Radiant Floor Heat

i do not have a boiler, but could I mount an ac coil in my hot air plenum and a circulator pump and run some plastic tubing in the concrete when I get the money to replace the floors. it would be nice to take the chill off of the garage floor when it is really cold, right now the floor temp is 62 degrees.....is this feasible? total area of floor space that I want to redo is approx 40x50...pouring new concrete sounds like a good time to install tubing, even if my efm hot air furnace isnt up to the task.....my idea is to raise the floor temps, not heat the whole building like that obviously.

I have a 40x60 shop with hot water floor heat. It will take a lot more BTU's to heat your floor than an AC coil in the plenum of your furnace.
My shop needs about 70K BTU to stay warm when it is cold out and the shop doors have been open or cold cars or trucks brought in.
I'd keep an eye open for an EFM boiler to go with your EFM furnace. You will need the BTU's from the boiler.

That said, hot floor heat is the most comfortable and efficient way to heat a building. Insulate the perimeter of the floor and under the floor. My slab has a 4" foam perimeter and 1" foam underlayment. The slab is 6" thick with wire mesh that the tubing is attached to. It's sort of a 'floating slab'.

Greg L

I currently have an EFM boiler and want to change my garage to radiant floor heating along with some other rooms in my house. My question is what water temps do you use for radiant floor with a coal boiler? I have cast-iron radiators throughout the house and the water in them is about 160-180. Would I have to use a heat exchanger for the radiant parts? I'll be replacing the garage floor this spring but I want to get all the info before I start as far as plumbing and what not.

The garage floor won't be as critical for comfortable temps as a floor in the house. In the house a floor temp above 80-85* give the people a 'hot foot' treatment.. it's too hot. In my ceramic floor'd bathroom, the parts of the floor that are about 75-78* are the most comfortable, only once in a while does it get above 80* and I do notice it. Especially in bare feet.

In my shop, I keep the floor at 55* or so, this makes for a comfortable 45-50* air temp, just fine for most work. Since the slab takes many hours to change temp, I don't try to raise the floor temp for a half-day of work, I just add supplemental heat with a propane radiant heater aimed at my backside at the work bench. Makes a nice hand warmer too.

My shop has 16' ceilings so getting the air a lot warmer is expensive. but with my feet warm, I'm warm. The little added radiant does the job. All the walls are insulated to R13, and the ceiling is R16, it is 2" foam with aluminum sheet facing down into the room, it reflects the radiant from the floor back down... it really works, you can feel it.

If you want warmer temps, then raise the floor temp the hottest I've ever had mine was 65*, a 55-60* air temp in the shop and toasty to work in.

You can buy tempering valves or mixing valves or you can make a manual one with several ball valves and a diagram to control the mix of hot water with recirculated floor water. I use a plate heat exchanger, 'cause I have a glycol mix in the floor, 'just in case'. I'm away for up to a week several times a month.. Can't loose sleep over a freeze up.

Hope this answers your questions... feel free to ask more.

Greg L

For hydronic radiant heat design you typically operate your boiler at it's most efficient operating temperature. This is almost aways greater than the water circulated in the radiant panels. To get the needed lower temperature you mix hot boiler supply water with cooler return water. There are many ways to do this. Here's Taco's way:

http://www.taco-hvac.com/en/products/Radiant+Mixi ... tegory=191

You will need some method like this because you will still need the higher boiler water temperatures for you exiting radiators. Regulating the temperature in concrete needs more than a simple thermostat because it's so slow to change. In a garage you may not care and a much simpler and less expensive system may be acceptable. You will find a lot of info using any of the internet search engines. Don't forget to insulate the slab, especially around the perimeter.

Another consideration is oxygen. There are two basic approaches to keeping oxygen out of your boiler water. Either use in-floor pipe with an oxygen barrier or use a flat plate heat exchanger to isolate the boiler water from the water in your radiant in-floor heat--then you can use less expensive pipe in the floor. As LS Farm discussed use of a flat plate heat exchanger also allow for the use of anti-freeze in the in-floor circuit.

Thanks for the info. I want to maintain a steady 50 - 60 degrees in the garage all the time. It looks like the heat exchanger/glycol setup would work good for me. Although I'm not sure about using non-oxygen barrier tubing, how would you keep taking the air out? An air scoop inline? I figure I will need a lot of tubing and the barrier is a couple cents per foot more but I think I'd just feel better with that installed.

I use plain ol' black plastic underground sprinkler tubing in my slab. It is NOT impermeable to oxygen. BUT, if you keep pressure on the system there is no migration of oxygen molecules through the plastic pipe into the circulating water.

I keep just a pound or two of pressure [gravity pressure of a supply/expansion tank 14' above the floor] on the tubing system, and use a water/glycol mix in the tubing. My first system designed the same way is still circulating after 20 years, this one is 9 years old.

Pex was VERY expensive when I poured my shop floor, now the cost of Pex is much more affordable, so it's not as difficult to use the better tubing.

Greg L.

mufwapo wrote:Thanks for the info. I want to maintain a steady 50 - 60 degrees in the garage all the time. It looks like the heat exchanger/glycol setup would work good for me. Although I'm not sure about using non-oxygen barrier tubing, how would you keep taking the air out? An air scoop inline? I figure I will need a lot of tubing and the barrier is a couple cents per foot more but I think I'd just feel better with that installed.

Dissolved oxygen is not much of a problem so long as the fluid doesn't come in contact with steel or iron in the boiler where it will cause corrosion. A simple air scoop, or a somewhat more expensive device like a spirovent (TM), installed in the circuit will remove gases that would otherwise make for a noisy system.

I've found that if the circulator pumps are run every month, that the cast iron pumps work fine. I've never owned a brass circulator pump. I've had only one pump stick from corrosion, and it sat in pure water for almost a year. I took the motor/impeller off [4 screws] and wire-brushed the housing where it had grown some rust that touched the impeller, and is still running 3 years later.

I'm not a fan of spending 3x the money on bronze pumps when a cast iron pump will work in most cases. However there are exceptions. Yanche has really acidic water and his water system eats parts every few years.

Greg L

Hi Ian, I don't think so either, but don't want to have some wierd microbe likes ethylene glycol to start growing in my hot water floor piping.

Do the rust inhibitors 'wear out' in antifreeze??

Greg L

I think that the rust inhibitors tend to loose effectiveness over time, esp. in the presence of rust or corrosion. The more of this that is in the system, the faster the inhibitors are used up. In gas engines that is what happens. The antifreeze is still there but the inhibitors are depleted and dirt is suspended in the antifreeze.
I may be wrong, that's just my opinion. You can probably add more corrosion inhibitor to the system.

Wood'nCoal wrote:I think that the rust inhibitors tend to loose effectiveness over time, esp. in the presence of rust or corrosion. The more of this that is in the system, the faster the inhibitors are used up. In gas engines that is what happens. The antifreeze is still there but the inhibitors are depleted and dirt is suspended in the antifreeze.
I may be wrong, that's just my opinion. You can probably add more corrosion inhibitor to the system.

sounds about right to me.....once that corosion protection is gone from antifreeze, galvanic reaction takes over......makes a mess of the cars cooling system.....it only takes about 5 yrs for antifreeze to turn into brown muck....wouldnt want to look inside a neglected closed loop boiler

I suppose the fluid from the system could be filtered and the rust inhibitors replenished, much the same way as with auto antifreeze today. Or you can just do it the old way, flush it out and let the ground absorb it, then add new antifreeze (remember in the fall, all the "do it yourselfers" would have their cars backed into the driveway flushing the cooling systems out with the garden hose?).

https://en.wikipedia.org/wiki/Ethylene_glycol