Water leak at an air bleed purge valve on a heating system (C) Daniel FriedmanHot Water Radiator Air Vent Leak Repair Guide
How to fix or replace leaky hot water heating system air bleeders & vents

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Leaky hot water heating system vent repairs - how to repair a leaky hot water air vent or bleeder valve: here we explain how to repair or replace both manual heating system air bleed valves and automatic float type air vents that are leaky. Float type automatic vents on hot water heating systems are normally left "open" to automatically purge air from the system.

But a float vent that leaks water will stop working, may leak onto and damage equipment, and is often left "closed" or shut off as a temporary measure. And a manual air vent that is leaky or that no longer vents due to corrosion and clogging is not going to do the job when your heating system is airbound and radiators or baseboards are cold.

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Air Bleed Valve Leak Repair: How to Inspect and fix or replace leaky float-type automatic or manual air bleeder valves (air vents):

Air bleeder valve corrosion (C) Daniel Friedman

Is there a little cap on top of the air bleeder? many air bleeders use an internal float and an air valve stem that is about the same as the valve stem of an automobile tire valve or bicycle tire valve. The cap over these valves looks just like the cap on the valve stem on a tire (it is).

But on an air bleeder the valve cap is normally left loose so that when the valve has accumulated enough air to move the internal float the valve can open to expel the air.

If the valve cover is screwed down tightly, or if the valve cover has become clogged with mineral debris left by leaking water, air cannot be released. Loosen the valve cap.

If air escapes when you loosen the valve cap on the air bleed valve, that's good.

If the valve cap on your air bleeder is badly corroded (photo above left) the valve probably needs to be replaced.

Water leak at an air bleed purge valve on a heating system (C) Daniel FriedmanIf the automatic air purge valve has become corroded and/or leaky: the air bleeder valve is of the automatic float and canister type rather than manual type; among these.

Automatic air bleeders, to work, require that their little screw cap (it looks like and is a tire valve stem cap) be left loose so that the float inside the automatic air purger/bleeder can release air when needed.

But on some of these devices the float eventually sticks or becomes waterlogged and the automatic air bleeder dribbles and leaks, as you can see in our photos above.

Usually automatic float type air bleeder valves are installed at the boiler where it's easy to see and replace leaky ones during boiler service.

I don't recommend using float type automatic air bleeders at heating baseboards inside the building where they may be hidden by baseboard covers and can leak into and damage the building for some time before anyone notices.

Additional automatic air bleeder valves may be installed at other points on the hot water piping, usually at a higher spot near the boiler and sometimes on upper floors in the building.

Look for and check the operation of these air bleeder valves too.

How to Protect the Building from Water Damage from a Leaky Air Purge Vent

Taco's Hy-Vent waste connector fitting protects the building from leak damage - Taco Inc.If water starts to leak out of the air bleeder valve continuously (photo at left) (it's ok for a drop or so to be expelled if the leak stops quickly) then screw the cap down tight again and ask your heating service technician to replace the valve.

Taco® provides a neat little attachment, the Taco Hy-Vent waste connector (sketch at left, adapted from Taco's product literature), that connects to the company's Hy-Vent air purger. The Hy-Vent waste connector allows the Taco Hy-Vent's outlet to be connected to a suitable drain location so that should the valve begin to leak through its vent opening water will be directed to a drain without damaging the building.

Watch out: Taco warns that while connecting a 3/4" flexible hose to the air vent outlet can suit to protect building areas from water damage, do not solder a connection to the air vent valve. Soldering the air vent outlet will cause it to fail to operate properly.

[Click to enlarge any image]

In our OPINION it would be smart to also inspect for leaks at any and all piping connections, including the threaded air vent mounts, and it would be smart to direct a waste connector vent drain to a location where somebody will notice that the vent is leaking so that proper repairs can be made.

Watch out: reading the installation literature from a variety of air eliminator and air vent manufacturers we observe that instructions about how to leave the air vent valve cap vary. Generally the cap is left loose to permit automatic air purging from float type air vents and partly loose to permit automatic air venting from some models of coin vents. But some instructions such as for the Honeywell EA79 Air Vent want you to leave the vent cap shut, stating

Make sure that the red vent cap is securely tight, and that the valve is in the closed position by turning the vent body clockwise. Use hands only [to tighten the valve] to avoid damaging the vent. Operate the EA79 by turning the vent body counterclockwise to the open position. Make sure the red vent cap is tightened all the way to the stop position for proper leakage guard operation. - Honeywell Braukmann EA79 Industrial Air Vent installation instructions.

How to Fix or Replace Corroded or Leaky Manual Air Bleeder Valves

Corroded air bleeder valve (C) Daniel Friedman

Is the air bleeder valve corroded? If the air bleeder is thick with corrosion or mineral deposits (photo at left) it has probably been leaking water when it should not, and it probably needs replacement.

If the air bleeder valve is badly corroded or coated with mineral deposits it's safer to leave the valve alone. Picking at a corroded plumbing or heating component of any sort risks starting a leak that you cannot stop without having to shut the entire system down. Call your heating service technician to have the valve replaced.

Inspect and use or replace manual air bleeder valves on the heat distribution piping:

Problem spot manual air bleed valves: If a building heating system has experienced previous problems with air blockage in the heating system a technician may have installed a manual bleed valve at a strategic location to get air out of a problem section of piping. Look for valves that resemble the one shown in our photo.

Problem finding all of the air bleeder valves: air bleed or air purge valves on hot water heating piping or baseboards can be hard to find. Usually, in addition to one or more automatic air purge valves found at or close to the heating boiler itself, you will find an automatic or manual air purger at the higher end of a section of heating baseboard that has become air-bound in the past.

Leaky air bleeder valves: I find leaky air bleeder manual air bleed valves as well as automatic or float type air vent valves on heating systems.

Sometimes the manual air vent is leaking just because the manual air bleed valve was left open or was abused by a heavy-handed operator who over-torqued its fittings and damaged screw threads or a seating gasket

Tips for Step by Step Replacement of a Damaged or Leaky Air Bleeder or Vent are

Manual Air Bleed Valve Cleaning & Parts

At below left you can see a disassembled manual air bleed valve. The smaller threaded stopper component screws into the center of the air bleeder body until its tapered tip is snug against the valve seat. At below right you can see the tapered air bleed valve seat against which the rotating stopper meets.

Air bleed valve components for hot water heat (C) Daniel Friedman Air bleed valve components for hot water heat (C) Daniel Friedman

Below you can see the vent opening in the side of this air bleed valve. When we unscrew the center stopper in this valve (turning the screw counter-clockwise or from right to left), we open the valve just enough that air can pass through the bleed valve body and out at this vent opening.

Air bleed valve components for hot water heat (C) Daniel FriedmanWatch out: You do not normally need to fully remove the bleed screw, and you shouldn't. If you remove the inner screw completely you'll have a hell of a time fumbling it back in place when hot water comes squirting in your face. Don't do it.

Just one full turn should be enough to permit air (if present) and ultimately hot water from the heating piping system (when there is no air present) to squirt out of the vent.

(On the air bleed valve shown here, about six full turns of its center screw will completely remove that component from the valve body.)

As soon as water begins to exit the vent body you should close the screw by turning it clockwise to a firm, closed position. Wipe off the vent body and check that it is not leaking. The vent shown and most of its sisters are made of stainless steel, are corrosion resistant (not corrosion proof), and should not easily clog with debris.

How to Clean Debris from a Clogged Air Bleeder Valve

If on turning the air bleed vent screw one full turn no air and no water vent out of the bleeder, either the baseboard, pipe, radiator, or convector being bled is completely empty of water and is under no pressure whatsoever (abnormal conditions) or the valve opening or its vent exit opening may be blocked with debris. Small bits of debris will normally vent successfully through the vent opening shown above when the valve is opened.

For an air bleeder that is covered with mineral deposits or lime from prior leaks, you could remove the valve, disassemble its parts, soak them in vinegar over night to loosen the lime, but frankly that's a risky approach. Given the trouble and disruption of removing an air bleed valve in the first place, and the risk that it will still leak on re-installation, just replace it instead.

Watch out: do not try to clean an air bleeder valve seat using abrasives. For successful closure, the air bleeder valve shown depends on a good fit between the tapered screw tip and the tapered valve seat in the valve body.If you scratch or scrape the valve seat or the mating face of the valve screw the valve may fail to seal completely. The air bleeder does not contain any plastic or rubber seals.

Reader Question: how do I solve problems with leaks at an air bleed valve and air remaining in the heating system?

I recently installed a new hot water furnace in my house. I did the job myself and everything is working ok except that  the automatic vent valve constantly needs to be opened to let out air.

 I left the cap loose so that it could vent automatically but then it leaked. Do you think that the valve is bad or is not placed in the proper location ? Any help would be appreciated. - E.M. 12/27/12


E.M. A competent onsite inspection by an expert usually finds additional clues that help accurately diagnose a problem with air bleeding or air leaks into hot water heating systems. (Incidentally, for clarity we call hot water heating systems heating boilers or hydronic boilers - furnaces are generally warm air heating systems).

That said, if you are continually seeing air bleeding out of your air bleed valve then I suspect that either you have not fully bled all air out of the system or there is a leak somewhere that is both letting water out and air in to the heating boiler or its piping and radiators / baseboards. I suggest checking all of the heat piping, baseboards, radiators &c. to see if everything is getting hot.

if your system is successfully circulating hot water through all of the piping and heating devices but there is still air in the system (and if there are no leaks) then it should be pretty easy for the automatic air bleeders (properly located at high points on the boiler and on the heat distribution piping) to purge the remaining air.

Air Coming out of the Bleeder is OK, Water Coming out of an Automatic Air Vent Means Valve Replacement is Needed

Water leak at a float type air vent valve (C) Daniel FriedmanObserving air coming out of an air bleeder is not an indication of a faulty air bleed valve. And if you are indeed seeing air venting out of the valve its location is a functional one. However that does not mean that it's the only air bleed valve needed.

What would be a fault would be water coming out of the air bleeder valve, such as I demonstrate in my photo at left.

[Click to enlarge any image]

Float type air bleed valves are supposed to stop venting once water enters the valve body.

I agree that if water is leaking out of the air bleed valve that's not an acceptable condition - the valve needs to be cleaned (try purging a little water through the valve to flush the valve seat) or replaced.

In an emergency I too would leave the air bleed valve cap shut tight to stop a leak but I wouldn't leave that as a long term solution, as leaving the cap tight also prevents the valve from bleeding excess air from the system. If you can't get the valve to stop leaking, try replacing the valve core stem or replace the entire valve.

Typically we see an air bleeder located close to (above) the boiler itself and one or more located at high points in the heat distribution piping. If you are having trouble bleeding air out of your system you might need to add another air bleeder at the high end of a heating distribution loop, or you might need to follow our suggestions for manually forcing excess air out of a system.


Reader Question: Leaky air bleeder on a Slant-Fin "radiator"

We live in a 45-year old house with baseboard Slant-Fin radiators. One of the air bleeder valves has started to leak, It's a slow leak. but enough to do some real damage if we don't fix it. Upon inspection, we found that all the bleeder valves in the house are horribly corroded and would like to have them replaced. I don't want to attempt this myself (I'm somewhat handy, but the thought of screwing up and losing our heat in the middle of the winter is horrifying) So what kind of specialist should I call? - Dave Selander 3/4/12


A heating service technician or a plumber experienced with hot water heating piping can do the job you describe. It's least disruptive to schedule the repairs for out of the heating season.

When Should a Heating System Air Vent Valve Cap Be Left Open or "Loose"

Opening up a an automatic air purge valve whose cap was closed

Auto vent No 67 from Maid O Mist - showing vent cap position (C) Daniel FriedmanIf loosening the valve cap does not make water leak out, leave the cap loose.

This permits float type air purge vents and some other automatic air vents to do their work. If the valve is leaking you will need to leave the cap closed tightly to avoid water leaks and damage, but you should then replace that float vent or auto-vent at next system service.

Shown at left, the air vent cap on a Maid-O-Mist No. 67 auto vent, automatic air eliminator. Notice the vent hole in the device cap? When this valve cap is left loose air can vent out of this opening.

[Click to enlarge any image]

Watch out: make a list of the locations of all of the air vents on your heating system and post it near the heating boiler. An occasional inspection of each vent for signs of leakage can stop trouble before it results in loss of heat or water damage to the building interior.

Watch out: for automatic and some manual air vents the operating water pressure in the heating (or cooling water) system must be higher than atmospheric pressure. Otherwise when the vent or valve is opened air may enter the piping system rather than being purged from it.

We describe this problem at AIR BLEED VALVE INSTALLATION.


Watch out: some air bleeder vents come with instructions indicating that the vent cap is to be left shut, not open. Be sure to read the specifications and instructions for your air bleeder vent before making a change in the tightness of the vent cap.

Reader Question: Installing hot water heating piping air bleeder valves: leave the bleeder cap loose?

There is a lot of great information here. Definitely helpful for some things and the images are also a plus.

I have one 8' stretch of baseboard pipe that isn't heating up. It's the middle room (my 1 y/o sleeps there) of 3 bedrooms. Both other bedrooms are fine. There are 2 new automatic bleeder valves on each end of the baseboard. I've tried slightly opening the screw valve with no luck. There is a supply and return on this stretch of baseboard (mono flow system?). The lower pipes get hot but it's not making it past the valves to the coils.

I am lead to believe this is the highest point in the house. I've tried turning the system on and than bleeding and still unsuccessful. As mentioned, both bleeder valves are fairly new with no corrosion or water leaks. What else can I do? Dad needs his sleep as his little one keeps waking him up so help is abundantly appreciated. - LB

I have cracked one of the valves and an ever so slight amount of air comes out but not enough to get the water flowing. I'm guessing I may need to bleed all the water from the system and start from scratch. I'm just glad that it appears to be an air blockage and not something more serious.

Just for reference, I should keep the caps slightly open to allow air to dispel when necessary or should I keep them tight and manually crack them from time to time? - LB [continued]

Reply: it depends on the air bleeder valve type


If a section of baseboard is cold but pipes leading to it are hot quite possibly it is airbound as you suspect. Possibly the air bleeders are defective; with the system at full operating pressure and temperature, careful (avoid burns, spills) manual bleeding might dump enough air to get things going, else you may need to try replacing one or both valves.

Or use the alternative air purge methods


If you're not getting the air out at a bleeder and you're sure the bleeder is not clogged, be sure that the heating system is up to operating pressure and temperature. If still you can't get the air out, you'll need to force water through the piping at higher pressure.

Finally, we can categorize air bleed valves on hot water heating systems into roughly two groups:

Automatic Air Eliminator - Auto Vent No. 67 from  Maid O Mist (C) Daniel Friedman Automatic Air Eliminator - Auto Vent No. 67 from  Maid O Mist (C) Daniel Friedman

Automatic air bleeders, often including a float valve that drops to open the bleed valve to purge air when air accumulates in the float body - on these valves the manufacturer instructs to leave the valve cap loose so that the valve can vent automatically. An automatic air bleeder at the boiler or in a utility area should pretty much always have its cap left loose so that it can do its job.

At above left is an automatic air vent, the Maid O Mist No. 67 auto-vent. Instructions for this air vent are shown in the second photo (above-right) For a Maid O Mist No 67 auto vent, normally the vent cap should be left open one full turn from its closed position.

If the valve is leaking water at that location you can close the cap to stop the leak (and prevent air venting) temporarily until you replace the leaky valve - a step that should be done promptly to minimize the risk of lost heat due to an airbound heating system.

Manual air bleeder on a hot water baseboard line (C) Daniel Friedman[Click to enlarge any image or photo at InspectApedia]

Watch out: an automatic air bleed valve with a loose cap, installed on a heating baseboard in the living area, can present a risk of water leaks into the building floor (or ceilings below), if it's not inspected and if necessary cleaned or replaced.

In this location some heating techs and plumbers avoid the leak risk by just keeping the cap tight on the valve.

The valve won't automatically purge air (you are relying on other air purgers elsewhere in the system for automatic air purging), but it can still be opened and activated if necessary to purge air from the particular high hot water piping loop that it serves.

Manual air bleeders (photo at above-left), usually don't even have a cap, but might. These are kept closed except when in use.

Leaving the Manual Air Bleed Valve Shut: a tricky way to "shut off" radiators in older homes by not bleeding air out of the radiator

Radiator air bleeder valve (C) Daniel FriedmanSometimes in older homes the heating radiator control valves are stuck and cannot be operated. Usually the radiator valve will be stuck in the "open" position because no one wants to live with no heat in cold weather.

When we lived in a 100 year old home in Poughkeepsie, New York, ever fall we found that some radiators had become full of air over the summer when the heating system was off.

For a few rooms where we didn't want heat, and where the radiator control valve was "stuck" in the "open" position, we found that if we simply neglected to bleed air from that radiator it wouldn't heat up when the rest of the heating system was operating.

Over at RADIATOR VALVES & HEAT CONTROLS you'll see more photos of radiator control valves.

On all hot water radiators the heating control valve that turns the radiator on or off will be at the hot water inlet pipe to the radiator and may be located near the radiator top or bottom. There are a few air bleeder valves that have a tiny round handle, maybe 1" in diameter, but they will be mounted right onto the radiator body itself, not on the piping.

This article series provides a detailed guide to using air bleed valves to get rid of unwanted air in hot water heating systems: fix cold or noisy hot water heating radiators or baseboards.



Continue reading at AIR BLEED VALVE SOURCES or select a topic from the More Reading links shown below.

Or see AIR BLEEDER VALVES for an explanation of how these valves work and how to use them.

Or see EXPANSION TANK AIR VALVE LEAKS for leaks at the air adjustment valve on a boler expansion/compression tank

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AIR BLEED VALVE LEAK REPAIR at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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