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bennelson 10-12-15 02:37 PM

Heated Garage Floor
Hi everyone,

I'm slowly trying to figure out how to HEAT my garage when I get it constructed.

I REALLY like the idea of a heated floor. A friend of mine has one, and it is SO nice to work on cars in there in the winter!

So, here's what I am thinking...

The proposed garage would be 27' x29', with a "storage area" upstairs. The storage area could be finished to be sort of a possible home-office/escape/hobby area. It would be low ceiling and less than half (maybe even only a third) of the floorspace of the garage itself, and accessed by a fold-down temporary stairs, similar to what many people use to get to their attic.

The construction would be typical - 2x4 walls and fiberglass insulation. There would be a layer of foam insulation on the outside, with siding over that, the roof would be metal standing seam.
The foundation would be a concrete slap with PEX tubing cast into it, and at least 2 inches of EPS foam insulation under it.

I think I would like to use an ELECTRIC MICRO-BOILER.

The garage is its own freestanding building. I have natural gas piping at my house, it WOULD be possible to get natural gas out to the garage, but would be a bit of a hassle (trenching, rerouting pipes, etc.)
Also, I would prefer to get away from fossil fuels as much as I can.

An electric micro-boiler could get its electricity from solar panels (proposed 5KW grid-tie) or from my regular electric connection, which could be set to time-of-day for cost savings. Because a concrete slab heats up and cools down so slowly, I think I could just run the boiler at night (Off-peak hours) to get time-of-day pricing on the electricity to heat it.

I tried making a basic heat loss calculation, and found that I would need somewhere in the neighborhood of 24,000 btus per hour.
I also found a "rule of thumb" saying that for electric heaters, watts times 3.41 = BTUs. So for 24,000 btus, I would need an 8KW electric heater.

In my area, one of the big box home improvement stores is called Menards. They have a store not too far from my house that has a pretty good selection of pumps, pex, boilers, and other components needed for a hydronic in concrete system.

I took a look through some of the equipment they have for sale, some flyers, and general info, and think I know what I need to get, but want your input.

They have a micro boiler called the Stiebel Eltron Hydro-Shark 3 Microboiler - It's available in a 7KW and 10KW version for $500 and $530 respectively. It's basically an instant water heater, only specifically designed for hydronic heat. It's 240V, and would be directly wired to a dedicated 50A circuit.

Basically, you just flow water through it, and it kicks on and heats the water (or anti-freeze) as it goes. Simple, right!?

It also looks like 1/2" Pex is typical for this type of installation, and that's what a lot of the manifolds use. Since the size of the building will be 783 square feet, and pex typically comes in 300' rolls, it seems to make sense to use three loops of 300 feet and do 12" spacing, that would cover up to 900 square feet.

So, I would want a manifold designed for 3 loops.

I've also seen panels that are pre-assembled with the pump, pump controller, fill and purge valves. They are more expensive than I thought, with the one I would be looking at costing $1600! The upside is that it is pre-plumbed and pressure-tested. The only things missing are the manifold and thermostat. That's also two pumps, for a primary/secondary setup, which is apparently what's needed when using a tankless water-heater.

Here's the pre-built panel:

Here's a pre-built panel that comes already hooked up to the 10KW version of the micro-boiler.
(In that one, I believe the image is wrong, it seems to show one of the higher output boilers.)

Here's a link to the 10K heater. The 7K is the same thing, just $30 cheaper:

Other things I would need:
Thermostat - $50
Manifold - there's several styles, but a NICE one, that's all built together and already has flow meters on it is $210.
PEX - 3 rolls of 300' each: $130/roll - $390 total.

Panel w/ boiler: $1840 and those other things, comes to right around $2500.

Does this sound about right to you folks?

What else should I be considering? Is there a particular reason to go with a 7K vs. a 10K heater? Some reason to look at some other fuel source? Other pex or concrete considerations?

Let me know! Thanks!

stevehull 10-12-15 03:09 PM

Great idea! Here are some thoughts.

1) 2 inches of high density foam under the slab will not be sufficient if you are heating with electricity. Your ground temp is about 45F and in the winter, it will drop even more. 3-4 inches is better (R18-24).

2) don't heat the entire garage floor - just the areas you are crawling on, perhaps the middle 2/3rds.

3) a lot of people in Wisconsin use wood fired water boilers. Since these are carbon neutral, it is "green" - perhaps even more so than coal derived electricity.

4) Peripheral slab insulation (R20), down at least 30 inches, is a must

5) Have you considered IR heat? It heats people, not air. Yes it might mean bring in the gas line, but these are INCREDIBLY efficient so long you are in sight of the IT tube heater.

6) I wish Menards were here in Oklahoma . . . . .

Lastly, how did you calculate the 24KBTU/hr? How large is garage floor and what do you want to heat it to?


randen 10-13-15 05:26 PM

Heated Garage Floor
2 Attachment(s)
I have lived with heated floors for 20 yrs. now. It is truly the holy grail of heating a space. I have them in both my home and office area of my shop.

It is very efficient in putting low energy heat were it needs to be.

My home uses both solar hot water and Geo-thermal to warm the floor and it is fabulous.

You may have a golden opportunity here as even a possibly better method is about to be tried. Direct Exchange Geo-thermal by Memphis91.

Copper tubing cast in the floor and another loop buried in the ground and a small compressor to move the refrigerant. This method would not require the addition of anti-freeze or control valves and pumps. Super efficient.

You could also have air-conditioning

The cost of operating would be minimal. If your thinking about powering with Solar PV even better but you would require a 3ph compressor and a VFD still not a deal breaker. The PV panels in this configuration could power the heat-pump directly and if need-be with line power as well.


JRMichler 10-18-15 02:50 PM

I built a heated garage/workshop in 1985. The heat was from two rows of heat lamps in the ceiling. Ten heat lamps at 250 watts each for 2.5 KW. That was enough to maintain 60 degrees in all but the coldest weather.

The floor was pitched 2" back to front, and an additional 1/2" sides to center at the front door. That worked perfectly. I could drive in my car with a full Wisconsin winter load of ice underneath, eat supper, and have the ice all melted off. The melt water drained out under the car, so the floor was dry on both sides and in front.

My current shop was originally built as a garage. I added 2" Styrofoam around the slab. The Styrofoam goes down 24", and out 24". There is no insulation under the slab. The wall studs were doubled to make room for 7" fiberglass batts, plus 2" Styrofoam on the outside. The ceiling was covered with drywall, foamed from above, then fiberglass blown in for a total R-value about 80. The 16' insulated steel door has 1" foil faced polyiso insulation glued to the inside. Heat is from a water to air fan coil unit that I built.

The floor stays warm enough that I have no desire to heat it. It would be even warmer if there was foam underneath it.

The total heat loss is about 10,000 BTUH. I leave the thermostat at 65 degrees. The highest inside temperature during an unusually hot summer was 70 degrees. A well insulated shop does not need air conditioning.

It will need a dehumidifier. A dehumidifier is especially important in an insulated building with minimal internal heat gain. Without it, the entire inside with be a moldy swamp whenever the outside dew point is above the inside temperature.

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