I have a gas fueled (converted from oil just a few years ago) boiler in my basement with an integrated tank-less water heating system for the showers/taps/etc. The output is three zones as shown in the picture below. Only zones one and two ever get used. Zone 3 maybe twice a winters gets turned on. Not shown in either picture is a drain spigot.

To describe what is happening here: The water supply comes from outside and splits. Half goes directly into the tank-less water heater part of the boiler and the other half move through the PRV and mixes in the air scoop with the return pipe from all three heating zones. There is one pipe and throughout the house the zones all drain into it. This enters the heating part of the boiler. One the left side of the boiler the water moves through a circulator (not shown) and then is split for each of the three zone valves. Each line after the zoning motor has a shutoff valve and a spigot.

Last year (late winter) I noticed my hot water heating pipes sounded like a river was flowing through them and efficiency was reduced. I had a plumber come out and he bled the system and recharged the expansion tank (hadn't been done since I bought the house three years ago). He also replaced the bleed valve on top of the air scoop. Unfortunately the baseboard radiators do not have any bleed valves (project for next summer).

This past weekend I was installing an ecobee and was shutting off the boiler periodically to do some wiring etc but now the pipes sound like a river again.

  1. What could be causing the reintroduction of air into the system?
  2. How do I bleed my system?

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UPDATE: I went down to check the expansion tank and on a lark loosened the bleeder valve cap a little more (it wasn't previously tightened down) and I got a huge spurt of air. Maybe making it a little more loose will help the system.

3 Answers 3


I ended up not having drain or replace anything. Just loosening the shrader valve cap a little let all the air out and the system is working normally again. Thanks for your help.

  • Hey, thanks for providing the solution you ended up using. This sounds so much easier than some of the others, especially since it worked. Be sure you click the check mark, too, so others know that this one has a working solution!
    – FreeMan
    Jul 31, 2020 at 15:35

I suggest you to check pressure of compensation thank on heating system (with system cold at pressure of 0bar, check that the tank has residual pressure of, about 1,2bar). My system was also noisy, i replaced the thank and now is much quiter (my had internal membrane broken so air from inner bladder slowly passed to water). NOTE: not the expansion thank on DHW, but the thank on heating circuit.

  • I am afraid I don't know which tank you are talking about?
    – salisboss
    Oct 22, 2018 at 20:50
  • There is only the one tank. The gray expansion tank on the boiler heating input.
    – salisboss
    Oct 22, 2018 at 21:05
  • I will check the tank though for a leak.
    – salisboss
    Oct 22, 2018 at 21:15
  • If it's connected to heating probably is that tank, leak, if present, is inside, you can only check it's pressure (with heating off and at 0 bar)
    – DDS
    Oct 23, 2018 at 6:07
  • The expansion tank feels good both from a sound and temperature perspective.
    – salisboss
    Oct 25, 2018 at 22:14

This system has a typical layout and is easy to bleed.

  1. Turn off the power to your boiler
  2. Turn off all three return line valves and hook a hose to one of the boiler drains above one of the valves you just turned off and run it outside or into a bucket. If into a bucket, you may have to empty it once or twice for each zone
  3. Manual add water to your boiler using the lever on the automatic water feed until the water pressure is 20-25 PSI. IMPORTANT: Be sure to watch the water PSI the whole time and don't let it get above 25 PSI as the relief valve will open at 30 PSI. If it goes above 25, release the lever.
  4. Once it reaches that range, turn on the boiler drain that the hose is connected to and run it until you start to hear or feel air come out of the line. If the PSI drops below 15, turn off the drain and let the pressure build back up and then turn it back on again.
  5. Run it until the air stops. If there is no air after a reasonable amount of time, turn the valve back on (the one you turned off earlier next to the drain) for a few seconds to bleed the other end and then turn off the valve and the drain and then repeat the process for the next zone.

When you're all done, bring the PSI down to 12 PSI, turn on all the valves you turned on (not the drains) and turn the furnace back on. Keep in mind that the pressure will rise once the water heats back up to normal temperature, if you happen to look at it later.

Generally the zones on higher floors will have more air than lower ones so I'd start there.

  • So just to be sure. Of the steps: 1. Hook hose up to spigot above a zone motor. 2. Turn off boiler and the crank valve above the zone motor. 3. Open spigot and drain two buckets while allowing water in to boiler to keep up pressure. 4. Also hook hose up to drain on the bottom of the boiler and drain that?
    – salisboss
    Oct 26, 2018 at 15:16
  • I just reformatted the directions.The amount of buckets you get can vary depending on how long the zone is. What you can do is wait until the water coming out has a noticeable water temperature change which indicates either boiler water has reached the end or fresh water you just added has. As long as the pressure has remained high the whole time, the air should have been forced out. Normally you'd hear the air or see the hose jump when air comes through. Oct 26, 2018 at 15:28
  • Do not hook up a hose to the bottom of the boiler. You want to hook it up to the drain above each zone valve (the silver boxes labeled 1, 2 and 3). Those drains are called boiler drains. The name does not mean it's at the bottom of the boiler. Oct 26, 2018 at 15:30

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