Why does baseboard heating continually get air-locked?

[AC2010]

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!

[akaothermick]

Automatic vent

Well air is getting in somewhere, if you got the room in the highest point just put in a auto-vent, in place of the key vent, and your done, aside from hearing it hiss once in a while, should be maintenance free.
Mick

[AC2010]

Hi Mick

Thanks for the suggestion. I actually have baseboard heat, not radiators, so there aren't any key-vents. I guess I could think about cutting into the baseboard pipe and inserting some kind of air scoop at that point, but I was hoping there would be another solution.

Best
Andrew

[HayZee518]

expansion tak

has anybody checked out the expansion tank? on top there's a float operated pressure release valve [ red in color] throughout the years the float hangs up giving an opening to the line.
if yours has a schrader air valve, check out the internal bladder.

[akaothermick]

Hi Mick

Thanks for the suggestion. I actually have baseboard heat, not radiators, so there aren't any key-vents. I guess I could think about cutting into the baseboard pipe and inserting some kind of air scoop at that point, but I was hoping there would be another solution.

Best
Andrew

No key vents? every section should have key vent, someone did It cheap, all hydronics, regardless of radiator or baseboard should have vents near the unit, relying on the air separator is not a good practice, non under the covers?

[AC2010]

Nope, no key vents. System was put in about 1968, maybe different practices/codes then? On the other hand, the system worked fine without needing this regular bleeding for the first 35 years or so - this problem started only after the boiler was replaced.

[AC2010]

No, I haven't checked the expansion tank - it looks like a sealed unit (Amtrol Extrol model D60), but I'll get someone with more expertise than me to take a look. Thanks for the suggestion.

[akaothermick]

Nope, no key vents. System was put in about 1968, maybe different practices/codes then? On the other hand, the system worked fine without needing this regular bleeding for the first 35 years or so - this problem started only after the boiler was replaced.

How do you bleed it? the boiler generally is at a low point (basement), air rises to the highest point. You will seldom vent 100% from the lowest opening, is your make up valve in good condition (internal screen clean?), when was system last blown down and flushed and refilled?,