*
What kind of provision would we need to protect an under-slab radiant heat system from freezing when the power fails?
Discussion Forum
Discussion Forum
Up Next
Video Shorts
Featured Story

Listeners write in about fostering trade work and proposed changes to Canadian code and ask questions about roof and wall insulation for an old house.
Featured Video
Video: Build a Fireplace, Brick by BrickHighlights
Fine Homebuilding Magazine
- Home Group
- Antique Trader
- Arts & Crafts Homes
- Bank Note Reporter
- Cabin Life
- Cuisine at Home
- Fine Gardening
- Fine Woodworking
- Green Building Advisor
- Garden Gate
- Horticulture
- Keep Craft Alive
- Log Home Living
- Military Trader/Vehicles
- Numismatic News
- Numismaster
- Old Cars Weekly
- Old House Journal
- Period Homes
- Popular Woodworking
- Script
- ShopNotes
- Sports Collectors Digest
- Threads
- Timber Home Living
- Traditional Building
- Woodsmith
- World Coin News
- Writer's Digest
Replies
*
Antifreeze in the lines compatible with the tubing or a secondary heat source or a generator....
*I've only had mine off for 3 days at 5F. (And it was on purpose). Couldn't notice the first day. Second day I needed a sweater. Third day was a bit nippy. Are you going to have a power failure for more than a week at temps below zero F? Unlikely.It takes a long time for 50,000 pounds of concrete to cool down when it is insulated and the soil on the other side of the insulation has come to 60 or 70?F I worry about the water pipes in the walls and in the heating manifold long before the ones in the slab.If I ever had an extended power failure, I'd run an invertor off my car battery. I only need a couple hundred watts to run the circulator pumps. But that would be for comfort, not freeze-protection reasons.If you wanted a totally automated system, I'd use some off-the-shelf emergency lighting or back-up pump system to run a 12-volt circulator pump off a thermostat set to 45F. Add some check valves so the 12-volt pump and regular 120-volt can each do their own thing. -David
*Short of checking w/ the mfg of the "system", I would use a non-toxic antifreeze. Almost all, if not all, non-toxic antifreezes are based on propylene glycol (PG). I believe the automotive antifreeze, Sierra Brand is also based on PG while the majority of automotive antifreezes are based on ethylene glycol which is toxic when taken internally. All antifreezes also contain corrosion inhibitors which would be highly advantageous in any "system". Unfortunately, the combination of antifreeze and water produces a solution which has heat transfer capabilities about 15-20% less than corrosion inhibited water. But this is a small price to pay for freeze protection. Use a 50/50 mix unless otherwise advised by the manufacturer of your system.
*Eric: I too am in favor of propylene gycol (which is found in name brand salad dressing) instead of ethylene gycol (tablespoon kills a cat, 1/4 cup kills a dog, intrapolate the dose for your child). I have used it in all systems I've installed.You are exactly right about Sierra brand versus other automotive antifreezes. Recently priced Sierra at K-Mart versus cans of propylene gycol at an industrial supply. And, of course, the mass marketed stuff is cheaper (and already has those anti-corrosion agents in it).Regarding reduced heat capacity: Wouldn't that push one towards 20-30% antifreeze concentrations? To retain the most heat capacity within freeze-protection required you'd rarely need to go to 50/50. Plus water is a lot cheaper than gycol. -David (Chem Eng)
*
What kind of provision would we need to protect an under-slab radiant heat system from freezing when the power fails?