Air in heating system: driving me nuts


Old 10-16-16, 12:37 PM
Thread Starter
Join Date: Oct 2016
Location: USA
Posts: 3
Received 0 Votes on 0 Posts
Air in heating system: driving me nuts

Towards the end of last year's heating season I began having a problem with air in the system. When it got to the point that it was sounding like living on a lake, I bled each of the 5 zones and was ok for a little while. I made a few quick attempts at figuring out the problem but then the heating season was over so let it go until this year. Now I've been spending all kinds of time trying to figure out the issue and have gotten nowhere.

The system is a 30-yr-old Weil McLain with 3 baseboard zones, one radiant zone and one zone that creates domestic hotwater with a Superstor. Equipment all seems fine, no obvious problems with valves,expansion tank, boiler itself. The radiant zone is newish but existed well before the problems started. (As I'm writing this from my basement office I can hear bubbling already, like I'm below the water line on the Titanic).

On one of the baseboard zones there's a section that runs under a slab. Thinking perhaps I had a leak under the slab I closed that portion of the circuit with a pair of valves someone had wisely installed. The zone continues with a bypass section that had also been installed way back. So I've ruled that out and actually haven't opened that section again.

The next thing I did was turn off the boiler, pressurized to 30psi, and turned off both sides of the boiler itself to check for a leak. There's no obvious leak but I thought that if it were slow enough it could be drying fast from the heat. Anyway, the boiler itself is fine as it held the 30psi for 2 or 3 days.

Next on the list was to isolate the radiant system. Did so by turning off the supply and return to that zone, bled the system, and turned the system back on. Within a couple of hours there was enough air that I was hearing the gurgling. So that zone is now just turned off and will leave it that way until I fix this or when it gets really cold.

I'm now checking the Superstor, having closed off the valves for the boiler circuit AND closed the valves for the potable water. Years ago I had a problem whereby the city water pressure was high enough to, through a breach in the Superstor, raise the boiler pressure so it was constantly spewing water from the pressure valve. The Superstor was replaced, pressure reducing valve fixed all was good for at least 18 years. Prior to that fix boiler water was mixing with potable through a tiny leak that developed on the inside of the Superstor. Luckily the potable was going into the boiler rather than the opposite. That experience is what led me to cut off the potable water through the tank. This isn't the problem - after 3 hours of running the system once again has air.

So now this leaves only the 3 zones that feed the various baseboards through the house. These 3 are, theoretically, closed loops, so I'm wondering where all of this water is going. My pressure gauge remains at a steady 14psi regardless of whether it's full of air or full of water, so the water has to be coming out of the system and being replaced by air. Logically there should be one heck of a flood somewhere by now, considering that I've bled and replaced water at least 6 times in the past 2 days. There are no valves, other than the Honeywell zone valves, on the system so I can't isolate them individually. I think I neglected to mention that the zone that develops the most air is the least-used zone feeding the living room and dining room, but air eventually finds its way into all of the zones. There's a full basement under that zone, actually the boiler is directly under that zone, but there's no water in either of those rooms or the basement. I've also replaced the air-eliminator that's on the boiler itself.

I'm out of ideas. My brother-in-law the plumber, who hasn't had an idea in 30 years, wants to add an air-eliminator (already have one that was replaced when this all started) but it seems to me that's only fixing the symptom and not identifying or fixing the problem.

Apologies for the length of this post but it was the only way to give a clear picture of the problem.

So, anyone have any ideas? I appreciate any and all

Thank you,

Sponsored Links
Old 10-16-16, 06:10 PM
rbeck's Avatar
Join Date: Jul 2007
Location: Pennsylvania
Posts: 2,449
Received 1 Vote on 1 Post
do you have 18" of straight pipe before your air separator if the cast iron ype and are you pumping away from the expansion tank connection?
Maybe post some pic's of piping.
Old 10-17-16, 07:46 AM
Thread Starter
Join Date: Oct 2016
Location: USA
Posts: 3
Received 0 Votes on 0 Posts
Thanks for your response. No, the separator is on the boiler itself. However, the system worked fine for 30 years as it is, so nothing really changed. The separator was changed about 2 years ago. I did however replace the expansion tank, but that was at least 6 months prior to a problem developing. Yes I'm pumping away from the expansion tank. ?

Old 10-17-16, 11:14 AM
Join Date: Oct 2008
Location: USA
Posts: 2,876
Received 5 Votes on 5 Posts
What is the boiler pressure, with the pump running, the pump not running, and just as the pump starts? The mystery is how the air is getting into the pressurized system.
Old 10-17-16, 04:56 PM
Join Date: Oct 2013
Location: United States
Posts: 2,698
Received 11 Votes on 11 Posts
I like your attitude about your brother-in-law. With the last idea he came up with you might be better off to wait another 30 yrs for his pearls of wisdom.
You might try to pressurize the system to about 25 PSI and then shut the main cold water valve off to the boiler. If you have a leak it will show in your boiler pressure. If your boiler pressure holds it sounds like you may be back to bleeding if you can go through your procedure to see how you're bleeding the air out. There is a certain procedure so the air doesn't get back in. You mentioned you have no valves except the zone valves. How are you isolating each zone to bleed. I would try that to start before I give you a bunch of useless info. Pics would be helpful.
Old 10-19-16, 08:56 AM
Thread Starter
Join Date: Oct 2016
Location: USA
Posts: 3
Received 0 Votes on 0 Posts
yeah, he's a gem. He's got a new boiler sitting in his shop, waiting for me to give the go-ahead, but there isn't a lick of evidence that there's anything wrong with the one I have except that it's 30 years old.

I had already done as you suggested, pressurizing the boiler and then turning off the valve. There was no drop in pressure.

As for bleeding, the three zones have a drain before the zone valve on the return side. So I close off the return manifold going into the boiler, open the drain on the zone I want to bleed, and allow the water to run until there's no air coming out. My pressure-reducing valve allows me to bypass the 15lb pressure it's set for, so while the drain is open I allow full pressure to run through the circuit. The other 2 zones connected to that manifold have the zone valves closed so the incoming water wouldn't be running through those zones. I've done each one sequentially in every possible combination (do top floor first then the others, do rear 1st floor first, front 1st floor first, then second floor, etc. ). If there's air in the system, and the system is full when I turn it on, that would mean that water is being displaced somewhere.
Old 10-20-16, 10:34 AM
Join Date: Apr 2014
Location: us
Posts: 603
Received 4 Votes on 4 Posts
My 60 year old high efficiency Weil McLain system with 6 zones has auto vent valves at the high point of each loop. Air is never a problem.

I look at each zone/loop as an air trap. Air in systems is a given so just deal with it. Found years back that air scoops and auto vents vales at top of boiler did not solve problem.

Just buy good quality vent valves, cheap ones cease to function after a few years.

See: PEX - Radiant Heat - Radiant Heating - Plumbing Supplies -

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
Display Modes
Ask a Question
Question Title:
Your question will be posted in: