max water temp for concrete

sw18x

My wife and I moved into our house last March and inherited an outdoor wood boiler that supplements a gas boiler in the house and heats a 4 car garage via radiant floor heat. The former homeowner ran the water temp at 130 for both loops - the problem being, at that temp, they were actually robbing heat from the gas boiler in the house. So I turned off the garage pump temporarily and I've been running the wood burner at 180 degrees, the result being a major improvement in the house. However, everything I'm reading says radiant floor systems shouldn't run water temps above 130 or there's a risk of damaging the concrete. I'm trying to get a mixing valve installed for the garage so I can run the wood boiler at 180 and mix down water temp for the garage, but the contractor I talked to said the high water temp might not make a difference if the concrete didn't contain fiber mesh. He said regular concrete with rebar would probably be ok, but fiber mesh would break down. I haven't got hold of the former homeowner yet to see what kind of concrete he poured, but if it didn't contain fiber mesh, what do you think? It would be a heck of a lot easier to skip the mixing valve and just run at the higher temperature. Sure, I'd like my play area warm but it's just a garage so it's hard to justify the added expense if it's not absolutely necessary.

NRT_Rob

the prevailing wisdom

is no higher than 140 in concrete. temp swings can be pretty bad with really high temps too.

Jean-David Beyer

there's a risk of damaging the concrete

I have a radiant slab. I cannot tell just how hot it got when I bought the place, but the asphalt tiles in the kitchen came unstuck from the concrete below. I replaced them with ceramic tile. Overheating the slab (probably not over 140F) did not seem to harm the concrete itself.



However, high temperature water in the slab leads to large temperature swings. Like a range of 66F to 75F. You could learn to live with that, but it is not ideal.



I now have a mod-con boiler with outdoor reset and the water temperatures I feed to it range from 75F when it is kind-of warm (50F and warmer) outside to 120F when it gets down to 6F outside. Design temperature is 14F, and I have never seen it go below 8F. It gets that cold only a few hours on a very few days, usually in February. The low supply temperatures are set so as to barely supply enough heat to overcome the heat loss. I do not get temperature swings. The indoor thermometer holds 69F almost all the time, though I do see 68F and 70F once in a while.

meplumber

Rob is correct.

The consensus appears to be nothing above 140. Concrete is also very susceptible to cracking if exposed to extreme fluctuations. Most radiant designs indicate a water temp somewhere between 95 and 110 deg depending on required heat output and climate.



A mixing valve is the proper way to control water temp in this application. There are several piping strategies that could correct the problems that you are describing.

sw18x

Thanks...

Thanks...looks like the mixing valve is the way to go.  I have some questions about that I'm listing in a second post - the contractor seems a little uncertain about my application.

sw18x

Thanks...

Thanks...looks like the mixing valve is the way to go.  I have some questions about that I'm listing in a second post - the contractor seems a little uncertain about my application.