Q: We've become interested in radiant flooring heating, partly because of frequent mentions of the system on This Old House. But what happens if the power is off for several days in very cold weather? Is there any way to prevent the tubing from being damaged?

— Don, Crownsville, MD

A: Richard Trethewey replies: Cross-linked polyethylene tubing performs better than copper in freezing weather: It can flex as much as twice its diameter as the water inside it turns to ice. But PEX is not indestructible. If an entire length of tubing were to freeze solid for 24 to 48 hours, it could split open, the same as copper. And if the tubes were embedded in concrete, it's possible that it could crack, too — water exerts a whopping 2,200 psi of pressure as it freezes. (By the way, freezing water can crack your boiler as well.)

How quickly the system freezes depends on how well insulated your house is, how cold the weather gets, and how long it stays below freezing. The location of the tubing is also a critical factor. If it's embedded in concrete, it will take a while to lose its heat, compared with tubing attached to the underside of a subfloor. The pipes most in peril are those in exterior walls, and in the vicinity of the boiler manifolds.

The best, though also most expensive, insurance against freezing is an automatic standby generator, one big enough to keep the boiler working. A cheaper solution would be to connect a small backup power supply to the circulating pump; moving water doesn't freeze as readily as still water. But the cheapest fix is to add antifreeze — the kind made specifically for radiant systems, not automobiles. A 50-50 mix works well down to ?28° F.

This protection does come at a price. For one thing, when the proportion of antifreeze rises above 40 percent, heating efficiency goes down. Also, antifreeze makes water more viscous, so you may need a more powerful circulator pump. And finally, fresh antifreeze has to be added periodically to keep the fluid from becoming too acidic. Neglecting this maintenance task risks corroding any metal in the system, especially the boiler's tank.
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