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-   -   Question re draining heating expansion tank (http://www.diychatroom.com/f17/question-re-draining-heating-expansion-tank-163716/)

Rav 11-17-2012 05:37 PM

Question re draining heating expansion tank
 
I have a natural gas hot water boiler (Weil-McLain series 8 CGM-5) and radiator system. The expansion tank is one of those older large metal tanks hanging from the ceiling near the boiler. As I do each year I drained a few gallons from the expansion tank today, closing the water inlet valve to the tank before draining.

There is no air inlet to the tank, only the water line. So, my question is, when I drain the tank, doesn't that create a negative air pressure in the tank, so that once I close the drain valve and re-open the water inlet valve, wouldn't the tank just fill up again in order to relieve the negative air pressure from draining? Otherwise, where does the air come from to keep enough of a void in the tank? I tried opening the drain valve just a little to allow air to suck back in, but only water came out. How is draining accomplishing the desired results? Thanks.

PS: If you want to see pictures of my system (including one of the expansion tank), see:

http://www.diychatroom.com/f17/quest...-noise-134214/

Bondo 11-17-2012 05:44 PM

Quote:

Originally Posted by Rav (Post 1054523)
I have a natural gas hot water boiler (Weil-McLain series 8 CGM-5) and radiator system. The expansion tank is one of those older large metal tanks hanging from the ceiling near the boiler. As I do each year I drained a few gallons from the expansion tank today, closing the water inlet valve to the tank before draining.

There is no air inlet to the tank, only the water line. So, my question is, when I drain the tank, doesn't that create a negative air pressure in the tank, so that once I close the drain valve and re-open the water inlet valve, wouldn't the tank just fill up again in order to relieve the negative air pressure from draining? Otherwise, where does the air come from to keep enough of a void in the tank? I tried opening the drain valve just a little to allow air to suck back in, but only water came out. How is draining accomplishing the desired results? Thanks.

PS: If you want to see pictures of my system (including one of the expansion tank), see:

http://www.diychatroom.com/f17/quest...-noise-134214/

Ayuh,.... After you've drained the tank, 'n closed the drain plug, the tank is empty, but full of Air....
As the water enters, the water compresses the air...
Most systems run 'bout 12 psi, so there's plenty of head-space....

Rav 11-17-2012 05:52 PM

Quote:

Originally Posted by Bondo (Post 1054534)
Ayuh,.... After you've drained the tank, 'n closed the drain plug, the tank is empty, but full of Air....
As the water enters, the water compresses the air...
Most systems run 'bout 12 psi, so there's plenty of head-space....

Thanks for the reply, Bondo. What I mean is: Before draining, the tank contained water and compressed air. After draining, the tank still contains air but it's less compressed (what I called "negative pressure") since much of the water is now gone. Since the air is less compressed, once I open the inlet valve, won't just as much water re-enter the tank and re-compress the air, as I had drained out of it? Or why not? Obviously there must be something I don't understand, but I'm trying to. Thanks.

how 11-17-2012 06:24 PM

There is a good chance that you didn't need to refresh the old exp tank unless your P&T was venting or the tridicator looked close to 30PSI everytime the boiler went from cold to hot
But
It didn't hurt anything and beyond tapping the side of the tank to confirm where the water/air level is, you now know it's set up right.

If you shut off the boiler feed to the expansion tank, open and drain it, close off the drain valve and open up the boiler feed to it, you've done the best you could do for it.

techpappy 11-19-2012 10:53 AM

you must drain expansion tank completely...this allows the tank to fill with air..the air is compressed after you close the drain valve and open valve to system. sometimes the air will leak out from fittings at tank thereby losing the air OR sometimes a pin hole which usually will drip water..make sure you tighten all fittings...if tank/fittings very old and rusted..recommend to remove and replace with bladder type EX Tank...can be floor mounted


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