I'm building a garage that is 28X20. We live in Saskatchewan, an extremely cold winter environment and I'd like to warm up my garage to a comfortable temperature. I've installed the pex tubing in two sections and am ready to install the "heater". I am leaning toward a tankless water heater, and have been thinking I'd like to use glycol instead of water since it will be a closed loop system and I would rather not trench water to the garage if I don't have to, but I will if water is the only option. Can anyone give advice on the size of water heater I might need, a brand that you'd recommend, and also any pros and cons of using glycol. Thanks, any help appreciated Deb

You need to first do a building heat loss analysis and then a HR design in order to properly size and then select the heat source. We have free DIY software on our website to allow you to do just that. We also have software on our website for sizing your expansion tank and a buffer tank should that also be required. You can change the percentage of glycol from zero to 50% to see exactly what effect is has on your required supply temp and circulator pump flow rate given your actual indoor/outdoor design conditions. In short, using glycol will provide increased freezing protection at the cost of less efficiency (i.e., higher supply temp and flow rate).

Assuming you have at least some insulation and the garage doors have reasonable air seals almost any natural gas or propane fired tankless will have sufficient output to heat the place.

To get satisifactory results out of it and get a reasonable lifespan out of the tankless will still require a real hydronic design, and it all starts with a realistic heat load calculation. Many systems designed around tankless water heaters are over-pumped and over-fired- don't make that mistake.

Good point Dana and why I mentioned a buffer tank might be a consideration here. When low thermal mass heat sources are combined with a small hydronic radiant distribution system or a larger zoned hydronic radiant distribution system, it is possible for the heat source to short cycle when a small system calls for heat or when only a couple of zones in a larger system call for heat. This occurs because the rate of heat produced by the heat source is much greater than the rate of heat used/released by these zones. Low thermal mass heat sources in combination with low hydronic fluid volume hydronic radiant distribution systems cannot absorb this excess heat rate without experiencing a rapid rise in temperature. This causes the heat source to reach its high temperature limit very quickly resulting in very short on times and short cycling. While electric heating elements can tolerate short cycling, gas valves, oil burners, ignition systems, and compressors will have a significantly reduced life expectancy. So a HR design is required to size and select the best heat source.

Thanks for the info so quickly, I am new to this so I may be back with more questions when I do the heat loss analysis. Deb

So I've started to do some calculations, but I need some help.

1. We have a standard garage slab, but under it we have 2" Styrofoam insulation. does that influence the number as well?
2. The floor will be concrete, no flooring installed as it's a garage?

2X4 framing, OSB walls and roof, roof paper, 25 year asphalt shingles, R20 batt insulation in ceiling, R12 batt insulation in walls, building wrap, 8 mil poly, and vinyl siding.
We have a double steel garage door, a steel insulated man door, and two small windows.

Can anyone help me do the calculation for the R value, and then help me convert for the U factor. I know this is a technical step necessary to help me determine our heating....but it's abit above my head as a homeowner. So much appreciated. This is at our cottage, and so it is really difficult to get tradespeople out to advise, or give estimates, so I am hoping that if I learn more myself, it will help us be more knowledgeable in dealing with the tradesguy who we will have to get to install.

deb

You might check code there in Saskatchewan. 2" of insulation under a heated slab seems a bit shy.

If you use our software, you only have to enter the building assembly R-values and the software does the U-factor conversion and calculates the BTUs:

Borst Heat Loss Analysis Software

Here’s the link to the HR design software:

Borst Hydronic Radiant Floor Heating Design Software

The software instructions explain all the input/output parameters.

With regard to minimum slab-on-grade under-slab insulation R-value, the equation is 0.125 (Ti-To) where Ti is the indoor design temp and To is the outdoor design temp. So an 80F delta T would require a minimum of R10 or about 2” of EPS.

I tried using a tankless hot water heater on a radient floor system and it DIDN'T work at all. The unit cycled on and off constanly because entering and leaving water tempuratures were so close that the heater couldn't run at a low enough btu setting without the water temp going to high and shutting the unit back off.

These unit are made to raise 50 degree water to 120 degrees in a fraction of a second. They can't run low enough to raise 110 water up to 120 degrees without over shooting and shutting off.

These days there are tankless water heaters specifically made for HR applications. A buffer tank can also be sized/added to a HR system to allow even the improperly selected tankless water heater that you apparently tried to cycle at an acceptable rate for its mechanical components to live a long life if necessary.

Borst Buffer Tank Design Software

I don't think Saskatchewan will allow the use of a natural gas water heater without using and additional exchanger. The fear is if it springs a leak, the flammable glycol sets the building on fire.

Posted By sailawayrb on 24 Feb 2014 08:54 AM
These days there are tankless water heaters specifically made for HR applications. A buffer tank can also be sized/added to a HR system to allow even the improperly selected tankless water heater that you apparently tried to cycle at an acceptable rate for its mechanical components to live a long life if necessary.

Borst Buffer Tank Design Software

With the thermal mass of a concrete slab as radiation it would have to be a pretty g'dawful system design to actually need a buffer tank. The tank money is almost surely spent re-configuring the system into something more reasonable.

Plumbing it primary/secondary and specifying the flow on the primary loop to be able to deliver the full heat load BTUs at somethingbetween 20-80F delta-T, (and not over-pumping the radiation-side either) will get you there.   Most hacker-design systems out there are ridiculously over-pumped on one side or the other, or both, or pumped direct with abyssmal problems.

It all starts with a heat load calculation. From your peak heat load calculations you can estimate the peak water temps and you'd need based on the surface area of the radiant slab, as well as the total BTUs, and you can keep working it back from there. It's a matter of setting up compatible flows and deltas on both sides that can deliver the full design day load (and not much more) into the system without over-firing the tankless. Generally speaking it's best to keep the peak-firing rate under half the full-output of the tankless if you want to have it last very long, and keep the flow on the tankless end fairly low, somewhere between 1.5-3gpm, even if it means a big delta-T on the primary loop.

A 2gpm flow at a 90F delta-T is about 90,000 BTU/hr (which is probably WAY more heat than this building is going to need) doesn't stress the tankless a bit.  Delivering the same 90K with a 6gpm flow @ 30F delta-T is guaranteed to wear out flow sensors on the tankless in short years.  Most tankless units out there can be set up with output temps well in excess of 150F, and all are tolerant of high temperature deltas by-design. Most slabs designs never need more than 110F water even on design day, but pumping direct rather than primary/secondary is likely to lead to a high-flow/low delta-T nightmare for the tankless.

But until you know the heat load, you're not even to square one for designing the system needed to support that load.

Posted By hogger on 23 Feb 2014 06:12 PM
So I've started to do some calculations, but I need some help.

1. We have a standard garage slab, but under it we have 2" Styrofoam insulation. does that influence the number as well?
2. The floor will be concrete, no flooring installed as it's a garage?

2X4 framing, OSB walls and roof, roof paper, 25 year asphalt shingles, R20 batt insulation in ceiling, R12 batt insulation in walls, building wrap, 8 mil poly, and vinyl siding.
We have a double steel garage door, a steel insulated man door, and two small windows.

Can anyone help me do the calculation for the R value, and then help me convert for the U factor. I know this is a technical step necessary to help me determine our heating....but it's abit above my head as a homeowner. So much appreciated. This is at our cottage, and so it is really difficult to get tradespeople out to advise, or give estimates, so I am hoping that if I learn more myself, it will help us be more knowledgeable in dealing with the tradesguy who we will have to get to install.

deb

For R12 batts in a 2x4 16" o.c. vinyl-sided OSB or plywood sheathed studwall with half-inch gypsum on the interior, figure on a U-factor of about 0.11 BTU/square foot per degree-F. And if installed perfectly, and air tight at both the sheathing and the interior side poly.

Assuming 100 square feet of garage door 20' of side door and 50 square feet of window, a 28' x 20' x 10' tall wall would have about 800 square feet of wall area. Assuming an outside design temp of -25F/-32C and an interior temp of 70F, that's a 95F delta, and


800' x 95F x 0.11= 8360 BTU/hr of wall losses.

R20 batts between 2x6 or 2x8 ceiling joists runs about U0.057, with a top-side air barrier to limit convective losses of performance at the low temperature extremes.  With 560' of attic area, that's a load of...

560 x 95F x 0.057= 3030 BTU/hr out the attic.

Assuming the doors are insulated to R4/U0.25 the 110' of door is worth...

110' x 95F x U0.25= 2600 BTU/hr of door losses.

Figure the 50' of window at U0.35 and you're looking at...

50' x 95F x U0.35= 1660 BTU/hr of window losses.

Slab edge foam needs to be attended to with at least 2"/R8.4 of EPS foam to somewhat match the wall's U-factor, and the sub-slab foam needs to be at least 3/R12.6 of EPS (4" would be better) to not add significantly to the heat load.

Add it up and you're at about 15,650 BTU/hr plus whatever infiltration & ventilation fudge factors, foundation edge and slab losses etc you can think of.  It'll probably end up  somewhere in the neighborhood of 20,000BTU/hr @ -32C, and not likely to exceed 25,000BTU/hr or roughly in the neighborhood of 40 BTU/foot of radiant slab, as long as you have all the windows and doors closed.  That should be pretty easy to hit with a slab.  Setting the tankless flow up for 1-2gpm and a delta-T of 25-50F should be pretty easy for delivering the necessary heat without overfiring or short-cycling the sucker.  The radiation side would have to be setup for a much lower delta-T and much higher flow.

But that's also within the range of what a much cheaper gas-fired tank can deliver.

Dana, your heat load numbers seem about right to me. So 25,000 BTU/H divided by (500 times 1 -2 GPM) is precisely a 25-50F delta T for the primary side. The secondary side would likely be 15-25F delta T and the flow rate(s) and manifold balance valve Cv settings would come directly from the design so as to generate the required upward heat flux at the supply temp using properly selected circulator pump(s) operating near BEP.

Dana, what exactly do you mean by g’dawful system design? Short cycling becomes problematic whenever you have too much heat source relative to the hydronic fluid volume being heated. So this can occur whenever your slab emitters calling for heat only require a small BTU charge, but your heat source provides too large a BTU charge. So a small HR system (or even a large HR system where only a small number of zones may call for heat for some period of time), can get you into this short cycling situation. A 28x20 HR floor area is relatively small and selecting the right size and type of heat source is always something to consider and properly sort out. I would agree that a buffer tank can and should be avoided when possible. However, if you manage to get yourself into this situation like the previous poster indicated, a buffer tank can often solve your problem.

Dana and all, thanks so much for the response and for the calculation. What are your thoughts on electric tankless rather than natural gas. I haven't looked at the energy consumption differences, but it would be easier to wire it for electric rather than running natural gas to it. Either are an option.

Again, thanks Dana for doing the calculations for me. I'm learning a lot as I go through this, steep learning curve.
deb

Sounds like you have you mind set on a tankless! I tend to agree with Dana that a tank might be a more economical choice here. You will need to sort out the utility rate differences at your location to pick the winner between gas-fired and electric-powered...although I suspect the gas-fired will be the winner. Electric-powered is certainly easier to install, is less prone to short cycling failure (although you will now avoid doing that) and doesn't have the flammable glycol issue FPPB indicated. Learning is always a good thing whether you do something yourself or hire it out...

"Short cycling becomes problematic whenever you have too much heat source relative to the hydronic fluid volume being heated. So this can occur whenever your slab emitters calling for heat only require a small BTU charge, but your heat source provides too large a BTU charge."

It's not just the thermal mass of the hydronic fluid, but also the thermal mass of the radiation. Slabs are moderately conductive and have substantial thermal mass- unless your feeding it a HELL of a lot more BTU/hr than it'll EVER need or pumping some ridiculously high flow/low-delta-T it's effectively self-buffering. Even when the duty cycles are miniscule it doesn't take much temperature hysteresis to deliver reasonably long burn cycles during low-load conditions.

Electric tankless units are a lousy choice. Electric boilers are designed to tolerate high flows, and a small electric boiler would be just fine pumping direct at flow rates and low delta-Ts that would ruin a gas-fired tankled. It would be cheaper to install than a gas-fired tankless, but unless you have super-low off-peak power rates considerably more expensive to operate. Most 6-8kw electric boilers are under $1500 at internet pricing. (A 6kw boiler is good for ~20.5KBTU/hr.) Keeping the place at temperature 24/7 with an electric boiler probably makes it cost effective to go much higher-R on the building construction. Controlling the boiler with a slab-thermostat to keep the slab from feeling too cold and supplying the bulk of the heat with a mini-split heat pump would be more expensive to install, but much cheaper to operate. A Mitsubishi -M series or Fujitsu XLTH series 1.25 tonner can deliver ~15,000BTU/hr @ -25C, using only half the electricity @ -25C that an electric boiler would, and the seasonal average power use would well under half. But you're probably talking $4-4.5 to have a pro install it, half that for 95% DIY install, only hiring the pro for the final system charging & testing.

Dana, I think you are perhaps forgetting that once a slab thermal mass reaches its operating temp (typically about 75F in a well insulated building), only the thermal mass of the hydronic fluid, the minimum heat output of the heat source, the heat source ON/OFF temp settings, and the operational heat radiated by the slab zone determine the cycle time. We don’t include the thermal mass of the slab when designing/sizing a buffer tank.

I fully agree that doing a ROI analysis after having a good HR design is really the only way sort out the best heat source. Other considerations may come into play too, but one should always consider cost. I am always amazed at what some folks are willing to spend on a plate HR system, but again, we live in a free country and folks can do as they choose.