The proper way to drain a boiler’s expansion tank

boiler

The expansion tank is supposed to have some air in it, but I'm guessing that mine doesn't. Let's suppose (i.e. I'm making this up in the hope that someone will correct me) that the tank should be half full of air and half full of water. If I just open the tank's faucet, some water will drain out (let's say one bucket), but I think I need to ensure that I get a bucket full of air to go back in the tank. I don't want to just decrease the pressure of the air bubble that's in there.

Or, do I keep the faucet open and drain several buckets? Will the water stop coming eventually, or will I just be draining water from the closed water circulation system?

Best Answer

  1. Turn off the valve that auto-fills the system from your cold water line
  2. If you have one, open a valve or bleeder on the top of your expansion tank. If you can't do this, it will still work, the water will just gurgle and drain slowly
  3. Open a drain valve and drain a suitable amount of water
  4. Close drain valve and re-open valve you closed in step 1

Amount of water to leave in tank: If the system is cold, I would leave the tank close to empty, that maximizes expansion space. If it's warm or hot, I'd go for about halfway. You can tell where the water is by feeling the temperature of various spots on the tank and the pipe that feeds the tank.

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