Q: I am an electrical contractor in the area and absolutely loved the article you wrote recently about how homeowners can put themselves...

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I am an electrical contractor in the area and absolutely loved the article you wrote recently about how homeowners can put themselves at risk by their own ignorant repairs or additions. What about plumbing and heating? Can you do a similar article along the same lines so I can see where I mess up?

Furnaces are rarely installed by homeowners, but peripheral work can hurt furnace operation.

This commonly occurs during a remodel of a basement with a gas or oil furnace. The furnace originally got air for combustion and exhaust from the greater basement and from drafty windows. When the furnace is confined to a corner, walled-in, starving for air, it creates carbon monoxide from incomplete combustion and will not vent that now-dirty exhaust correctly up the chimney.

The same phenomenon can occur with furnaces (or water heaters) in closets with plugged combustion air grilles. Newer homes have this issue in tightly constructed garages, so some municipalities are requiring vents in the sides of garages.

Add a dryer next to a water heater or a furnace, and suddenly you have a powerful fan pulling air in and spewing it outside. Where does that air come from? We hope not from the water heater or furnace exhaust.

Adding an electronic air cleaner or altering other nearby duct work can allow air to leak at the furnace, and this air likely contains furnace exhaust. The cold-air return needs to be sealed tightly, especially near the blower fan.

Plumbing is rich with homeowner-related issues, the most common being the leaking sink drain and trap. Caulking, plumber’s putty, glue, duct tape, baling wire or chewing gum are not necessary to install a leak-free drain.

If the drain is leaking, it is not aligned right or is loose, or the washer is in backward. There are no other reasons. More caulking does not align the pipes, tighten the nut, or reverse the backward washer.

Under this same sink, you could see every variation of trap imaginable. There is really only one correct way.

Put the trap too high, and it will siphon and won’t stop the flow of sewage gas. Put it too low, and it will be prone to plugging. Put the trap arm too far into the drain and it blocks the vent and the drain. Leave it short and it leaks.

Homeowners doing their own supply piping commonly leave out the dielectric fitting between steel and copper plumbing, allowing corrosion to occur more quickly. This same homeowner-installed pipe also might not be supported correctly, might lack an air chamber or water hammer arrester, and might bang when the valve is turned off.

Water heaters require ¾-inch supply and relief pipes — not the ½-inch pipe sitting in the garage. One more time: Use the ¾-inch.

Vent pipes in the attic need to run continuously uphill through the roof. Let them run downhill, and they will fill with rainwater and sag. Then no air gets through at all.

Do you have proper cross-connection protection on that dishwasher drain and yard sprinklers? Freshwater contamina-

tion is possible if you lack these devices on fixtures where fresh and contaminated water converge.

Have you created a funnel anywhere in your plumbing system? Drain pipes need to get larger as they progress downstream. This fundamental rule is sometimes ignored in amateur-installed systems.

With the advent of flexible gas lines, more homeowners are installing their own gas appliances and running the supply lines for them. But no gas line should ever be run through an air duct, plenum, chase or chimney.

And what about that washer hose? How old is it?

Darrell Hay is a local home inspector and manages several rental properties. He answers reader questions. Call 206-464-8514 to record your question, or e-mail dhay@seattletimes.com. Sorry, no personal replies. More columns at www.seattletimes.com/columnists.