Our radiant heat boiler tends to "flush" very loudly every night. It seems to go on for awhile too. We have three zones and I think it just happens with the one zone in our bedroom. From my understanding, this may be caused by low pressure. First, could this be a cause of this flushing or running sound in the radiators? If so, what should the pressure be or does this vary? According to our gauge the pressure is at about 25psi.

I would like to get some idea of what the issue could be before we call in someone to look at it and fix the problem. Thanks.

  • When you say flush, do you mean you hear a water trickling or running sound?
    – Mister Tea
    Nov 29 '16 at 14:57
  • It is a running sound. I was thinking about it last night when I woke up hearing it and we have our thermostat drop during night. It heats back up a little before we wake up, so maybe it happens when the system is firing back up?
    – junta
    Nov 30 '16 at 2:33
  • Can you post a picture of your boiler setup?
    – Mister Tea
    Nov 30 '16 at 20:43

Air trapped in the lines. There is usually an automatic air vent in the system but not always. It should be mounted to an "air scoop" or air separator which may also connect the expansion tank. Though that setup can vary. One name brand air vent is the Hy-Vent. If you don't have one, you should have one installed. If you are handy, install it yourself.

And this could be a sign of bigger problems as air doesn't magically appear in a hot water system. Have you noticed any water on/near the boiler? Your expansion tank diaphragm could have failed and air has leaked into your lines. You can test that with with a tire gauge as the fitting is the same as a car tire and is under a cap on the opposite end of the pipe. CAUTION! Let the system cool down a bit before you do this otherwise you might get sprayed with scalding water.

Another thing to check for is leaks around the baseboards and piping. That could be admitting air into the system.

  • I do have an air separator off of the expansion tank. The expansion tank was replaced two years ago. Would a tank typically fail with in that time period? I do not see any leaks or water on or near the boiler.
    – junta
    Dec 10 '16 at 19:02
  • Is that bedroom zone above the boiler or on the same level of the house?
    – Mister Tea
    Dec 11 '16 at 14:33
  • That zone on the floor above. This only happens with this zone. There is one zone on the same level (in the basement) as the boiler and then there is a mid-level (below the bedroom level, but still above the boiler) that does not have any sounds. So it is only happening in the highest zone. Thanks for the help with this. I never had a boiler before so this is all new.
    – junta
    Dec 12 '16 at 15:44
  • I'm leaning towards air. If you think about it, air floats up in water. So it stands to reason that air from some event has filled the pipe and it's not getting pushed out. Low flow on that circuit can also be a problem as the water is being pushed hard enough to push the air along and catch it in the trap. And each circuit can have its own air valve, sometimes it is on the end of a baseboard or on a pipe somewhere. Have a look around and see if you find one. Should be a cylindrical looking thing screwed into a pipe fitting.
    – Mister Tea
    Dec 12 '16 at 19:46

sounds as if you have air in the water lines. If you have baseboard hot water heat then air is probably the problem. In this type of system air is hard to vent. There should be air vents where the piping turns down.

  • Do you mean where the pipe turns down coming into the baseboards?
    – junta
    Nov 30 '16 at 2:34
  • 1
    Normally the air bleeder is at the highest point in the system.
    – Mister Tea
    Nov 30 '16 at 19:23

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.