LARGER VIEW of hot water heating air bleed valveHot Water Heat Air Bleeder Valves
Guide to Air Bleeders for Radiators, Baseboards, Convectors

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A 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. 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

What is a baseboard tee, how do they work with baseboard air bleeder valves, how to install air bleeders, vents, purgers.

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A Guide to Air Bleeder Valves for Hot Water Heating Systems: Radiators, Baseboards, Convectors

Photograph of an oil fired heater stack relay

Why You May Need to Bleed Air from a Hot Water Heating System

If a hot water heating system develops too much air in the piping you may hear bubbling or gurgling in the heating pipes when the heating system is operating, or worse, so much air may be in the heating piping, radiators, or baseboards that heat may simply not circulate at all.

[Click to enlarge any image]

Here we explain how to locate, inspect, use, or replace automatic and manual air bleed valves on hot water heat, and we explain methods used to remove air from air-bound hot water heating systems by finding and repairing or using automatic or manual air bleeder valves, or by using two different service procedures to force air out of air bound pipes in a hot water heating system.

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.

Hot Water Heating System Air Bleed-out Procedure

How to Open Manual Air Bleeder Valves on Heating Radiators and Baseboards to Bleed Out Air - step by step guide

Here we describe the procedure for using manual air bleeder valves to remove un-wanted air in a hot water heating system in order to correct noisy gurgling pipes or to correct loss of heat due to an air-bound radiator, heating convector, or section of hot water heating baseboard.

Step 1: turn on and turn up the heat. First make sure that your thermostat is calling for heat and that the heating system boiler has been running for ten minutes or so - to insure that the system is warm and up to normal operating pressure.

Honeywell round thermostat (C) Daniel Friedman

This step is necessary to ensure that heating system pressure will easily push out air from the air-bound radiator or baseboard, and to subsequently force hot heating water into the previously cold radiator or baseboard, confirming that you have successfully removed air that was preventing heat from rising into that unit.

In our photo, the room temperature (bottom scale) is at about 70 °F and the thermostat is set to 65 deg. F - in this condition the heating system will be off and removing un-wanted air in the system would be more difficult.

Step 2: find the air-bound radiator or baseboard: if you have not already done so, once the heating system is up to operating temperature and pressure, check each radiator, convector, or heating baseboard to see if it has warmed up.

If you find one or more that remain cold, and provided that the cold heating radiator's valve is in the "open" position (counterclockwise), proceed to step 3 to see if that unit was air-bound.

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

Our photo above shows a manually-operated air bleeder on a heating baseboard. Manually operated air bleeder valves are opened (turn counter-clockwise) using either an air bleeder valve key (if the bleeder valve stem is square) or a simple flat-bladed screwdriver (if the air bleeder valve stem is slotted).

In a building whose heat is delivered by hot water radiators or by hot water convector units every one of these may have its own individual manually-operated air bleeder valves installed.

If one radiator or convector is not heating up, find and open the air bleeder valve to let out air. Close the valve immediately when water begins to come out.

We hold a cup under the valve spout during this operation so we don't spill water in the living area. (Photo below.)

Step 3: bleed out excess air: Open the air bleed valve (turn it counter-clockwise) and listen for the hiss of escaping air. A "roller skate key" like device called a radiator air bleeder valve key may be needed to turn the recessed square end of older manual air bleeders on radiators and heating convectors.

In pinch we've been able to open and close the valve using needle-nose pliers (photo at left) that have a point fine enough to reach into and grasp the square end of the valve control.

How to bleed air out of a heating system (C) Daniel Friedman

Question: where to buy a radiator air bleeder valve key

2016/10/19 Harry Rust said:

I want to bleed the air "if there is any trapped" in all the old heavy radiators in my house. I don't know the proper name of the tool used to do this but I think it looks similar to a key. Anyway, I need to know where to get one. Thank you.


Harry, the little key tool you described is widely sold in several sizes of which 1/8" square is standard; it's described as a Radiator Air Vent Key or Radiator Air Bleeder Key, for less than $1.00 each.

You'll find these at your local hardware store if your are lucky enough to have one that's not been squashed by the big box stores. Air bleeder valves are also sold by local plumbing suppliers, by building supply stores such as Lowes and Home Depot, and from online plumbing suppliers.

Below you can see a typical radiator air vent key or "radiator air bleeder key" produced by Durst.

Radiator air bleeder valve key (C) Daniel Friedman

Slotted type air bleeder valve stems use a screwdriver to operate the valve. They're often sold under the description "Coin Air Vent Key Bleeder" but as the slotted valve stem is usually recessed you may find that a coin doesn't work well.

Watch out: If no air comes out of the air bleeder valve valve, just water, then the radiator or baseboard served by that valve is not air-bound.

If nothing comes out of the air bleeder - no air and no water, the system may not be hot enough or it may be air-bound.
See NO AIR & NO WATER COMES OUT in this article just below.

Step 4: close the air bleeder valve. When water begins to come out of the valve, close it.

Step 5: feel the heating pipes entering the baseboard, radiator or convector. Provided that the room thermostat is calling for heat, in a minute or so the pipes and radiator should begin to warm up and eventually become hot. If this does not occur either the heating system is off, there is another airbound location, or there is a separate problem with the heating system.


Manual air bleeder valve on a heating convector (C) Daniel Friedman

Other air bleeder valves are opened using a simple flat-bladed screwdriver.

Separately at AIR BLEEDER VALVE FUNCTIONS you can see photos of all of the various types of hot water heating system air bleeder valves including both manual air bleeders and automatic or float-type air bleeder valves.

Watch out: if the manual, screwdriver-operated air bleeder valve is badly corroded such as the one shown in our photo of a manual air bleed valve at the bottom of a heating radiator (photo at left) chances are it has been leaking and it be stuck.

This particular valve was one we handled with great care. If it was not easy to open and shut it again with a screwdriver (without much force) we'd have chosen to leave the valve alone until it could be replaced.

How Do we Know That the Air Bleed Valve Operation Has Been Successful?

What if I Open the Air Bleeder but No Air & No Water comes out?

Your system may be air-bound. First though, be sure that the thermostat is calling for heat and that the system is up to full operating temperature. That alone may put enough pressure in the system to successfully bleed air out of the cold radiator or baseboard.

If even after the heating boiler has been running for ten to fifteen minutes and is up to full temperature (burner has cycled off) you still get nothing coming out of a radiator or baseboard air bleed valve then your system is probably air-bound. At that point you need one of the two air-bound heating system repair approaches described by the two blue links just below.

11/27/2014 Anonymous said:

-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

AIR BOUND HEAT SYSTEM REPAIR by PUMP - live link just below


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

What if there are no manual air bleed-out valves on my heating system?

If your hot water heating system has become air-bound (one or more sections of heating radiators or baseboards are staying cold even though the boiler is on and the circulator pump is running), and if your system does not have an air bleed valve to remove air blocking water flow, you probably need to call a heating service technician who will use one of the methods we describe

It is also quite possible to add manual or even automatic air-purging devices onto existing hot water heating systems when and where they are needed. Below we illustrate the two main types of air purge or bleeder valves: at below left is a float type air purge valve mounted atop an American brand air purger. At below right is a manual air bleed valve on a baseboard tee.

[Click to enlarge any image]

American air scoop with float type air purge valve atop (C) Daniel Friedman American air scoop with float type air purge valve atop (C) Daniel Friedman

But before rushing over to one of those cold radiator or baseboard fix-it articles, check the article summary
at AIR-BOUND HEATING SYSTEMS to see if there is an automatic air purging device that needs repair or replacement on your heating system.

What if nothing comes out of the air bleeder valve when it is opened?

Reader Question: Hi - I have baseboard water heat. Nothing comes out of the air bleeder valves (All but one is manual, they fit a flat head screwdriver.) I just ran water through the whole system to get rid of all the air bubbles.

The pressure on the little gauge looks good and pretty consistent. Now the pipes seem to be hot, but still when I open the bleeder valves I get no water and no hiss of air. Is this normal? Is it ok? Do I need to do something? Thanks very much for your informative articles. - Abby 10/7/12


If no water comes out of the air bleeder valves I suspect the valve body is clogged or blocked.

What if I can't open the air bleeders on some of my radiators

Reader question: The bleeding screw in some of my radiators have rounded off so cannot unscrew them. Can I buy replacement plugs and if so would they be simple to fit myself. Combi boiler system.

Maggie 10/14/12


Maggie I have two possible solutions to rounded-off air bleed screw fittings.

First, try using a mini Vise-Grip tool that can reach inside the opening of the air bleeder and get a good bite on the air bleeder. I've successfully opened, bled, and closed tight air bleeders in this condition, allowing us to defer replacing the device.

The other solution is to shut down heat, cool down the system, and replace the entire air bleeder. Prepare the new replacement unit with teflon tape or pipe dope. Put down a towel, Remove pressure from the system (but you shouldn't have to actually drain it), work fast, and don't cross-thread the new fitting.

Question: can I change a manual air bleeder to an automatic one

Can an automatic bleeder be screwed in where the manual bleeder is on my radiator or will i have a puddle of water on my floor when auto bleeder starts working? - Darrell 11/20/12


Yes you can change a manual air bleeder to an automatic one provided you choose a device that works in the position in which you need to mount it.

You are correct to worry about unintended leaking, since most automatic air bleeders require (to work) that their screw cap be left loose. So it makes sense to use these conversions where someone will quickly notice if there is a leak, and to avoid them otherwise. In my OPINION.

Question: how to bleed a heating convector with a broken vent key: Steps in replacing a broken air bleeder valve

2016/10/27 Nicci said:

Hi, my son has a convector radiator in his bedroom which desperately needs bleeding, however the inner part where you enter the key has broken off so i'm unable to turn to let out air. Can anyone please advise whether it's still possible or whether the radiator would need new part?

This question was posted originally at RADIATOR VALVES & HEAT CONTROLS

Reply: how to replace a radiator air bleeder valve


In an emergency, such as over a holiday weekend when your son is freezing to death and all of the heating and plumbing suppliers are closed, one could bleed air by removing the vent entirely, then screwing it (or a pipe plug) back into the opening when water appears.

The air bleeders on radiators are rather standard in thread and dimension. You can simply replace the bleeder device by unscrewing it and screwing in a new one with a bit of pipe dope or teflon tape. BUT first you'll need to let the heating system cool down and you'll need to drain enough water from the system that you don't have a flood from water squirting out of the air bleeder opening during the repair.

Air bleeder valves are described in detail at InspectApedia in this article AIR BLEEDER VALVES

Here are the steps that a heating service technician would follow - more or less, as we don't know a thing about your specific heating system

  1. Turn off the boiler
  2. Let it cool down
  3. Turn off water supply into the boiler
  4. Drain water until the pressure drops down to maybe 0-5 psi. You do not need to fully drain the system, just drain enough that there is no water pressure.
  5. Prep the replacement bleeder device so that when you unscrew the old one if water comes out anyway you can minimize the mess by acting promptly to place and screw in the new one.
  6. Place a bath towel on the floor where your flood is going to occur (maybe)
  7. Unscrew the bleeder device - DO NOT PANIC if water starts coming out. Keep calm, carry on.
  8. Screw in the new air bleeder device - don't panic or you may cross thread it
  9. Mop up the water
  10. Take a deep calming breath
  11. Turn the water supply to the boiler back on
  12. The pressure/reducer/water feed valve should automagically refill the boiler to the proper cold-starting pressure.


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


Or see these

Radiator Air Bleeder & Vent Articles

Or see these

Articles on Fixing Air-Bound Heatiing Systems

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


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