Need help calculating water heater size for a garage.

d9canada

I have a concrete slab on grade with steel building on top, cemented in on the sides. The end walls are steel stud framed with steel cladding. The total floor area inside is approximately 700 SF. I've installed four 1/2" PEX circuits in the concrete - each is about 225 feet in length continuous (no fittings). I believe that each circuit involves approximately 1 gallon in volume. I have to make in/outlet manifolds. I have a 110v utility pump that I already tested with a jury-rigged solar heater, so I know it's capable. The building is on my acreage where I only have 110v power, although I do have 220v 20 amp close by in my well pump house that I may end up extending into the building, but I'll have to avoid starving my well pump for power. I have no gas available.

I want to do is heat the floor to approximately 50-55 F during the winter even when it's freezing outside. I'm in Canada, on Vancouver Island on the Pacific coast, so we don't usually get much below 20F at any time but we do hover around freezing for months through the Winter, at least at night. The building houses my motorcycle collection, is almost 14 feet at the ridge, 70 degree sloping side walls. I'm tackling insulation glued to the inside of the structure now.

Please tell me how I can calculate the BTU requirement or the specs of water heater that would work. It will not be used for domestic water heat as well. And please understand, the main goal is to keep the floor (and thereby the building interior a bit nicer than freezing cold - preferably with a 110v system (I don't ask much!) - but it's not like heating a home to be really comfortable.

If anyone wants to tackle and added twist - I would like to also use my solar heater during the day, but I don't know how to combine it and the other system so that the solar can be isolated at night (so it doesn't act like a cooler) and yet not have it freeze up. I don't want to be using antifreeze if I can avoid it but I will it is needed.

Thanks in advance for any suggestions!
Brian

Larry Weingarten

Hello, Just a question, which will likely affect your system design; Did you insulate under the slab? And if so, what sort of insulation and how thick?
Also, you might want to go here: http://www.slantfin.com/slantfin-heat-loss-calculator/ and have a look at the Slant Fin app for determining heat loss from the building.

Yours, Larry

d9canada

No this is was a soil/cement process but, it's well drained, on gravelly soil on a saddle-back of bedrock. So no insulation but no water issues.
Thanks for the link, Larry!

d9canada

Just to follow up, I tried the Slantfin app but it's predicated on boiler systems running 180F and does not cover steel buildings. Nonetheless, it did give me a suggested BTU at 12,500 and I figure I'd say 18,000 to be safe. I've read enough to realize that despite much discussion to the contrary, people have very successfully used a standard hot water tank to heat a floor.

So saying I need 18,000 BTU, how do I translate that to determine an adequate hot water tank?

Larry Weingarten

Hello, Water heaters fail early if they are worked too hard. Doing this creates lots of condensation and rusting of the tank on the fire side. Getting something with a big enough burner so it does not run hours at a time will help reduce that condensation and prolong tank life. Even condensing water heaters have problems if they are kept in condensing mode for too long. I know this doesn't answer your question, but it's a piece of the puzzle.

Yours, Larry

Brewbeer

You will be using an electric water heater?

d9canada

I found calcs online, saying 3.41 BTU/hr from each Watt/hr. So it would seem I have to go 220v with 2-3000 Watt elements.

Comments?

Zman

18,000 BTU / 3.412 = 5,285 watts.
If you divide that by 120 volts, you would need 44 amps
Divided by 240 volts would require 22 amps.

A standard 4,500 watt water heater would probably do the job on most days. You would need to bring a 30 amp 240 volt circuit to the building. Better yet, bring over a 100 amp subpanel and run everything comfortably.

For the solar, a simple drainback system would do the trick. I would suggest a heat exchanger to separate it from the the water heater and loops.

hot_rod

not a lot of solar available from a cold winter sky, it might not add much to the system without a large array

d9canada

Hot Rod, you should do more research. Go on Youtube and see guys out in the snow, and their solar heaters are running well over 100 degrees. Of course daylight hours are restricted but it's hardly useless. I've got 5 parabolic reflectors on 24" X 1.25" copper tubes and yeah that thing heats water.

Zman, thanks - our local Home Depot has a 40 gallon tank, 220 on 20 amp breaker with twin 3000 watt elements. I agree it seems under powered on a 20 amp circuit - will have to investigate.

hot_rod

quantity not quality when it comes to solar thermal. I use T-sol simulation software it can model cities most anywhere, much more accurate than You Tube Diyers

Sunny SW US designers shoot for a 20-30% SF, The simulation will model very accurately.

To bad the sun doesn’t shine at night where loads are usually the highest🤔

I’m a huge ST fan, just be realistic in you expectations If you do the labor yourself and have access to low cost components, it’s a fun technology

Jamie Hall

d9canada said:

Hot Rod, you should do more research. Go on Youtube and see guys out in the snow, and their solar heaters are running well over 100 degrees. Of course daylight hours are restricted but it's hardly useless. I've got 5 parabolic reflectors on 24" X 1.25" copper tubes and yeah that thing heats water.

Zman, thanks - our local Home Depot has a 40 gallon tank, 220 on 20 amp breaker with twin 3000 watt elements. I agree it seems under powered on a 20 amp circuit - will have to investigate.

Electric water heaters use either one element or the other. They do not -- unless rewired -- use both together. So twin 3000 watt elements will not produce 6000 watts. That is how that heater you quote can run on 20 amps/240 volts. However, if you need 6000 watts, it won't produce it.

On the solar heaters. Indeed they can run well over 100 degrees. With concentrating or parabolic collectors, they can run well over boiling, in fact, which produces its own interesting set of problems. I quite agree, also, that they are hardly useless. However, you would do yourself a favour if you actually looked up the effective daylight hours of sunshine, particularly in your area. You may find that that it is less than you might think. In the Vancouver area, the best information I can find for the winter months is an average of 2 hours of sunshine per day. Assuming that you have tracking parabolic collectors, that translates to an input of 2 kilowatt-hours per square meter of collector aperture per day -- a number you may find useful.

hot_rod

And the ambient air temperature It grabs some of the harvest before you can get it in the bank

Jamie Hall

I might add that if your collectors don't track in both altitude and azimuth, your actual input is considerably less... never mind, as Bob just pointed out, the output...