Air-Bound Hot Water Heating System Repair
& Air Bleeder Vent FAQs

Index to Q&A on removing air from hot water heating boilers, pipes, radiators, baseboards or convectors
Air bleeder valve FAQs
Airbound heating system repair FAQs
Air vents on hot water heat FAQs

Boiler & Circulator Runs But Baseboards or Radiators Don't Get Hot

Heating zone circulator runs but doesn't move water

I have a 2 year old boiler using Taco circulators for 2 heat zones and hot water storage tank.

Each of the last 2 years when the thermostat called for heat for the first time after not having run all summer (except for personal hot water), the first floor circulator would run but not pump water.

I bled each (cast iron baseboard and radiator) on the first floor, which immediately spouted water, not any air.

I then went to the boiler, closing off the first floor return, connecting a drain hose and opening the drain petcock while opening the cold water quick fill valve.

Water flowed right away, seemingly without air. There is a Spirovent brass microbubble resorber on the supply line right out of the boiler and just before the first floor circulator.

There is no noticeable leak or corrosion of any of these lines. Any thoughts? On 2019-10-02 by Don

Reply by (mod) -

Don:

A couple of diagnostic questions:

- are there zone valves or just circulator pumps on your system? (a zone valve could be stuck closed)(

- do you ever hear gurguling water or burbling bubbling sounds in hot water heating pipes, radiators, etc?

- how has the problem of no-heat been resolved before?

by Don

There are just circulator pumps and no zone valves. There is some gurgling in the pipe/radiator system but goes away when individual radiators or baseboards are bled.

The 2 times that the circulator was air bound the problem was remedied by purging the system right at the circulator. This seemed to get water into the circulator and hot water would then pump through the system.

by (mod) - recurrent airbound heating system solutions

Because the problem is recurrent, I'd

1. be sure that the air purger at the boiler is working - or replace it; also be sure it's properly located (high on the outlet side above the boiler)

2. install automatic or manual air vents higher in the building- at high end of high runs or rads or baseboards

3. take a close look at all accessible boiler connections and hot water heat piping for corrosion or other signs of leaks out - that might also be air-leaks-in when the system cools.

See AIRBOUND HEAT SYSTEM REPAIR

If it's not an air-bound heat zone but really an individual circulator problem the issue could be a damaged circulator impeller itself (broken blades, mineral clogging) or a broken circulator drive connection between the actual circulator pump motor output shaft and the circulator impeller assembly.

Will air in hot water heat lines increase heating cost

Do you know if there is unwanted air in the forced hot water heating system, would it cause the functioning to become inefficient and therefore cost more to heat the space?

Also, does replacing the air purger cause unwanted air to come into the heating system? Thanks! On 2021-02-11 by BlueSkyNA

by (mod) - possibly yes

Blue Sky

That's an interesting question that I can answer in theory but not quantitatively:

Yes, there are two cases in which air in a hydronic (forced hot water) affect heating cost and efficiency:

1. Worst case:

one or more baseboards or radiators are actually blocked by air so that hot water doesn't circulate in that rad, baseboard, or zone - the thermostat keeps calling for heat and is never satisfied so the circulator and boiler run more than normal.

2. Subtle case:

a small amount of air in a baseboard or radiator may not be enough to block hot water from flowing through the radiating device but nevertheless its presence will both reduce the flow rate (as in effect the internal diameter of hot water heat piping or internal volume of a radiating device is reduced by the air volume) and therefore will at least slightly reduce its heat radiating capacity.

I suspect that this effect is quite small.

About replacing an "air purger" I'd need to know more precisely what part you're replacing.

An air scoop - shown below - is a part whose replacement requires draining the heating system pipes - which introduces air into the system that would have to be removed.

But usually it's possible to replace a simple float vent or air vent (shown below) without introducing air into the system.

Why are Some Heating Zones Still Cold?

Of 2 heating zones, it's cold downstairs, warm upstairs

I have 2 zones for heating. I have two thermostats - one upstairs and one down. The upstairs is nice and warm. Downstairs is freezing and not actually heating when heat is being called for. I have Honeywell TH421D thermostats.

The furnace is in the attic. It has some sort of sensor thing on it and it shows zone one with a blinking amber light and zone two is greensand the heat shows green as well. Is there something i can do myself to get zone 2 to work? On 2018-01-01 by Brenda

by (mod) -

Brenda

Let's start by finding the brand and model of furnace you have in your attic, then find the installation and operation manual for it. There we will find a specific guide to error lights and codes.

For other readers: while many people use the word "furnace" for any sort of heating system, strictly speaking "furnaces" usually refer to forced hot air systems while "boilers" usually refers to hot water or steam heating systems.

Air bound systems and the need to get air out of a heating piping zone to fix cold baseboards or radiators is a problem for boilers and hot water heat, not hot air heat.

But from just your question we don't know if the problem is a bad circulator, zone valve, thermostat, or air in the heating pipes.

So without any more clues I'd start at

• NO HEAT - BOILER

One baseboard zone is hot others are cold

1 baseboard is hot the other four is cold . Check the system and water to the baseboard is coming in hot . The boiler and thermostat work fine On 2018-12-28 by darryl

Reply by (mod) -

Indeed that sounds as if your heating system maybe are bound. Take a look at the suggested procedures in the article

• AIRBOUND HEAT SYSTEM REPAIR

How do I Re-Fill One Heating Zone Loop?

Exactly how do I re-fill one heating zone pipe and radiator loop that I emptied?

I have a 1991 New Yorker coil hot water boiler to cast iron radiators.

I had to empty and repair the second story radiator and empty that loop.

I am going attempt to refill the loop.

Should I keep the inlet valve closed to the boiler on the loop and bleed the air out of the spigot right before it goes back into the boiler and then open it to the boiler when air free?

I understand all I have to do to refill the loop is turn the water back on to the boiler which is now off. The boiler is off too. Is there anything else I should know? I have my bucket and towels ready. Thank you! On 2018-06-06 by Jeff

Reply by (mod) -

Jeff,

I may have misunderstood your situation, bit if you keep the inlet valve closed to the boiler, NO water can enter the system and it will have too-low water pressure in the system, and it won't be able to purge air.

by Jeff

Then is the best way to purge air maybe just out of the radiator bleed valves on the second upstairs loop as I fill it or should I open the boiler drain and/or the loop outlet spigot to purge air as it fills? Thank you.

Reply by (mod) -

That's something to try first but keep in mind that the system needs to be at normal operating pressure and up to full temperature.

How do I isolate and purge just problem piping loop in air bound system

I have three zones: two on te first floor and one on the second. The second floor zone is made up of two loops. One comes up and around the "front" rooms and one for the "back" rooms. The loops leave the boiler as one pipe and then "Y" before traveling vertically to the second floor

They rejoin as one before returning to the boiler. The front loop heats well but the back loop is not heating at all. I think it is air bound. Do I need to drain all three zones to purge the air or can I isolate it and just purge the second floor zone?

Using air bound heat remedy one if the boiler is off how can the valves stay open so the water can forcefully run out. As soon as I shut the boiler off the valves feeding the pipes close. What am I missing in this scenario? (Oct 22, 2014) Alan

Reply:

If the zone valves have a manual lever, latch it in the OPEN position. Else you have to power the zone valves and thermostats.

Air Bound Hot Water Heat Problem Keeps Coming Back

Bleeding my radiators (two zones) about every three or four days but air keeps returning.

I've been bleeding my radiators (two zones) about every three or four days but air keeps returning. With the New England winter fast approaching I want to get this under control before I get an airlock when I am away and my pipes freeze.

My boiler is in my basement, the zones are 1st and 2nd floors. There are two bleed valves above the furnace, just below the ceiling in the boiler room, one for each zone. Both valves have the caps undone on top, loose.

The system has a circulator pump right before the boiler on the return pipe....I read somewhere that this could cause negative pressure in the system and air could be sucked in through the bleed valves or the pressure relief valve. They look like this -...

Should I just replace them or is there something else more likely the problem ? On 2015-11-29 by Will:

by (mod) - Signs that not all air is being removed from an air-bound heating system

Will, see AIRBOUND HEAT SYSTEM REPAIR by WATER FEED VALVE

or for our other method for gettting air of your hot water heating system

see  AIR BOUND HEAT SYSTEM REPAIR by PUMP 

and also see AIR BLEEDER VALVES - how to locate, inspect, use, or replace automatic and manual air bleed valves on hot water heat to fix cold radiators or baseboards caused by air trapped in the heating system.

to read about the type of air bleeder you are describing.

The problem you describe may not be in the air bleed valves but rather I suspect one of the following:

- you have not successfully purged a large volume of air from the heating system and it is making its way slowly in doses to the radiators where you are bleeding it

- there is a water loss out of or air leak into your heating system

It is normal to put the circulator pump on the return side of the boiler, as lower temperatures there may extend the life of the circulator pump and its motor.

The lift capacity of circulator pumps on residential buildings is very modest - the pump circulates rather than lifts, and relies on the boiler pressure to enable the necessary lift or elevation of hot water through higher heating pipes in the building, so suction or negative pressure is not normally a problem on residential hydronic systems.

However there might be an odd case of a system leak (such as a defective air bleed valve) that DOES let air back into the heating piping.

If that's happening, however, I suspect the root problem is the presence of the leak, not the placement of the circulator pump.

The boiler pressure ranges from just under 30 PSI when the boiler is at full operating temperature down to 12 psi when the boiler is cold. That pressure change itself (30-12=18 psi) is more than enough to draw air into the system piping during cool-down cycles.

Start by temporarily tightening the (normally left loose) valve caps on your air bleeders and making sure that no other bleeders are leaking. Keep us posted.

by Will

I closed the two automatic bleed valves and still got air in the system. Whats more, when I opened them up again and ran my heat one of them is now leaking constantly. It drips every few seconds.
I

purged a lot of air, ran the water for a full 5 minutes. It will be ok for a day then will be full of air again.

I don't see any evidence of water loss other than a small damp patch under the pipe that leads to the non return valve. It just kind of hangs down close to the floor and must drip occasionally.

Is there a more effective way to purge the air ?

I shut off the valve just past the purge spigot, connect the hose and run it to bucket over a drain. Then I turn on the outside water supply and open the spigot.

I run the water pressure reducer wide open until I reach 30 psi then shut it off again so as not to trip the pressure release valve. When the pressure drops I open it again until 30 then off again and so on. I keep doing this until I see no bubbles in my bucket that I have the other end of the hose in.

Should I have the purge spigot wide open, partially open maybe ?

If I have an inward air leak how can I possibly find it ?

Thanks

Will

Reply by (mod) -

Yes Will,

see  AIR BOUND HEAT SYSTEM REPAIR by PUMP 

The pump approach can force air out of an air bound heating system when other methods fail.

by Will

Unfortunately my setup doesn't have service drains on the zone valve side, only on the returns where I have been purging air to date.

by (mod) -

Sometimes pushing the temperature and pressure up a bit can help purging air. Just watch you don't dump the TPR valve. Scalding hazards.

by Will

When you say pushing the temperature up a bit do you mean the boiler temperature or the thermostat temperature ?
I pushed the pressure to 20 psi and that hasn't helped.

I've isolated the 1st floor zone (turned off the basement and 2nd floor and closed the shut off valves) to see if the first floor on its own gets air introduced.

Is there a way I can post a picture of my set up ?
In the meantime I will try to describe it.

Cold water in :
Cold water from outside goes straight down into the furnace. There's a valve that is wide open right above the furnace. Just above that valve there is a T that leads to another valve that is closed.

Beyond that closed valve is the "no return" valve then after that is the pressure regulating valve and after that it tees into another large pipe that goes down into the furnace and up to the zone valves.

Between the t into that main pipe and the zone valves is a t off to the expansion tank (brand new).
Question....the valve that is closed (the one between the cold water and the pressure regulator (not the one right above the furnace that is wide open), should that be closed or open ?

Seems that it should be open if that is there to "auto fill" - is that the auto fill ? Should I have an auto fill ? I don't know. If I could post a picture it would be easier to describe.

by (mod) - difficulty getting all air out of an air-bound radiator

The boiler temperature - but don't set above 200F

Then call for heat and keep the boiler running long enough to reach max temp and pressure.

by Will

I've adjusted the HT to 200, should I do something with the LT ?

Reply by (mod) -

Will

See AQUASTAT HI LO DIFF SETTINGS for the best advice I can offer; let me know if questions remain.

 

I keep getting air in my system and have to purge it every couple of weeks

I keep getting air in my system and have to purge it every couple of weeks. I opened my automatic bleeders and nothing comes out, should I screw the caps down tight after purging the air using the shutoff valve and drain spigot method ?
Since nothing comes out of the bleeders should I replace them ?

Also, my furnace is in my basement, the bleeders are on elbows maybe 3-4 feet above the furnace but probably 2 feet below the run of radiators so obviously not the "highest point". Does this matter ?
Thanks On 2016-11-21 by Will

Reply by (mod)

Will

I'm not sure what's going on at your system, but

1. the screw cap on float type air bleeder valves is normally left loose all the time so that the valve can automagically vent air when it grabs some.

2. If you remove the screw cap and press down the schrader valve pin in the air valve and water squirts out, then there's no air at that point to be vented

3. If the air valves are really stuck and not venting air - say from build-up of corrosion, yes certainly I'd replace them.

4. GOod last question. Yeah there are actually air purgers at several high points in a system.

Air can get trapped such that it is grabbed at *local* high points such as right over the boiler.

For example air might be pushed or pulled through a zone pipe into the boiler on the return side, then it bubbles up the boiler output riser and hopefully is vented there.

So you want both: air vents at the boiler on the outlet side and on some systems on the return side too - as well as at other high points in the piping system.

by Will

Thanks for the info

1) OK, the caps are loose

2) I didn't know there was a pin in the valve, I'll check that tonight. However, if I have to push the pin to let the air out how is it let out automatically ?

3) I'm probably going to do this anyway

4) I don't think I can have both since each radiator is fed through the floor (hardwood) and the returns go back under the floor and on to the next radiator.

There are no sections of pipe visible worth mentioning. I will think about adding bleeders in the return side above the furnace though.

My guess is that the bleeders I have are stuck and that the air isn't being vented - where is the air coming from, city water presumably. I have no visible leaks that allow air in (and water out) although in reality it could probably quite easily be leaking unnoticed.

I'll swap the valves and see what happens.

Thanks !

Reply by (mod)

Will:

Air in a float valve pushes an internal float that in turn opens the valve from the inside.

Your radiators may also have air bleeder valves.

by Will

No bleeder valves on my radiators but the 2nd floor bleeder valve was closed, that was the zone that had a lot of air in it last night, after I bled the first floor. (I forgot to check the second)

When I opened the second floor valve and pushed the pin I got water. When I turned the heat on I got heat and no gurgling.......so where did all that air go ? Has the bleeder valve worked whilst I was upstairs waiting for the heat to work it's way upstairs ?
I guess I'll leave it a couple of weeks to see what happens.
Thanks for your help and suggestions !

by Will - How I finally got rid of the air problem in my hot water heating system

Well, I left it a few days and still no air in either zone.

One other thing I did was manually operate the fill valve while the system was running, the pressure is up to 30-35 psi now.

Not enough to trip the relief valve and it seems to be working.

Either that or I finally got all the air out.

Reply by (mod) - safety warning about high boiler pressure

Watch out: a residential hot water heating boiler normally operates at pressures no more than 30 psi, usually a bit under that. Otherwise the pressure-relief valve should be opening and spilling to relieve unsafe pressure or temperature.

Double check or get a more accurate reading of your boiler pressure and be sure it's below 30 psi - or replace the gauge and the relief valve with proper parts. \

The risk is a dangerous boiler explosion.

I bleed air out of my heating pipes but they keep refilling with air!

As I bleed the system it keeps on refills with air? On 2016-11-22 by Anonymous

Reply by mod

Anon:

Very good question. Something's wrong for sure. Either you're not getting all of the air out of the system, or there is an air leak into it.

I'm doubtful about air leaks into hot water heating systems as the system is usually under pressure, but it's possible that during cool-down an opening, such as at as bad circulator mounting pump flange might leak air in as well as other times leaking water out. Look for signs of water leaks around the boiler.

An over-charged bladderless expansion tank might also inject extra air into the heating system. BUt that would find its way out through float type bleeder valves or manual air bleeding.

Most likely: you've got a larger air reserve elsewhere in your heating system - such as at a long horizontal nearly dead-level piping run, and it's taking some time for the circulator to push air through the system to the point where you're trying to bleed it out.

Try the diagnostics recommended at AIR BLEEDER VALVES

Where / how does air get into a hot water heating system?

Where does the air get in , in a hot water system - On 2017-11-29 9 by Anonymous

by (mod) - where to start bleeding air from a cold hot water heating system

Great question, Mary.

The most-efficient way to figure out where to start bleeding air from an air-bound heating system in which some radiators or baseboards won't get hot is as follows:

Starting at the heating boiler, follow the hot water supply pipes leaving the boiler enroute to radiators or baseboards.

Feel along the pipes (don't get burned) moving ever further away from the boiler.

At the first point where you feel pipes are cold that's where there's probably trapped air.

This presumes that

• You turned up all thermostats
• The boiler ran and got hot
• The circulators are running
• The zone valves are open

I haven't seen an authoritative answer to "where to start the air bleed process", but if you cannot follow all of the heating pipes, I'd either

• Start at the lowest cold radiator or baseboard in the building and use its air bleeder valves
• Start at the lowest air bleeder first, figuring that that helps avoid pushing more air upper in the building during the procedure .

Air is most-likely to be at the highest points in the system but don't let theory fool you.

Air can become trapped in longer horizontal piping runs or in lower-floor radiators just as easily as on upper floors.

You will probably find that air does not come out of every manual bleeder.

Watch out: you will have trouble bleeding air from a hot water heating system if you don't first turn up the thermostats to get the boiler and system up to full operating temperature and pressure.

See AIR BLEEDER VALVES for details.

I bled some air out of my boiler system: the temperature doesn't get hot - could the system still be air-bound?

Just removed a bell and gosset pump from heatinging system to replace coupler when i reinstalled it my zone will not heat higher than 63 degrees.

I flushed the line to bleed trapped air still wont go past 63. Ihave an auto vent on top of return line 7 feet above pump.

Could it be bad and still trapping air and not letting temp rise to over 63 but water is circulating because it will go from 58 to 63 but no higher. On 2018-01-23 by jim kaiser

by (mod) - Yes

No if you haven't waited long enough for you boiler to heat up and to circulate hot water entirely through your heating system pipes.

Yes if you are saying that the boiler has run up to its temperature limit, the circualtor pump is running, any zone valves are open, but the radiator or baseboard never gets above a low temperature, then the system could still be air-bound.

Often there is a bolus of air in piping well ahead of or between baseboards or radiators, so you may need to bleed air more than once to get it all.

I feel along the pipes and start bleeding air at the first air bleeder closest to (after) the first cold radiator or baseboard.

Try the procedure at AIR BOUND HEAT SYSTEM REPAIR by PUMP

Airbound Hot Water Heat Repair Mistakes / Warnings

How much draining of the boiler water is needed before opening the water feed bypass

I am not sure - do you drain the system entirely prior to opening the water feeder bypass? (Nov 4, 2012) Alex

Reply: none, you've got it backwards, we need to remove air, not water!

No - none.

Alex, we are not draining water out of the heating system when purging air, we are pushing water in and air out.

You open an air bleeder (high on the system piping or at the high end of a run of pipe that we think is air bound) and allow air out until you begin to see water, then STOP.

If the heating system isn't hot you may need to use a water feeder or a pump to push water through the system to push out the air.

After I purged my baseboard heat I don't get any hot water at my faucets

I purge my baseboard heating system but now I do not get any running hot water from any of my taps , The cold water is fine ( when i say no hot water I mean no water at all when i turn on the hot water handles )

Not sure what to do next. (Jan 5, 2013) Keith C Lenox Mass.

Reply by mod

Keith: purging air from the heating pipes would have no effect whatsoever on the presence or absence of water flow out at your hot water taps. (No hot water pressure).

I am guessing that while working on your system to purge air or feed water into the boiler, someone closed a cold water supply in to your water heater or closed a hot water supply valve out of it.

Daniel F.

After I did some piping work and a lot of water came out we don't get enough heat

I changed an adapter that connected the hot water baseboard pipes together in my lower level the other day an a lot of water came out now that it's back together it won't heat enough to satisfy the thermostat upstairs but it will heat our lower level. Why's that? On 2016-03-10 by Zac taylor

Reply by (mod) -

Zac,

Perhaps you need to add water and bleed air from your heating system.

by Zac

I have what looks like a bleeder on one line an a pressure pull in one pipe best way to explain it that lets water out but no were I see has a spot to connect a hose I'll give it a shot an let you know.

How fast should my heaters warm up once I've bled air out of the system?

I found bleeders on the base boards them selfs I have bleed them till water was steadily coming out now how long will it take for that water in those pipes to heat up? On 2016-03-11 by Zac

by (mod) - in minutes

Zac

If the system is NOT air bound somewhere, you should feel the pipes warming up in a few minutes after the circulator starts - longer for very long piping runs. If not, start at the boiler and follow the piping to find where it's cold. That may be an air-bound section.

What's causing a loud air release at boiler and steam in the boiler room?

So holiday company shows , we run our oil burning boiler more for heat/hot water.

Then after a few days , the upstairs radiator still carry heat but then we get significant loud air valve release in boiler room filling it with steam , and the floor by the boiler relief is wet .

We turn off system, restart. later and still within 1/2 hour we get a another air valve release and a room full of steam . ? what next 4 Jan 2015 ken

Reply: turn off the system and call for repair

Ken what valve is releasing steam:

Watch out: if it's a pressure/temperature relief valve then the system is unsafe and should be shut down for repair.

Uneven Heat vs Air in Heating Pipes

I think my heating zones are not airbound but one room is much hotter than any others

I have gas boiler with water based board heating.

One room is significantly hoter than any other room in the entire house. All base boards have been bled and the boiler was serviced last year. Do I need to get into the crawl space to shut off one of the base boards water supply? Is that even the answer? Thank you. (Oct 6, 2014) Billy

Reply:

Billy

Take a look at the heating system distribution piping.

Sometimes one baseboard loop is much shorter than others, or closer to the boiler, and can be hotter than wanted IF it is on the same thermostat as other heating pipe loops

If that's the case you can have your plumber install a flow balancing valve to slow or restrict the excessive hot water flow to the too-hot loop.

If the too-hot area is on its own thermostat then that's were to go to adjust the temp.

Watch out: on occasion we find a partial air blockage in hot water heating lines can reduce the heat output from that zone's baseboards or radiators without totally blocking it.

So don't rule out a problem with air blockage in the heating system pipes.