Thoughts on draining and recharging hydronic heating system (long-ish)


Old 02-10-12, 11:33 AM
Thread Starter
Join Date: Dec 2010
Location: USA
Posts: 332
Received 0 Votes on 0 Posts
Question Thoughts on draining and recharging hydronic heating system (long-ish)

Disclaimer: I知 not currently changing any components of my system, instead I知 trying to educate myself on how to do it when I need to, so I知 not forced to learn it at the worst moment. Yes I知 abusing collective wisdom of the board again for my own educational gain And I知 thankful to the board for that.

Here is my system (I hope picture is readable):


It痴 a hydronic heating system with gas fired boiler. I have 2 separate zones with zone valves and one recirculator. There is a discharge valve on each zone above zone valves. Cold water is connected to zone returns going to the recirc. Air scoop is integrated in the boiler. Expansion tank is connected directly to the pipe with no valve and doesn稚 have an air scoop on top. No bleed valves in any radiators. Radiators are in series, zones are in parallel.

I want to understand what is involved in draining/recharging the system in case I need to do some repairs like changing recirculator/cartridge or expansion tank.

Please correct/comment on my points below..

Draining system:

0. Turn the boiler off, let it cool. Close cold water intake valve.
1. Open the drain valve at the bottom of the boiler. This will drain as much water as I would need in order to disconnect recirc and/or expansion tank. Depending on how far the air could get into a system it might drain exactly half of each zone (all the way to the highest point on one side of the loop).
2. Opening valve zone will drain the entire system. This might not be needed for tank/recirc replacement, but I think it will help with air purging during recharging of the system (more on it in recharging).

Recharging system:

0. Close the bottom boiler drain.
1. Open 1st zone drain valve above the zone valve (connect a hose to it first).
2. Open cold water intake valve and let the water circulate thru the zone and push majority of the air out of the zone drain valve. When the stream of water out of the zone drain is steady close the zone drain valve
3. Repeat 1st and 2nd for second zone.
4. Equalize the pressure in the system to desired level. At this point there should be very little air left in the system and it will be extracted by air scoop during normal operation.

If the system is not drained completely, I think I would still have to do the same process of recharging and step 2 would effectively drain the system completely anyway. So I see very little value of not draining it completely in first place. Am I missing something? Is it possible to partial drain of the system, leave it with a lot of air, equalize it and restart normal operation hoping that the scoop would remove all the air? I would be afraid to have too much air in the system, starve the heat exchanger of the water and overheat it.

Thoughts for new system:

1. Install ball valve and drain between the system and the expansion tank for painless replacement.
2. Install ball valve right after expansion tank so the system drain will be limited to section between expansion tank and cold water intake valve. This way boiler can be drained and recirc could be serviced/replaced. Recharging should be painless open intake valve, let the air escape thru the scoop, equalize the pressure and you池e done. Correct?

Thank you for reading all this, I hope to get some constructive criticism on this

p.s. no fiberglass on the recirc anymore, it's just an old pic..
Sponsored Links
Old 02-11-12, 08:30 AM
NJT's Avatar
NJT is offline
Join Date: Oct 2006
Location: USA
Posts: 23,539
Received 3 Votes on 2 Posts
Everything you've outlined is basically correct...

Is it possible to partial drain of the system, leave it with a lot of air, equalize it and restart normal operation hoping that the scoop would remove all the air?
Sometimes, but not often. Get enough air into the zones and the pump won't be able to move that air through the system and back to the boiler. Ever try to sink a beach ball in a swimming pool?

Without bleeders, you will have to do a system purge as you've described in order to use city water pressure to move the air out.

Adding full port ball valves to the piping going up to the zones from the boiler supply would allow you to totally isolate the system from the boiler for servicing.

If it's the circulator you are mostly concerned about, they make 'isolation flanges' for mounting pumps that would allow you to change the pump with NO draining of the system.

image courtesy

There are many manufacturers of these... some have swivel flanges (shown) which make alignment a bit easier, some have fixed flanges, solder or threaded, some have drain valves built in, etc...
Old 02-11-12, 01:08 PM
Thread Starter
Join Date: Dec 2010
Location: USA
Posts: 332
Received 0 Votes on 0 Posts
NJ Trooper,

Thanks for your reply. It looks like having a ball valve on the tank and right after it would allow enough flexibility to do possible repairs without any major draining. I will keep this in mind.

Thanks again.
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

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