Radiant Floor slab Question

[Guswhit]

Okay guys/gals. I have had an OWB for many years, but never had radiant heat(just water/air exchangers) before so here's the big question.
When do you guys start to warm up your concrete? Obviously, I don't want to wait until it's freezing out and give the cement a shock, but I don't want to start it when its in the 60's outside either. Basically I'm taking a poll to see what sustained outside temps you guys are looking for outside before firing up the radiant heat? I'm talking about just in slab concrete by the way. One more also. How long does it usually take you to get the slab up to heat? I have a 1500 sq. ft. garage and a 2350 sq. ft basement, what would you expect time wise for those?
Thanks.

 

[ChoppyChoppy]

I just heat the slab when I need it. Thermostat set to 45*, so whenever the garage gets colder than that. The house I keep the thermostats at 60*. It only comes on if the stove goes cold.

Takes about 12-24hrs for slab to heat up and warm up, depending on temps. Like in the garage if I wanted it to be 70 and it's -20* outside, it'd take about a day.
I have a Modine heater for those times, plumbed to boiler water, not mixer water.

 

[Guswhit]

I talked to a couple other guys last night that said they fire up there floor heat after having a weeks worth of 40 degree weather. They said from their experience it seemed to stay cool enough and then the house didn't overheat. Sounds good, I guess I will have my own experience in a couple months, if I every get it finished.

 

[ChoppyChoppy]

House won't overheat too much if you have a good mixing valve. It adjusts water temp based on outdoor temp. Like at 50* outside, I'm running 70* water. At -20*, 135*.

 

[lindnova]

At 60 degrees I wouldn't worry about it at all.

I usually keep my garage around 55 unless I am planning on working out there for a while I might raise it to 62. It is actually warmer near the floor and can get plenty warm working under a car. I would keep it cooler but don't want to have any water lines freeze near the overhead doors.

I have a mixing valve and run somewhere around 105° water temp. I have turned it on when it is close to freezing and with 105° won't hurt the concrete. I would not put 160° water into concrete just to avoid cracks.

My garage is well insulated and has southern windows. It heats pretty fast. I would say 10 hrs to raise it from 50-60. Once it gets there it usually overheats up to 63 degrees before cooling down.

 

[huntmaster86]

lindnona-- what is the max temp you will run through your concrete? Im new to boilers and trying to get mine finished for this season. 2300 sq ft garage is all ill be heating with mine. I hadn't planned on a mixing valve for my system but like i said im new and still learning

 

[NSMaple1]

I would say a mixing valve is a must.

Most floors are happy with 100-110 water. Most boilers are happy with return water above 140.

Which might actually call for two mixing valves - one at the boiler to mix in hot bypass water to maintain +140 return temps, and another for the floor to mix in cool return water to maintain -110-120 floor water temps. Is there a bypass loop on the boiler?

 

[lindnova]

I agree with NSMaple1. My thought when installing mine was concern for the floor, but the boiler also likes warmer water returning so as not to cause corrosion or expansion type problems in the boiler. I have the old type insulated pvc with 2 - 1" pex pipes inside that tough each other, so if the return water is too cold it has 100 ft of pipe to do some equalizing before getting back to the boiler.

 

[huntmaster86]

Im still finishing the boiler. I have been contemplating putting extra ports in for a bypass loop on it.

 

[huntmaster86]

NSMAPLE-- how can my return water be 140 if the water is only 100-110 for the floor?

 

[NSMaple1]

NSMAPLE-- how can my return water be 140 if the water is only 100-110 for the floor?

That's where mixing valves come in. And boiler bypass loop.

The boiler return water is kept above 140, by mixing in hot water from the supply via a boiler bypass loop. And the mixing ratio is usually handled by a mixing valve. Some instead use a second circ pump on the bypass loop, that runs off & on as return temps raise & drop. ('bang bang' control).

Maintaining 140+ boiler return temp is quite important for preventing firebox corrosion arising from condensing of creosote in there. Most of the early years OWB failures I think were from that. Most OWBs now incorporate some sort of mixing loop, already built in, usually on the back.

Then on the other end, another mixing valve & circ mixes cold water returning from the floor, with hot water coming from the boiler, before the mix hits the floor.

The net result of all that should be, not a lot of water actually flows between the boiler, and the floor - only what is needed to maintain the desired floor temp. (And what is needed to account for whatever heat losses there are in between - like to the ground from underground piping).

 

[huntmaster86]

Ah ok. Makes sense. Thank you!! All my plumbing is indoors. Boiler and All so any heat loss will just escape into the heated garage. I was going to have a reserc loop on top of the boiler to keep the water in the boiler constantly moving hoping to have a more even temp within it. Have it opposite top and bottom from the supply and return