Go Back   DIY Chatroom Home Improvement Forum > Home Improvement > HVAC

CLICK HERE AND JOIN OUR COMMUNITY TODAY...IT'S FREE!

Reply
 
Thread Tools Search this Thread Display Modes
Old 11-17-2012, 05:37 PM   #1
Rav
Member
 
Join Date: Dec 2011
Location: Bethesda, MD
Posts: 237
Rewards Points: 154
Question

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:

Question for forum member "how" on banging noise

Rav is offline   Reply With Quote
Old 11-17-2012, 05:44 PM   #2
Master General ReEngineer
 
Bondo's Avatar
 
Join Date: Dec 2007
Location: Chaumont River, Ny.
Posts: 3,727
Rewards Points: 2,088
Default

Question re draining heating expansion tank


Quote:
Originally Posted by Rav View Post
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:

Question for forum member "how" on banging noise
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....

Bondo is offline   Reply With Quote
Old 11-17-2012, 05:52 PM   #3
Rav
Member
 
Join Date: Dec 2011
Location: Bethesda, MD
Posts: 237
Rewards Points: 154
Default

Question re draining heating expansion tank


Quote:
Originally Posted by Bondo View Post
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.
Rav is offline   Reply With Quote
Old 11-17-2012, 06:24 PM   #4
how
Member
 
Join Date: Feb 2011
Location: Vancouver BC
Posts: 1,531
Rewards Points: 578
Default

Question re draining heating expansion tank


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.

Last edited by how; 11-17-2012 at 06:26 PM.
how is offline   Reply With Quote
The Following User Says Thank You to how For This Useful Post:
Rav (11-19-2012)
Old 11-19-2012, 10:53 AM   #5
Member
 
Join Date: Jul 2012
Location: Toronto ON
Posts: 599
Rewards Points: 500
Default

Question re draining heating expansion tank


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
techpappy is offline   Reply With Quote
The Following User Says Thank You to techpappy For This Useful Post:
Rav (11-19-2012)
Reply

Tags
draining, expansion, heating, tank


Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Tankless water heater need expansion tank? charliebarkin Plumbing 3 06-04-2012 06:21 PM
questions about plumbing hot water tank bubbler Plumbing 13 01-18-2012 07:43 AM
Water softener tank not draining BigPat Plumbing 7 02-08-2011 12:41 PM
Pressure relief leaking and expansion tank full themow HVAC 1 11-25-2009 05:49 PM
Trane XE80 Heating Question, Please help!!! Stephen1977 HVAC 3 01-30-2007 07:34 PM




Top of Page | View New Posts

Copyright © 2003-2014 Escalate Media. All Rights Reserved.