How to bleed air out of a heating system (C) Daniel FriedmanHot Water Radiator Air Bleed Vent Diagnostic Guide
Troubleshoot Hot Water Heating System Air Vents

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Air bleeder valves & automatic air purging valve troubleshooting & repair for hot water heating systems:

FAQs: here we focus on how to find, install, diagnose & fix these devices.

Here we explain how to diagnose and repair problems with air bleed valves and we describe methods used to remove un-wanted, air from noisy or air-bound hot water heating system pipes, radiators, convectors, and baseboards. We illustrate how to buy and add air bleeders at baseboard elbows using a baseboard tee and air bleeder valves.

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Hot Water Heating System Air Bleeder & Vent Troubleshooting FAQs

Manual air bleed valve on a heating baseboard (C) Daniel Friedman

How do I Know if the Air Eliminator or Air Purger is Working on my Heating System?

As we explain beginning at AIR BLEEDER VALVES , virtually every hot water heating system has one or more air bleed valves installed. On most hot water heating systems there is at least one automatic air bleed valve, usually located on the heating boiler itself, or close to the heating boiler on a nearby check valve or flow controller.

If the automatic air bleed valves are installed in the proper locations on the heating system, typically at high points such as at or near the heating boiler and at one or more high points in the radiator or baseboard piping system, then small amounts of air in the heating piping will be purged automatically.

First, is there evidence that the heating system is not working due to air trapped in the hot water heat piping, baseboards, or radiators?

Check hot water baseboards, radiators, or heating convectors: if some of these heating devices are hot and others cold, are they all on the same heating zone? Feel the hot water piping leaving the heating boiler - it should be hot when the boiler is running and the thermostat is calling for heat.

If the building has multiple heating zones each zone will be controlled by its own thermostat and each heating zone will either have its own hot water circulator pump (controlled by a thermostat and pump relay switch), or each hot water heating zone will have its own thermostat and a zone valve that opens to let a common circulator pump send hot water through that individual heating zone. Are all of the thermostats turned up high enough to call for heat in each heating zone?

If some heating baseboards or radiators are hot and others cold and we're sure that they're on the same heating zone, then the system is probably air-blocked.

Second: even if heat is working throughout the building and the heating system is not air-bound, there may still be air purge or air bleed valve repairs needed. Sounds of air gurgling in the heating piping is a sign that trouble is brewing, and leaks anywhere in the syste are also leading to trouble.

But a variety of problems can cause the automatic air purging valves to fail to do their job. The effort to fix these problems ranges from trivial to a bit of work. Failure to fix the air purge system can ultimately lead to loss of heat in all or parts of a building.

Question: bled air but still get gurgling sounds - where will I find secondary air bleeders

I have an under 10 year old baseboard hot water radiator system. I bled the main tank at the furnace but am still getting gurgling and water flowing sounds in some of the radiators. I assume there are secondary bleeder valves. Where are the likely places these might be located. Thanks - Jim 1/30/12


AIRBOUND HEAT SYSTEM REPAIR by WATER FEED VALVE has more thorough and aggressive means to get unwanted air out of a hot water heating system.

Question: I don't think my hot water heating piping air-bleeder vents are working

I have this type of radiator For someone reason the one closest to the boiler needs to be bled every day. I do not see any leaks. All other radiators function fine. It is a one floor house with one zone. Any thoughts? - Chris 1/3/13

I have three automatic air vents - one Spirovent and two Hy-Vents for three paralleled heating needs off oil-fired boiler. Former is on main whole-house distribution loop for Infloor radiant heating and one of latter is for DHW storage tank distribution loop. The last is on the far-side distribution loop of heat exchanger for sunporch with Infloor radiant heating. This loop has 50:50 mix with antifreeze because the temperature control is set for 50 degrees in Gypcrete slab temp and not air temp in glass/extruded aluminum sunporch.

This automatic air vent seems not to be working automatically anymore because this heating season I'm hearing entrained air 'tinkling' and 'squirting' through the circulator pump.

Even with pressure checked at 20 PSI. This vent sits atop Taco air scoop with expansion tank under scoop and these three are downstream of pump which is piped in very soon after boiler outflow manifold. I have checked that screw cap cover is backed off and have now depressed Schrader valve to release a 'highly carbonated' bubbly froth three times today and still haven't gotten all air out.

Other Hy-Vent on piping to DHW tank delivers only water when I depress Schrader valve. Can I clean this [faulty] unit without depressuring that loop? If so, how do I take it apart. Taco manuals I've found show nothing. Just replace it? Is the antifreeze to blame? Specifications said I could use it. Suggestions welcome. - Christopher D'Amico 1/14/2013


Christopher I think you might first take a look at our air bleed valve diagnostics beginning
at AIR BLEEDER VALVE DIAGNOSIS to be sure we're on the same page.

As you are continuing to have trouble you will want to
see AIRBOUND HEAT SYSTEM REPAIR by WATER FEED VALVE where we describe methods to be sure you've gotten all the excess air out of all of the system components and piping.

Question: cold wall panel

Wall panel hot at top but cold at bottom. Cause and remedy please - John 7/29/12


John, so little information, sorry, no idea. I don't even know what kind of heat you are discussing.

Take a look at how the equipment is piped; if hot water is entering but not exiting a hot water radiating device it m ay be air-bound, or an exit pipe, valve, or trap may be clogged with rust or debris. For steam radiators that don't get hot the diagnostic procedure is different, as we may need to check the steam vents.

Air Bleed Valve Installation: How & Where to Install Manual Air Bleeder Valves on Hot Water Heating Systems

This topic has moved to a separate article: AIR BLEED VALVE INSTALLATION

Watch out: if the air vent or air bleeder valve is not installed properly, in the proper position (upright or horizontal depending on valve type and model), and in a functional location (where air can enter the valve or vent) it will not work reliably, may not work at all, and worse, may actually allow air into the water piping system, making the system air-bound frequency worse rather than better. Details are in the air bleed valve installation article cited just above.

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

This topic has moved to a separate article: AIR BLEED VALVE LEAK REPAIR

Air Bleed & Air Purge Valves for Hydronic Heating - Where to Buy Them

Question: Where can I buy canister type air bleeder or air purge valves?

We are unable to find the air bleed valves that you have listed on your web page (canister type). We are looking to buy 10 of them and am wondering if you have a contract from which to purchase these? W.M. - Wacol, Australia

Reply: W.M. please see AIR BLEED VALVE SOURCES where we list air purgers, air bleeders, manufacturers and sources. For a description of the different types of air bleeders or air eliminating valves you can also see AIR BLEEDER TYPES & LOCATIONS

What's the difference between an air bleeder valve and a radiator on-off valve?

Reader question: how do I differentiate between the valve to bleed the baseboard radiator of air, and the on/off valve? I just bought my house and I notice that there are 3 baseboard radiators in one section of the house that are cold. I think that perhaps they are turned off, but I don't know how to tell. Looking at the radiators, they have pull chains coming out of the top louver, but there's no tension on the chain like there is when you use a pull chain to turn on and off a light, so maybe the pull chains are for opening and closing the louvers?

When I take off the lower cover, the part that covers the fins, I see a valve on the pipe where the water comes in (or out?) that has a slot for a flat-head screw. Would I use that to turn on or off the radiator, or is that the bleed valve? I think it must be the former because I don't see where air or water would escape.

My heat is provided by a gas boiler forcing hot water to baseboard radiators, and I have only one thermostat. And another pertinent detail: In the basement the pipes going to those 3 radiators feel hot, so water is circulating.

In trouble shooting, I want to try the easiest thing first: are the radiators on? Then I'll move on to the more complicated procedures if that's not the problem. But how do I tell if they are on?
Thanks! - Anastasia

Reply: How to tell an air bleed valve from a radiator control valve ?

Radiator manual air bleeder valve (C) D Friedman

At left we show a manual hot water heating radiator air bleeder valve that has a round black handle. Because of the handle shape some folks may be confused about just whether this valve is an air bleeder control or a radiator on-off valve.

But it's easy to see the difference. The valve shown at left is attached directly to the hot water heating radiator at its top at one end. It is not connected to hot water piping, so it cannot be controlling the flow of hot water into or out of the radiator.

Now for more details:

At below left we show a common radiator control valve found at the top of a hot water radiator. Other radiator control (on-off) valves may be located close to the floor at the bottom of both steam and hot water radiators.

At below right our sketch (courtesy Carson Dunlop Associates) illustrates an air bleed found at the top of some older radiators - a model that has a tiny round handle. The advice in the sketch to leave air bleed valves alone is for home inspectors.

Radiator valve with air bleeder (C) Daniel Friedman

The worry is that opening an air bleed valve could lead to a leak if the valve is damaged or defective. But in fact manual air bleed valves on heating radiators are a control intended for use by a homeowner, need to be functional to get an airbound radiator back into operation, and need to be fixed if they're defective.

Radiator air bleed valve (C) Carson Dunlop Associates

The reason a home inspector might not operate the valve is that during a home inspection s/he doesn't want to risk starting a leak that can't be promptly shut off.

Other air bleeder valves have a t-handle or a square fitting operated with a "skate key" wrench and still other air bleeders use a flat bladed screwdriver for their operation - illustrated in the article above on this page. In the sketch at above right you can see the radiator on-off control valve at the lower right.

Notice that the radiator control valve will always be connected to both the radiator and a hot water (or steam) pipe, while an air bleeder valve will be connected directly to the radiator.

Well almost. Our photo at above left is tricky because that particular model of radiator control valve also happens to include a little bleeder fitting - that hexagonal brass nut shown at the center of the radiator control valve body.

Question: difference between air vents and vacuum vents

(Dec 4, 2011) DanH said: Is there a difference between an air relief valve and a vacuum relief valve?


Indeed because people discussing plumbing parts may not always be precise nor use exactly the "official" name for a part, the two valve names you cite might be used by some to refer to the same device. But to me, if we are discussing well and water supply systems, then these are different devices.

An air relief valve would allow excess air out of a pressurized system.
A vacuum relief valve would relieve vacuum - the opposite of air pressure.

Can you tell me what kind of equipment on which the valves you name are used? If so I can write a more accurate reply.

Question: attic air bleeder vent not working

(Feb 15, 2014) Anonymous said:

We have hot water boiler system and there is air in the lines. The bleeder vent is in the attic and I am not sure how to bleed it. I has a stem that comes out of the top of the bleeder.



The air bleeder in the attic should be removing air automatically; If it is corroded or leaky it needs replacement; If it looks clean, try pressing gently on the center pin or valve stem (you may have to unscrew a cap that looks like an automotive tire air valve cap).

Just press the exposed pin briefly with a pen tip or similar instrument. If AIR comes out then the bleeder was not working automatically.

If water comes out STOP PRESSING as you're making a mess and no air is coming out at this location anyway.

If you mess with the valve stem pin as I described and now the darn thing won't stop leaking, just screw the valve cap back on as a temporary fix until the valve can be replaced.

If no air, just water comes out of this valve but your system is air bound, there may be other air bleed valves, automatic or manual, that need to be checked, tested, or replaced; on occasion we have to add a bleeder at a problem spot, and in the worst case we use other methods (described in this article series) to force air out of the system.

Watch out: My consultant Paul Galow suggested I add this warning: in the event of having created a catastrophic leak you'll end up having to turn off heat and turn off water supply to the boiler - an event that seems to me unlikely as long as you keep that valve stem cap to close off the valve if it's leaking or dripping.

But DO NOT use great force turning anything - some of these air bleeder valves are made of thin-walled copper. You can break one right off if you're heavy handed.

Question: drumbeats from the hot water radiators

(Oct 28, 2014) Anonymous said:

After what sounded like Congo drum playing occurring in my hot water radiators, I tried to investigate. All but one radiator was heating up. I have been bleeding the radiators - hear the hissing air until it finally stops, but no water coming out. I've bled them years past where water would escape. Not sure what is going on. The one radiator that wouldn't heat up now is. Do I need to keep bleeding the radiators until water comes out? Thanks.


See BANGING HEATING PIPES RADIATORS - for diagnosis and repair of this problem

Question: how long do I keep the air bleeder valve open on a radiator?

(Nov 13, 2014) Anonymous said:

do I need to keep bleeding the radiators until water comes out


es but ... If you find that bleeding air put of a radiator gfets it heating normally but that after less than a whole heating season the air problem returns, then further investigation for a source of air leaks in or water leaks out is warranted.

Question: heating system shuts off with a loud bang and rattle

(Nov 19, 2014) Shelley said:

Our system shuts off with a log bang and rattle. Right at shut off is the only time we get air to purge from the pipes but it seems the more we do it the louder it is getting to where I worry we will have a joint blow apart



Watch out: I'm not sure from just your note what is going on - but it COULD be a *dangerous* puffback or fuel problem. I'd ask for an inspection and service by a trained heating service tech.


Question: opened bleeder valve, no air came out

(Nov 19, 2014) marc said:

I turned the little skate key device I have to Open the air bleed valve and there was no hiss air or water coming out it mentions that the valve body is clogged or blocked is there any way to easily fix this problem or is it a complicated fix? thx



The heating system needs to be on and up to operating temperature - check that and try again.

Details of what to do for this situation are at AIR BLEEDER VALVES

Reader comment:

Anonymous said:

11/27/2014 Anonymous said:

Couple of things-[about what to do if no air comes out of an air bleeder valve] -If your system is not filled with water, you can't bleed the air out of the topmost radiators. You may need to open the fill valve in the basement, then bleed until air comes out. Typically, the pressure limiter valve in the basement will drip a little bit after the system is full of water, but usually this stops after a day or so. Also--if your system has the "scoop" type of automatic air purger, it can still get air bound in the top story of the house. Then you simply have to open the valves in the system in the basement until all the air is out. Takes a while to figure this out.



Good point and thanks for the comment - indeed in More Reading links just above you'll find two articles


that describe how to get past an air-bound system when the bleeder valve alone won't do the trick.

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. Hydronic heating air vents and air purge devices: types, where to buy; How to diagnose and fix heating system noises & air in hot water heating system pipes. Service Procedures to force air out of an air-bound hot water heating system. How to bleed a hydronic (hot water) heating system: how to purge air out of heating system boilers, radiators, baseboards, or piping. What is a baseboard tee, how do they work with baseboard air bleeder valves, how to install air bleeders, vents, purgers.

Question: reasons to add air bleeder valves at the end of zones

(Dec 3, 2014) LarryM said:
I've got a 3-zone baseboard heating system that keeps getting airbound. There's an automatic bleeder by the boiler, but no sign of any bleeders anywhere else. I'd like to add bleeders at the topmost point of each zone, but the pipe comes up from the floor into an elbow, then immediately into the radiator, then there's another elbow and the pipe goes back through the floor. It seems to me that there's no slack to be able to unsolder the elbow in order to replace it with a baseboard tee.

What are your thoughts on adding the automatic bleeders to each zone in the basement, where I have free access to the pipes? Also, what are your thoughts on using a saddle tee to add a bleeder?

(The circulator pumps are on the far end of each zone, if it makes a difference.)



Sometimes the problem is that we didn't successfully purge all of the air out of the system in the first place. Sometimes it's hard to get all the air out of long horizontal piping runs.

A second possibility is that there is a leak somewhere.

Sure you can add air bleeders at the ends of high end of each zone.

Question: persistent cold radiator

(Feb 21, 2015) joe s said:
One radiator keeps getting cold even after bleeding. When bled water came out but air bubbles still present. Do I keep bleeding till no bubbles? Thanks


Joe, if you're getting mostly water out at the bleed valve I suspect the problem is ahead of the radiator, at it's control valve or air in the heating lines ahead of the radiator. You may need tonuse one of the procedures discussed at AIRBOUND HEATING SYSTEMS in More Reading above.


(Feb 18, 2015) Anonymous said:
Here is my situation. I own a 3 story commercial building. It is a boiler heat system. This system was probably installed somewhere in the 1930's or earlier, if that's possible. Anyway,on the third floor, the radiators that are first in line receive hot water. The last three or four do not. I have opened all air bleeders on the the radiators on the third floor at the same time. The first eight radiators all bleed fine.The last three or four do not. Sometimes on these last four, two of them will bleed water and air and the last two do nothing. These last radiators are in the area that we use as our offices and we need heat. I understand that there is air lock here somewhere. The problem is that I can't get all the air bled out from these radiators and they remain cold. I've read your articles here and they provide great information. Thank you.Any ideas as to how or what I can do to get these radiators up and operating?
Wayne in Latrobe, Pa.



For bleeders that do nothing they're probably clogged and should be replaced.

Check that the radiator valves actually work rather than just the handles spinning. Check for air bound piping ahead of the cold rads or clogged radiator return piping.


(Feb 17, 2015) SteveF said:
A few years ago we had a section of our hot water heat removed because we were planning to remove a wall between the kitchen and living room. To make up for the lost heating or those radiators, they put PEX under the floor in the entry way and kitchen only. But ever since then I can't bleed the system out like I used to. It actually sucks air into the bleeders. So I've left it alone but I still hear gurgling in the pipes and radiators although it seems to heat fine. But I know it's not as efficient and probably not good for the system. Why would it suck air in? I can take water on my fingers and turn the bleeder and see it get sucked in. Thanks a bunch.



From your description it sounds as if those air sucking bleeders are not in the right location -leave them closed. Try bleeding from higher bleeders,
Or install a higher located bleeder, or use one of the other methods we discuss for forcing the air out of an AIRBOUND system using a pony pump.

Question: not enough heat from radiator

(Feb 21, 2015) lester said:
I would like to know what can cause my radiator to give out little heat. I checked the pipes and they are hot


In the More Reading links just above please take a look at one of the COLD RADIATOR ... Articles for some diagnostic suggestions


(Mar 1, 2015) david said:
I have been trying to purge my 2 heat pump zone system with little luck getting all the air out. I have put city water pressure into the boiler @ the boiler drain, forcing the water through each zone pump individually and together with the loop closed and the bleed drain open, all with the gas valve off. ran until no air bubbles were present. Then I started the boiler up (turned on the gas valve and called for heat) There is still a lot of air in the system. Can a boiler make its own air when cold water is introduced to the hot system by the fill valve when bleeding?



There could be a water leak out and air leak into the system, or it could be that you need to use a more powerful air purge approach to force air out of horizontal piping runs.



in the More REading links above

(Mar 3, 2015) David said:
Thanks for responding Dan. I've checked for leaks and I never hear the fill valve open. I've used city water pressure at the drain valve as i've said, and there is little or no air. I've checked the city water for air(none). It seems to me there is only 2 possibilities:The fill valve is sucking air when i'm bleeding, or as you stated, I'm just not getting enough pressure to force the air out. I also tried using a "L" shaped 1/8" fitting @ the highest point of the upstairs zone with a hose attached and opening an 1/8" valve there and open it when the air comes by. This seems to work as a lot of air comes out. But it seems to make the air worse, hence my ? about cold water causing air to be released when bleeding. If I shut the gas valve off and try the above method, air comes out for a while then stops. Turn the gas back on, then more air appears.

How can I check my fill valve for "air sucking" and how can I do a more powerful air purge?


When the heating system is cooling you can feel with a wet finger whether or not there is suction at the air vent opening.

Question: American Air Purger as illustrated in this article, which is automatic? I have excessive gurgling, bubbling, and banging

(Feb 2, 2015) GorRiv said:
I have an American Air Purger as illustrated in this article, which is automatic? I have excessive gurgling, bubbling, and banging in my baseboard system though... Can/should I crack the valve at the top to bleed off air? Should I make sure the water intake for the boiler is open, and how much/when? Everytime I try to access other pages about Air bleeding they come up with "error, problem loading page". Help, please.


GorRiv -

Check that the valve cap is loose and that the valve is not leaky or clogged with mineral salts from prior leaks - or just replace it.



Continue reading at AIR BLEEDER VALVE FUNCTIONS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

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

Suggested citation for this web page

AIR BLEEDER VALVE DIAGNOSIS at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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