Expansion tank water logged, filling system

Old 01-25-08, 08:24 AM
Thread Starter
Join Date: Jan 2008
Posts: 7
Upvotes: 0
Received 0 Upvotes on 0 Posts
Expansion tank water logged, filling system

Expansion tank is full of water, or at least close to full (sounds solid with water). How do you empty expansion tank without draining the boiler (the tank is above the boiler)? Tank is not bladder type and has drain valve underneath and a valve to turn off the water flowing into the tank. When I try to drain a vacuum forms in the tank and it drains a gulp at a time.

Next, how is the system filled with Expansion Tank feed valve open or closed? How should the expansion tank work? Seems like it gets filled when filling the system due to low pressure readings on the water. gauge.http://forum.doityourself.com/images/smilies/wall.gif
Old 01-25-08, 09:23 AM
Join Date: Dec 2007
Location: A Galaxy From Afar
Posts: 337
Upvotes: 0
Received 0 Upvotes on 0 Posts
To help with the draining try inserting a small piece of pipe or tubing into the drain. If it is a straight shot into the tank and can get the end of the tubing at the top of the tank that should break the vacuum lock. Be sure to have the burner OFF, and the valve to the tank closed.

Once the tank is empty, close the drain valve, open the system fill valve a little (crack it), then the same with the tank valve (crack it). As the system and tank fills the tank valve can be moved to the full open position. You don't want to fully open it at first as this can cause the system pressure to drop too far. Which can then pull air out of the water in the upper radiators. So go slow and keep the pressure changes minimized.

Once the system is back up to pressure the expansion tank should be about half full of water. The other half being air. As the system heats up the water expands and compresses the air in the tank. This prevents the pressure from sky-rocketing.

My system has the same type of expansion tank, and they are a pain. What I did to eliminate the need to drain it is to add a tire fill valve to the drain. Then in the Fall before I bring the boiler on line I add air back into the tank. With the system at room temperature it is supposed to be at 12 psi. In the Fall it never is, a lot of air gets pulled from the tank and bled out of the radiators during the previous heating season.

So I just add air until the pressure is back up to 13 - 14 psi or so. Go around and bleed all of the radiators. Then adjust the system pressure as required (more air for 12 psi, or let a little out if too high).

Of course I did this trick AFTER plumbing in a shut off, and a drain valve and a vacuum break valve.

Old 01-25-08, 09:55 AM
Ed Imeduc's Avatar
Join Date: Aug 2002
Location: Mountain Williams Missouri
Posts: 17,505
Upvotes: 0
Received 1 Upvote on 1 Post

Like said drain and refill tank .

Seems like it gets filled when filling the system due to low pressure readings on the water.
I wonder how often this happens???? Do you have a leak some where or what. Boilers dont use much water once they are filled?????
Old 01-25-08, 03:09 PM
NJT's Avatar
NJT is offline
Join Date: Oct 2006
Location: USA
Posts: 19,710
Upvotes: 0
Received 8 Upvotes on 6 Posts
One thing to keep in mind with that type of tank is that when in operation, it will (and should) be approximately 2/3 full of water !

When you drain the tank and fill it with air at atmospheric pressure, close the drain, and open the tank isolation valve, the boiler system pressure will push water into the tank and compress the air that is trapped in the tank. This is exactly what you want to happen. That compressed 'bubble' of air in the tank is the 'cushion' that the water pushes on when heated, and prevents the pressure from skyrocketing.

When you drain the tank, it helps to use the largest diameter hose you have ... 3/4" works best... and the SHORTEST hose you can. If it's all coiled up on the floor, it makes matters worse.

Sometimes you can simply loosen the hose on the drain, and allow that to suck air... rather than the air having to travel all the way back up the hose to get into the tank.

When it starts to GULP ... close the drain, remove and drain your hose. Put a bucket under the drain, and open it to break the vacuum. (this may or may not work... if it does,
close drain, put (empty) hose back on, and open drain again. Repeat as necessary.

If this trick doesn't work, you can remove your drain hose, and using a short hose (a washing machine hose works good) attach that to the drain. Open the drain and with your HEALTHY lungs, BLOW! back into the hose. Have a bucket handy ! Get your face out of the way quick when you stop blowing. OR, if you have a small air compressor, use that instead of your lungs.

Take a look at your drain valve ... does it say "Drain-O-Tank" on it ? if so, there's a vent on those that you can open to allow the tank to drain fully, quickly.

Why the tank manufacturers never saw fit to place a valve bung up high on the tank, I'll never guess ...

Thread Tools
Search this Thread
Ask a Question
Question Title:
Your question will be posted in: