I have a baseboard heating system which continually gets air-locked and has to be bled. Why would this be happening?

I have a two-story single family home with gas-fired boiler in the basement serving both baseboard heating and domestic hot-water.

Baseboard system has two zones (upstairs and downstairs) and four isolatable flows (up and down, front and back). The upstairs front part of the system becomes air-locked and has to be bled on a regular basis, was every year, now it’s more like every two or three months. The bleeding itself is not too much of a problem, since that section can be isolated and the air removed – typically takes two full buckets of bleeding, then no more air coming out, baseboard is toasty hot and all is well for a couple of months.

So why does this keep happening? The system appears to be well sealed with no leaks or anything that could be a source of air other than the regular mains intake. I understand that fresh water coming into the system will contain some dissolved air which will come out when heated, but why would that happen months after the fresh water is introduced rather than within a few days? The system has an air-scoop that should be venting any such air anyway, that has been replaced, and the problem existed both before and afterwards.

I’m stuck at this point! Any ideas at all would be appreciated!

Hi AC

Does the automatic vent on top of the air scoop work...is the cap loose enough so air can escape? If the auto. vent doesn't work then it can't do its job, right? Check that to start...post back.

Mark

Hi Mark

Yes, the air scoop seems to vent okay. Plus, it was replaced a year ago, to try to solve the problem, and the change didn't seem to make any difference.

Andrew

Andrew,
What you are experiencing may be due to the location of the circulating pump. Circulators that are located on the return piping, pumping towards the expansion tank, can create a pressure drop that will cause some of the automatic air vents to suck air into the system. The boiler feed valve location is also important; it should be connected where the expansion tank is tied in - this is the point of no pressure change ( ponpc ).. .

I first learned of the ponpc in the late 1980's, and have been installing the circulating pump on the supply side of the system, pumping away from the expansion tank ever since. With the circulator in this location, pressure will be added when the pump starts, which gives the fluid a much better chance of overcoming the friction ( head ) caused by elevation and length of piping runs. It is typical for the upper portion of piping systems to have the least amount of flow, and the most potential for air binding...

Thanks for your answer Mike, it raises some good thoughts. From what I can see, the flow is:
This seems okay from what I understand you're saying (circulation pump IS on the return piping, but it's not going toward the expansion tank because that's on the outbound side from the boiler).

But it sounds like you'd expect the expansion tank to be on the return side, which it's not. Is that an issue? This problem didn't exist with the original plumbing of the baseboards, before the boiler was replaced a few years ago, so it would not surprise me if the issue had to do with the way the boiler is located relative to everything else.

Andrew,
Your circulator is on the return, pumping torwards the expansion tank through the boiler; this is the least desirable location. The circulator should be installed on the supply pipe, right after the air scoop. The water feed or makeup valve should be connected between the air scoop and expansion tank - you can reference the boiler installation manual, or consult with the boiler manufacturer, ALL boiler makers understand the advantages of " pumping away '', and purposely include piping diagrams in their install instructions that demonstrate ideal near boiler piping strategies...

Hi Andrew

Mike45 presents some very useful information, but be clear that the information he provides is relatively new and has taken awhile to catch on, but yeah, most boiler manufacturers nowadays recommend putting the circulator on the feed as pumps like to push more than they like to pull (oversimplified on purpose)!

However, with that being said, I don't know if I would go repiping your system over this as there are multi. MILLIONS of boilers that still have the pump on the returns and they are not having the problem you are having...;)

As Mike said, consult your owners manual and see what the recommended set up is...always interesting to see if things got piped correctly. I do think piping the water feed into the air scoop (with shut offs isolating the pressure reducing valve and the backflow preventer and an additional shutoff isolating the expansion tank (all for future swap out should the part(s) need to be replaced)) may be helpful here.

Finally, do you have shut off valves with drains on all returns? If not, that would be helpful here as it would allow you to "power purge" the main lines and then you can purge each radiator/baseboard individually after that. This is an ideal setup (regardless if circulator on feed or return). Heck, if you go this far in re-piping and the manufacturer would allow it, moving the circulator to the feed isn't really that much more work...;)

Mark

Andrew,
The old style, two cavity air scoops work better when they are located at least 12" from the transitional elbow - 18" is optimal. Continuous infiltrating O2 will cause greater problems than the nuisance of poor, noisy flow and low heat out put. Oxygen is the arch enemy of all ferrous components found in most closed loop heating systems, and, gone unchecked, corrosion and premature failure can be expected...

Andrew,
If you are operating your system with plain water, try adding a couple of shot glasses of liquid dish soap ( Dawn ) to your system during your next purging. Somehow the soap alters the surface tension of water, which reduces flow friction and the size of the air bubbles; often resulting in a smoother, quieter flow...