Keep getting air from a single zone.

Woozle

I have been having trouble with one of the zones in my hydronic baseboard system. I have three zones, and two zones are working fine. The third zone isn't returning hot water though.

It started out with hearing air in the lines when I started up the boiler for the season. I found that the pressure reducing valve wasn't working, so I replaced it. After that, I filled up the system, bled the air from the lines, and I noticed that I wasn't getting hot water to return on two of the three zones, and none were really heating the areas they were supposed to. I bled things again, and still had two zones without hot water returning, so I decided to replace the circulator pump. After doing this, I was now getting hot water returning on two of the three zones, and those zones warmed up to temperature, but the third zone was still not returning hot water. I shut off the two working zones, so that the only path the water could take was through the third zone, and still it wasn't returning any hot water, so I decided to try bleeding that zone again, and I got a ton of air from it. Thinking I may have just not bled it long enough previously, I let it go for a while, and after about an hour, I checked, and the return was again not hot. I bled it again, and got a ton more air from it. At this time, I decided that the automatic air bleeders looked old, and had obviously leaked in the past, so I replaced all six of them. I then bled the system again, and the same results. Two zones working fine, and the third one not returning hot water. Every time I bleed the non-working zone, I get tons of air from it.

Considering that I have 12 PSI in the system, my thinking is that unless I have a hell of a leak in the system, that I should see water from, I shouldn't be getting air in the system at all.

What could be causing this problem that I haven't addressed?

hot_rod

Do you have an air purger installed to take care of remaining air after you purge? It is almost impossible to get 100% of the air out by just purging. More air comes out of solution as you heat the boiler. A central air purger grabs that air to keep it flowing and quiet.

Send a picture of the near boiler piping.

HVACNUT

Is the problem zone going into a floor slab?

Woozle

I don't have the big air scoop that I've seen on some systems. All that exists are the air vents at the ceiling, as seen in the pictures. There was a major water leak over the summer that took out part of the ceiling and hasn't been fixed yet, but you can see all the work I've done, as those are the shiny and clean parts. I replaced the pressure relief valve, pressure reducing valve, circulator, air vents, and added valves to purge the zones, as they weren't there originally. Previous work was to replace the actuators for the zones, which was done two years ago, and is why those are shiny.

As for the zone going through a floor slab, all the pipes are run in the ceiling. In fact, some of the ceiling on the other side of this boiler has fallen down as well, so I can see quite a bit of where things are run there.

hot_rod

Really no reason to have all those high vents, a central air purger and one vent at the highest radiator or highest piping run, maybe.

In fact those auto vents can also allow air into the system. After purging, close the caps on all of them and see if it helps.

If the circulator is pumping into the boiler, I'd have that pex expansion tank line connected to that tee at the reducing ell above the pump. Looks like a manual bleeder there? Then you would be "pumping away" and it may solve some chronic air issues with that one change.

Ideally a good air purger at the boiler outlet , Discal or other style. expansion tank connection moved and all the high auto vents closed.

It's not really about the quantity of air vents on a system.

Top jacket on the boiler is ugly? Leaks from all the air vents above?

Woozle

There is a vent on the outgoing line and on the incoming line for each zone. 6 in total, and that is the highest point in the system. It is all on one floor, and the pipes are all perpendicular to those or lower.

Those are new vents though, and changed a few days ago, after this problem was already rearing its head. I'll tighten the valves down though, and see if that helps. It just seems strange that they were able to get it to work at one time, and it's not working now.

As for that manual bleeder, there is a single 3 foot long baseboard in the bathroom that is fed by that, and it is always open.

I am curious how they bled the system when they set it up. The only place there was a hose bib was on the bottom side of that circulator before it goes into the boiler. It seems to me that that the water would simply go into the boiler and back out to the hose.

Woozle

And the cover on the boiler is a mess for multiple reasons. There was a water leak in the roof that leaked down on the boiler. That's why the sheetrock is falling down by the air vents. It wasn't the prettiest before that, probably from leaks from the air vents at some time. Not sure when. They weren't leaking when I replaced them.

I've had to do lots of stuff to this building since I bought it. This boiler nonsense is just the most recent.

hot_rod

Air removal in hydronics trips up a lot of people. it's possible the system has suffered air problems from day one.

The sheer number of auto vents tell me someone has tried the "shotgun" method of addressing chronic air problems.

Grab a copy of Pumping Away at the book store here. Read and understand it and pass it on to anyone that works on that system.

It could, and should be this simple. Air purger near boiler, a couple well placed purge points.

unclejohn

Is the pex. oxygen barrier? It looks like you have a 4th return coming up from the floor. What is that? Also get that rag off the flue pipe.

Gordy

Pump away, and lock down all those air vents after bleeding. Like @hot rod_7 suggests. You can check them for air now, and then when pump is off, and system is hot, but once burped close them off.

Woozle

Okay, I bled the system and let it run overnight. When I got back this morning, the return for the zone was warmer than usual, but not hot, and when I bled it this time, much less air than normal came out.

As for the pex, I have no idea if it is air barrier or not, and don't know how to tell. It has been there since I bought the place.

The lines coming up from the ground are water lines. This used to be used as a house/business, and at that time, I believe they used those lines for hot and cold water to a washing machine, but they are capped off now.

As for the rag on the vent pipe, that was there because I had taken that picture right after I had installed those air vents. I used the rag to get a better grip on the old ones as I took them out. It's no longer there.

One other problem I just noticed this morning is that in another zone, about half of the baseboard isn't getting warm at all. It worked fine in previous years. I checked the bleed valves on those baseboards, and only water comes out. It's like this entire system has decided to give me hassles.

Woozle

It looks like the aquastat is bad. I decided to test the power to the circulator pump again, and I got nothing. When I disconnected the circulator, it shows 120 volts. Somehow putting a load on the power line is stopping the power, so I ordered a new one, and it should be here in a couple days.

I still have no idea where the air is coming from though, and it really seems strange that I am getting heat from anywhere though.