I have a baseboard heating system with an old Weil-McLain PC-G 6 Series 4 gas boiler. Recently had the pipes bled and soon after noticed a load banging sound coming from the boiler that echoed through the pipes. It was most noticeable when the system fired up from a period on prolonged inactivity.

After checking for various causes on the web, self diagnosed that it might still be air in the system. Since I no longer heard the sound of rushing water in the pipes I assumed they were properly bled. Saw a posting about opening an air vent to release air in a heating system and decided to give that a try. I unscrewed the cap of the "tire type" air vent on top of the furnace and depressed the valve until water started seeping/dripping out.

The air vent is an American Tube and Connector Series 700 (appears older) and the label states that for proper functioning, the cap must be unscrewed at least two times. I followed the advice, but water continues to drip out slowly from the valve. There was some scale and mineral deposits on the top of the vent, so it looks like it had leaked in the past.

Am I OK just tightening the valve cap all the way and leaving it "closed" completely to stop the leak? The banging noise appears to have been quieted, so I'm hoping I don't need to replace the air vent just yet.

 

By closing the cap you are defeating the purpose of the auto vent which is to vent air automaticaly. You will do no harm by closing the cap, esentially making the auto vent a manual vent.

 

Yes, you can leave that valve closed ... obviously it won't be able to let any air out, but it won't be leaking!

You can occasionally open the cap manually to let any air out ... if you hear any more banging... then, in the spring, put a new one on.

What does the temp and pressure gauge on the boiler read when the system is cold ? and hot ?

 

Thanks for the advice. Turns out the banging is still happening, only not as loud. The temp when fired up is around 180 F with 18-20 PSI and cold is 120 F with15-16 PSI.

 

Describe the 'banging' ... does it move around the system ? or can you localize where it's coming from ? i.e. always the same place...

 

The banging noise is coming from the boiler itself and resonating throughout the pipes. It doesn't move around, but is really hard to pinpoint, but definitely from the boiler. If I had to guess it would be right below the pipe where the heated water exits the boiler. Kind of sounds like a hammer hitting the pipes when I am in other areas of the house. Anywhere from three to 10 "bangs" before it stops. Usually is worse when the unit first fires up, but happens throughout the day. It started just after I had the system bled for heating season and is loud enough to be annoying, especially when it starts up in the AM.

After this last service, the pipes are the quietest they've ever been so this new noise is really bothersome. From what I researched it sounded like it could be either air still in the system or scale/mineral deposits in the boiler. I tried checking the air vent, of course after I opened it it leaks. I'll give my service guy a call to come out, but was hoping I could fix it myself.

 

Just curious, is this a cold start boiler or does the aquastat equipped with a low limit? If it has a low limit, try raising the low side to 150 or 160 (Still keep the high side at 180) and see if the banging noise goes away.