What is the proper way to drain a boiler's expansion tank?

Question

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?

Answer 1

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.

Answer 2

You are correct. The expansion tank is usually about half full of water and half full of air--by volume. One tank full of air at atmospheric pressure will be about half its volume when pressurized. When in doubt, drain it. First, shut the valve between the tank and the boiler, or the rest of the system. Then open the drain valve on the tank. The compressed air will push some water out. If hardly any water comes out, the tank is waterlogged. When the water stops, you're not done. The point is to fill the tank with air. Removing a bucket or two of water does no good. Sometimes, you'll need to hang your bucket on the drain valve and wait for the water to gurgle out little by little. Usually, there's a pipe union between the shutoff valve and the tank. In that case, connect a hose to the drain valve and let as much water out as you can. Then take a pair of wrenches and crack open the union. Air enters the tank through the union and water is siphoned out by the hose. When the tank is full of air, tighten the union, shut the drain valve, open the tank isolation valve and pressurize the system.

Answer 3

Plumbing is often a messy task. It often involves needing to drain a water line. (Can I tell you about the time I needed to drain a sewage line? NOT fun.)

The point is, you may well need to drain the system, so that water does not come out when you open that valve, or at least it is not under pressure. Run a hose to a proper drain, as these drain valves usually have a garden hose connector on them.

If the air bladder on the tank is ruptured, then there is essentially no air left in the tank. I would probably take an air line to the needle valve on the tank to try to purge out some of the water that may remain in the tank. That water will drain out more slowly otherwise, but it should drain out even without help, but more slowly.

Using a screwdriver handle, tap the tank. Does it sound hollow? If not, then you need to get some of the water out, or it will be quite heavy to handle.

Now, remove the tank.

Install the new tank, using either pipe dope or teflon tape thread sealant on the threads. If there is PVC pipe involved and you are using pipe dope, be careful to use a PVC approved dope. Some pipe dopes could cause problems with the PVC overtime.

Re-pressureize the system, bleeding out any air in your lines. This may take some effort.

Clean up your floor, as I always leave behind at least a small puddle of water.