My heater stops pushing hot water through the heating loop. The symptoms can be addressed by bleeding the system which always produces a lot of air coming out. The heating works for about 24 hours until air block is developed again. The heater pushes water from a basement to 3rd floor. The pressure shows about 24psi.

So far I've done:

  • replaced air vent assembly and installed two additional small Caleffi air vent on radiators upstairs.
  • replaced old 007-F5 circulation pump with a new one.

without any sign of improvement. I wonder if anyone could point me to the right direction.

  • @hellomoto They're referring to a forced hot water boiler, not drinking water. Nov 17, 2018 at 20:45
  • How are you bleeding out the zones? Are they split zones? Nov 17, 2018 at 20:46
  • m.youtube.com/watch?v=wG-CqFr6FLM
    – user70085
    Nov 17, 2018 at 20:53
  • Shut off valve above pump, shut off valve out of boiler, open drain above and open pressure regulator breaker, usually it takes about 10 min to get the bubbles out. My system has only one zone.
    – bioffe
    Nov 18, 2018 at 5:52
  • 1
    Being a (nearly) two year old question, you could tell us how you resolved it, as you presumably have by now.
    – Ecnerwal
    Sep 3, 2020 at 11:15

2 Answers 2


This is, in my experience, nearly always an indication of leaks in the system, so make-up water is being added by the boiler feeder (adding new dissolved air) or, if the boiler feeder is shut off, air gets in after water leaks out. In a system without leaks, once the dissolved air introduced with new water is removed there should be no further need to bleed it.

Apart from the inconvenience of having it stop working, the constant addition of new water with dissolved air also violates the assumption that when the system is sealed there's no more oxygen added to promote rusting of the iron components once it's either bled out or rusted a little.


Your description of how you beld out the zone wasn't 100% clear so make sure you follow the steps below for a standard zone with no splits anywhere, including at the radiators:

  1. Turn off the power to your boiler
  2. Turn off the return line valve and hook a hose to boiler drain above the valve you just turned off and run it outside or into a bucket. If into a bucket, you may have to empty it a couple of times . IMPORTANT: This is the drain on the zone pipe and not on the boiler.
  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 between the boiler drain and boiler and then release the automatic water feed lever and turn off the drain.

When you're all done, bring the PSI down to 12 PSI (open the drain valve to release pressure if needed) 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.

Keep in mind that the boiler value you're dealing with is the one on the zone pipe and not on the boiler.


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