I’ve got a 5-6 yr old water heater (gas) that has developed a leak at the drain valve. It’s a plastic hand tighten valve with hose threads. How big of a job is it to replace this? I’ve changed out a gas heater before, but I’ve never fooled with the guts.
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It's not difficult at all, unless the old valve breaks off while you are changing it.
A good hardware store or plumbing supply house can steer you to the right valve to buy.
Shut off the gas, and the hot and cold water valves to the heater. Open the drain valve and drain the heater all the way down. (The water will be hot. You can wait a few hours first if you wish.) When completely drained, unscrew the valve. Brush the dirt from the threads on the tank with an old toothbrush, and blow off any loose material. Apply pipe dope to the threads of the new valve and screw it on. Snug fairly tightly with a wrench. Shut the valve.
Turn on the cold water supply. Turn on the hot water outlet, and a hot water faucet nearby so the air in the tank has somewhere to go. It will snort all over until the air is out. When all the air is out, check for leaks. If ok, turn on the gas and relight the heater.
If it's the typical "coaxial" drain valve (the outlet is in the middle of the valve handle), to remove it you first unscrew it all the way, then, while pulling on the valve, you turn it the opposite direction. This will leave a metal stub that you wrench out.
Replace it with a "boiler cock". Depending on the thicknesss of the insulation you will need one with either a long stem or an extra-long stem. The boiler cocks are available in either plastic or brass. Plastic's a heck of a lot cheaper and probably will work just as well.
Yep, that's it - thanks guys.
For a nice upgrade, get a boiler drain with a ball valve. Much less likely to leak, and more likely to turm when you need it.