How to bleed air out of a heating system (C) Daniel FriedmanAir-bound Hot Water Heating System FAQs
Q&A on finding & fixing air-bound heating systems

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FAQs on fixing cold heating baseboards or radiators caused by air trapped in the hot water heating system.

FAQs helping diagnose & fix trapped air in heating systems causing cold baseboards, convectors or radiators. Air trapped in heating system pipes, baseboards, radiators makes a gurgling noise and ultimately leads to loss of heat.

This article series provides a detailed guide fixing no-heat calls in which the heating boiler and circulator are running but heat is not being delivered to one or more building areas.

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FAQs on Removing Air In Hot Water Heating System Pipes, Radiators, Baseboards

Pony pump used to force water through a boiler (C) Daniel FriedmanThese questions & answers about repairing air-bound hot water heating systems were posted posted originally at AIR-BOUND HEATING SYSTEMS - topic home. Be sure to check that article if you haven't already seen it.

These FAQs help find and remove air trapped in hot water heating system pipes, radiators or baseboards. Trapped air or an air-bound hot water heating system can slow or even prevent hot water movement through the piping and radiation system.

The use of a pony pump to force air out of a hot water heating system (photo) is described at AIR BOUND HEAT SYSTEM REPAIR by PUMP .

[Click to enlarge any image]

On 2017-12-14 by Arthur - how can I cure bubbling gurgling noises at my heating system?

I have been eagerly reading your contents regarding noises in a heating system. If the answer is within, I have not found it yet. Hopefully you can help. We have a gas fired boiler with baseboard heaters. Relatively small house, it's a 3 bedroom cape built around 1920-1940, and the boiler was replaced just before we moved in...2013.

It's a Burnham hydronics series 2, 80.1% efficiency, which is at the very low end of the spectrum. I started by having a plumber drain the system, there are three zones. He flushed each one as I looked on trying to learn how to do it. When he was done, he said there wasn't much air in the pipes at all. And of course, it did not make a difference with the noises we experience.

The three zones appear to be

1. two bedrooms and bath upstairs;

2. front half of first floor, living room and dining room;

3. rear half of first floor, kitchen, bath and rear bedroom/den;

The noises range from loud gushing of water through the pipes! Not just trickling, gurgling or bubbling/burbling. Almost sounds like someone is squeezing a large diaphragm/container of water though the pipes, then a little less noise while the diaphragm/container refills and gets squeezed again...over and over.

Another noise sounds like metal on metal clanking, and it seems to be coming from the boiler room itself...i.e. not in the baseboard radiators throughout the house. I've tried to run downstairs to see if I could see exactly where the noise is coming from, and I just can't seem to isolate it!

I've checked to make sure all the exposed copper pipes running horizontally in the basement are secured and not loosely hung. Other times it’s a loud vibration noise that softly rumbles the house. If I can explain it better by sending or attaching pics, let me know. Thanks in advance.

PS my plumber said he would have to be here to hear the noises to offer any other real help…since we don’t know when the noises will happen, and they don’t happen all day long, it doesn’t make sense to pay him hourly to sit around and wait for the noise to happen…or does it? 

This Q&A were posted originally at ELECTRIC MOTOR NOISE DIAGNOSIS

Reply: a little summary of curing heating system air noises + other noise sources


1. Air noises, bubbling, burbling, gurgling, rushing water:

I infer from your question that there may be air trapped in the system. Such air, essentially a hydronic heating system that is not completely filled with water, can cause sounds like water gushing through pipes, also bubbling and gurgling.

It's also possible that during boiler re-fill, instead of removing previous trapped air, we are left with new, worse air trapped in the system.

Just what sort of noise air makes in a hot water heating system will depend on the orifices through which it is being forced. I speculate that that can create a wide range of sounds, not all of which would be polite to describe.

There may be two other sounds from your system that bear thought

2. Vibration noises, for example transmission of normal vibration from an oil burner motor through oil piping or other metal parts into the building structure or piping.

3. Creaks and clanks can occur where pipes move through walls and floors as they expand and contract during heating system operation.

All of these sounds can be fixed.

I'm not sure why you drained the system in the first place. Saying "there's not much air in the pipes" is like saying "our cat is only a little-bit pregnant".

That's not the best analogy since even if there were only a little air that was observed, there could have been a greater amount of air that the plumber didn't find and thus didn't observe.

In any case, draining (and presumably re-filling) the boiler with water is not a repair for noises. And for hot water heat there are some small disadvantages in doing so (increasing the level of minerals in the system, for example).

That suggests that maybe your plumber was not an expert on hydronic heating and air purging.

She should have

- discussed the sounds and where they were heard

- inspected for the location and operating condition of air bleeders throughout the system, making sure they're properly located and working

- tried opening manually operated air bleeders

- if that didn't work, tried one of the methods we describe in our AIR-BOUND HEATING SYSTEMS article series to force air out of the system.

See HEATING SYSTEM NOISE DIAGNOSIS for our full catalog of heating system noise & sound causes & cures.

On 2017-06-17 by Anonymouswilliam

How can I know if my rotating hot water pump is the correct capacity for my gas furnace?

On 2016-01-03 by (mod) - squeaking noises vs air gurgling noises

The squeaking creaking sounds you describe may be due to thermal expansion and movement of piping through tight openings or along tight wall mounts

I'd try to track that down with a mecanics' stethoscope so that you can see where to give space or better mount piping.

The gurgling is indeed typical of air in the heating system - eventually it can lead to loss of heat. AIR-BOUND HEATING SYSTEMS describes 1 of several approaches to removing air from the system.

All of the key links are given at the end of this page..

On 2016-01-03 by Neil McKenna

We have two zone gas fired hot water baseboard heat and I do not think there are automatic bleeders.

In the upstairs zone, the pipes are making some very loud squeaking noises (like a cracking sound almost sounding like wood squeaking when prying loose from a structure) when the heat comes on. There is also a gurgling sound as well as a sound like water trickling when the water is circulating.

Although the pipes do get warm, they don not get as hot as I've known them to get in the past. I am thinking that there is excess air. There is a bit of gurgling in the first floor pipes but not much and those baseboards do seem to get hot enough. Any suggestions on a cure?

The noise issue has been worsening over the past few years, actually waking us up at times. The circulator is new (<1year) but the noise preceded its replacement.>

On 2015-11-06 by (mod) - circulator pump runs backwards? causes of pressure increase in a boiler.

Could your circulator be running backwards, say after a power surge ir a failed starting capacitor? You think not.

Yes a bad water feeder or could be upping pressure. Turn off water feed and watch pressure.

A leaky tankless coil in the boiler will also push up water pressure to abnormal highs.

On 2015-11-05 by Howard J

Thanks for the reply, I replaced the flow valve and inspected the ball valve they are fine. Still no luck. And now I have no heat (from OWB heater exchange) I have determined that the water is flowing backwards through my system even over powering a pump (plus the pump that brings the hot water from the exchange.

The pumps are in the correct direction. I found my pressure guage, checked the expansion tank and did find it was too high. I lowered it to 12 psi but I tested it several times later and each time the pressure was higher. Could the expansion tank be bad and have too much pressure in it, so much that it reverses the flow?

It doesn't seem likely but at this point I'm lost. Also, I can't seem to keep the boiler pressure at 12. It wants to go to around 20 to 25 psi. Maybe a bad fill valve? and it is sending in too much household pressure in? My fill valve is on the return side just in front of the inlet to the boiler and before the pump manifold.

I thought there might be a physical blockage in the radiator circuit but it seems to be flowing fine. Again, any help would be appreciated and thank you.

On 2015-11-04 by (mod) - correct tests to diagnose cold radiators and an air-bound system

Howard, it sounds as if you are making the correct tests. When you feel hot heating pipes on one side of a control and cold on the other it's a good bet that the control is closed.

On 2015-11-03 by Howard J

Hello, I have a no heat situation with my hot water radiators (cast iron). I have an oil fired/outdoor wood furnace system. boiler is up to temp, pressure is good, it had seemed I had an air bound radiator as I did have over half the radiators working (first 5 were hot last two were not and would not bleed)

a day later no radiators are hot,

BUT now all have been manually bled at each radiator and have no air coming out of them and now have steady water.

I also tried the air bound technique using the water feed valve but to no avail.

Physically checked the TACO 007 and it is running. All my pipes are hot except for the feed to the radiators and the return from the radiators.

There is a green TACO FloChek in the line and it was hot at the valve itself but not after. Then I manually opened it and it started to get hot after playing with it.

There is a ball valve next and the pipe is hot just before it but not after it. It doesn't seem like a ball valve could be bad but maybe?

I also noticed my pressure was hanging around 12 psi earlier but now is at about 28 psi. I have been opening the pressure relief on the boiler but I don't want to open it too much as I know this introduces cold water in and don't want to risk cracking the boiler. The other thing I am not sure about is the expansion tank

. I can't find my pressure gauge to find out where it is at and the valve on top is leaking rusty water. But this is before the flochek valve so there is hot water before and after. Any help would be appreciated. At least it is warmer this week, but we are in NH and that won't last long.

On 2015-01-07 by (mod) -


I'm not sure what you mean by "the head gets warm"

Feel the heating pipes before and after the appropriate circulator pump for the cold zone. If the circulator is running and pipes at the circulator heat up but no heat is delivered into the zone itself I suspect that the system is air-bound, or a check valve is stuck.

On 2015-01-07 by Mike C.

I have a 3 zone hydronic system oil fired. Not getting any heat in basement zone. First and second floors are ok with heat. Thermostat works and have replace the taco power head. The head gets warm. Any suggestions? Thanks.

On 2014-10-31 by (mod) - signs of an airbound hot water heating system

Yes Paul
from your description I think that the circulator is not able to push hot water along the water piping. There could be any of several causes:

air bound piping - the most common problem

a broken circulator impeller - not uncommon

another blockage in the hot water piping - uncommon

something else I have not thought-of

On 2014-10-31 by Paul Wood

Water entering the circulator pump is hot and also hot leaving the pump but only for a short stretch of pipe where it becomes cold could this be air in the system or a faulty pump

Question: no heat after fixing a burst pipe

(Feb 12, 2015) Amy said:

We had a pipe burst and shut off the water valve that sends the water through the pipes while we repaired it.

Now when I went to turn the furnace on again after the repair, I opened the valve and it makes a continuous water running noise and no heat is getting to the baseboards. I don't see any more leaks, and the thermostat is set correctly. We do have hot water heating capabilities, just no heat in the baseboards.

When I shut the furnace back off after I could see I was getting nowhere, I could hear water in the pipes that sounded like it was draining, for lack of a better term. My expansion tank needs replacing but I am unsure if that has any bearing on anything that happened. I am going to call a burner guy in a few days, but in the meantime was wondering if there's anything I can check/do.

This question was posted originally at DIAGNOSE & FIX HEATING PROBLEMS-BOILER



When the boiler has re-filled along with the system piping, radiators, baseboards, and you have no heat, the system may be air-bound.



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