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My house has baseboard heating from a natural gas water boiler and I am hearing some rattling when I turn on the heat at night on the 2nd floor. My house has 2 zones and I believe the issue is that air is trapped upstairs in the baseboard.

I inspected all the baseboard units and found that there is no bleeder value anywhere.

I also tried draining air from the boiler (which is in the basement) and that didn't help. Each zone has a drain but it is located in the basement. I did open each zone in a bucket of water and small bubbles came out but that didn't help.

Also I noticed that there is an upside down U of piping hidden behind a false baseboard in a small closet - I am not sure of its purpose - there are no valves there - just pipe and elbows.

My questions:

  • Should I installed a T or bleeder value to remove the air on the 2nd floor?
  • What is the purpose of the upside down U plumbing?
  • Beside air, could anything else be causing the noise?
  • I am no expert but there were traps in the hospital I worked in like your U they had a ball that when air was there would vent the air, I saw the plumber working on them a few times, the ball would get stuck and the pipes on that leg sounded like little jack hammers untill he fixed them. – Ed Beal Nov 13 '15 at 19:36
  • Pictures, particularly of this U, and the start of the loop with the cover off the baseboard. I'm going to bet there's a bleeder here someplace you are overlooking. – Ecnerwal Nov 13 '15 at 22:41
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Beside air, could anything else be causing the noise?

Depends on exactly what the noise is, but I would be on the lookout for thermal expansion. If you have long, straight runs, the pipe can expand significantly when heat is delivered. If there are places where the pipe binds on the supports, it can make pinging and popping noises as it alternately binds and frees itself. If it's attached to a loose metal airflow director, it can rattle.

Because this noise isn't coming from inside the pipe, pulling or shifting the pipe as you turn it on will change the sound significantly, but shouldn't make much of a difference if there is air inside.

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Do you know what you are looking for, valve-wise? A manual bleeder valve on a hot water loop is not usually a particularly large thing - it's normally just a small screw fitting on an elbow. If you are looking for a full-size valve, you'll miss them. If you are only looking for an automatic bleeder, you may also miss them.

a manual bleeder valve

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