving. or is their an equivalent WH tank that would be another option? I believe I need around 19,500BTU I also want to use rebar to suspend the pex close to center of 6" slab. Thanks for any insight anyone wants to add. This is a long time coming!

I can't advise you directly, but I can tell you this. I have not seen a customer of that particular company ever post anything good on this forum. It is pretty well always about how things either aren't working, or it's costing them a ton to heat, or some other bad thing. I'd steer clear of them entirely.

Also, if you are doing new construction and need 19,500 btu's for 650 sq ft, you are doing something very, very wrong. I'd suggest that number is off based on the construction you posted. Half that is probably closer to reality. The heat load is going to be tiny. The smallest boiler made is going to be massive overkill, but that's about the best you can do.

Another thing is to make sure you have realistic expectations for this. Is this a part time use space? Are you expecting to bring it up to temperature quickly? The answer to some of these questions will make sure you have realistic expectations for what you are installing.

yellowdog

@glandjr Stay away from Radinatec. Nothing good will come from your install and the products they push are junk.

hot_rod

20 btu/ sq ft is a WAG, depending on your design temperature, so maybe closer to 13,000 btu at design. the small mod cons like I showed turn down to 8,000 btu/hr.

You may be able to direct pipe a small mod con, lock the firing to 10,000 btu/hr

you can compare fuel cost here if electric is cheap in your area.

https://coalpail.com/fuel-comparison-calculator-home-heating


Call it 97% efficient on LP or natural gas

glandjr

thanks for the input, I feel pretty discouraged now.lol I clearly don't have the correct heat loss specs. It's unanimous that the Radiantec is junk. I just want to heat the slab so it's not so damn cold on the floor and everything in it. I'm in WI. 650sq ft is too small for any in-slab system to work efficiently? Nobody makes an appropriate boiler for this application? lochinvar nkb080? I have about 23machines-say 5-6000lbs of woodworking machinery, TS, jointer, planer, etc. no cars,(and yes my wife is cool with that) I plan to keep the temp low all winter. maybe bump it up a few degrees depending on the day or how well it works. it'd be more than part time hobby, more like full time furniture making. I figured spend the money on insulation now and not regret doing more, bc like we know, prices of materials only go one direction! Thanks guys!

Hot_water_fan

650sq ft is too small for any in-slab system to work efficiently? Nobody makes an appropriate boiler for this application? lochinvar nkb080?

No 650sqft is fine. That boiler (really water heater) is just too big. 

If you’re going with a gas condensing boiler (many are wall hung), you don’t actually need a heat loss calc: the 80kbtu unit that turns down to 8kbtu is the best you can do. 

The part I’d explore is skipping the gas: an electric boiler would be right sized (and would need a heat loss calc). If the install cost is low enough, then potentially higher energy costs of electric might be justified. They’re simpler products and would be a better size fit. 

hot_rod

My shop is a tad bigger, partially earth sheltered so close to the same heat loss I'd bet.
I have a Lochinvar Knight 55. It turns down to 8,000 btu/ hr, similar low end as the Nobel 80 I showed above.
The Knight has a more robust control to really dial it it to a small system.


Use this calculator to compare electric to gas or lp. This will show you the realistic fuel costs.

https://coalpail.com/fuel-comparison-calculator-home-heating

I keep my shop around 66 when I working, 68 when I do desk work.

Don't talk yourself out of radiant, regardless of what heat source you chose.

Dave Carpentier

I have 6" slab in my 600 sqft attached garage. We just tyrap'd the 1/2" to mesh and then used hooks to lift it up approx halfway after the pour. I made the mistake of doing the whole garage in one loop.. so dont do that (lol).
10ft walls , 2x6 R22, and R40 in the attic. I'm north of you in Canada.
We keep it at 60f. The slab doesnt feel "warm", but it's certainly not that bone chilling "cold" that an unheated slab can be. Laying on the floor with just a light jacket or covies works fine. You're going to love it.
Some things I'd love to have...
-a booster heater (maybe a wall-mount unit heater ?) for when we bring a frozen vehicle in to work on it.
-an exhaust fan for the garage. Opening the doors on a northern summer mosquito filled rural life evening isn't an option lol

Edit - and dont forget to scratch the year and your initials in the corner. Some future person is gonna like that.

GroundUp

I will second the electric boiler suggestion. If you have NG, it may be worth exploring an 80k mod/con like the Noble Bob mentioned above but if LP, the odds are really slim that you'd ever see any ROI. I build and install these systems all the time in MN, and at least with our rates, electric is always a clear winner for something that size.

PDTech

I have radiant floor heat in my shop and really like it. My shop is 30' x 40' x 12 high. Approx. R-22 in ceilings and walls. Insulated overhead garage door. My calculated heat load was about 17.500 Btuh using Manual J. I heat it with a 50 gallon tank type electric water heater. The calculations and the actual performance have matched well; no surprises. I live in Nebraska and can keep 60 degrees down to 0 degrees F. The room also cools decently with a 14K window air conditioner.