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Q: There's a small puddle of water near my boiler. Fortunately, it seems to be coming not from the boiler but from an open-ended vertical pipe, like you might see on a water heater. How can I fix it?

—Terence McCafferty, Manchester, N.H.

A: Sounds like you need to check the backflow preventer. This device, which I'm installing here, connects the water-supply pipe to the boiler and makes sure that water flows in one direction only—toward the boiler—whenever the heating system needs to be topped up with water.

A preventer can provide years of service without any problem, but if either of the two check valves inside the device becomes clogged with debris or a mineral buildup, it spits water into an open-ended vertical pipe. I suspect that's where your puddle comes from.

Although it's not an emergency, a clogged backflow preventer is not something to be ignored. Here's why: Normally a preventer just sits there, not doing a thing, until the system needs more water. Then, pressure on the supply side moves the water toward the boiler. But if that supply pressure ever drops for some reason—when the fire department opens a nearby hydrant, for example—the valves inside the preventer stop the water in the heating system from being sucked into, and contaminating, the supply pipes. That's why plumbing codes require a backflow preventer on hydronic heating systems, outdoor hose bibbs, and sprinkler systems.

To remove a malfunctioning preventer, shut off the water-supply valve and purge the heating system so that its water level is below the preventer. If it's connected to the supply pipe with a threaded fitting, use a pair of wrenches to back off the union nuts on both ends of the preventer. Then simply screw a new preventer of the same make onto the supply pipe. But if your unit has no threaded fittings, grab a hacksaw, cut off the old unit, and solder a new one onto the same spot.