Hot Water Radiators: How to Bleed Them

Hot Water Radiators. “Second winter” is upon us, and we are trying to prepare for the home heating season.  It’s about to get pretty chilly here in the ‘Burgh. So we thought it was a good idea to talk about what to do when you are not getting enough heat in different areas of your hot water-heated home.  These very efficient hot water boiler systems are common in the Homestead and Squirrel Hill areas.

What to Check First

Is your hot water radiator system heating your upper floor(s) adequately? This may help with fixing a hot water heating system (hydronic) radiator that is not heating up as it should or if you have heat in some areas but not others. If individual hot water radiators provide your heat, usually there is a control valve at each radiator.  Ensure that the heating radiator’s control valve is “open” or “on”.

Is your radiator getting hot? Check if the hot water radiator control valve has been turned off.  Try turning the valve counterclockwise to see if it will open. The control valve is most often found at the floor level.  Sometimes, the manufacturer adequately marks the “open” and “close” directions for this radiator valve.

The Radiator Valve

First, if the hot water radiator valve does not turn counter-clockwise, try turning it in the other direction. You should be cautious though.  Don’t use excessive force to try to turn a “stuck” radiator valve.  You may be trying to open a valve that is already in its fully-open position. Second, the valve could be jammed.  Excessive force can break a jammed valve or even cause a leak.  If the valve won’t turn at all counter-clockwise towards “open”, again try turning it the other way – clockwise, towards “closed”.  If the valve now turns you’ll know it was already in its open position.


Perhaps you decide to have the control valve on a hot water radiator replaced. Consider installing a new valve that incorporates a thermostat as well, such as an automatic radiator valve.  This (more expensive) radiator control valve lets you treat each individual radiator as a “heating zone”.  As long as the thermostat is calling for heat, each radiator can be regulated automatically.  As always, Proudfoot Plumbing Heating & Air can help you with this kind of replacement should you choose not to do it yourself.

Maybe only some of your hot water radiators aren’t getting hot. If an individual radiator valve is open (turned counter-clockwise) then you may need to bleed air out of the radiator. This is so that hot water from the boiler can flow into the radiator.  Many hot radiators have an air bleeder valve that the homeowner can operate. As long as they take care to avoid possible scalding risks when bleeding just air.

Hot Water Heating System

By looking at the radiator control valve, one can conclude that they have a hot water heating system and not a steam heat system because there is no automatic air vent found on steam radiators.  Secondly, the radiator control valve includes an air bleeder, a nut on the side of the valve body. Also, you’ll need to be careful if the radiator valve appears to be “open”, that is, turned fully counter-clockwise.  The valve may be broken internally and you may be just turning the knob with the valve staying closed inside.

Looking at the Valve Stem

Turn the radiator valve from “closed” to the “open” position. Look closely at the valve stem – which is the metal rod or shaft extending below the knob in your hand and also extending into the body of the valve itself. You’ll see that as you “open”, the valve stem gets “longer” and is often less-oxidized.  The shinier part of the valve stem will become exposed as it moves outwards.

That’s a great way to convince yourself that yes, the valve is probably opening internally, and you’re not just turning the knob.  If the valve body has broken loose from the valve stem, that’s an internal problem you can’t see, and turning the radiator valve knob, even if it rotates, will not open a broken, stuck, or frozen valve.  Again, Proudfoot Plumbing Heating & Air can help with this problem.


Do not attempt to take apart a radiator valve while the heating system is on and hot; you risk getting sprayed with hot water and/or starting a leak that’s hard to stop and ultimately having to shut down the whole heating system.

How to Open Manual Air Bleeder Valves on Hot Water Radiators

This short procedure for using manual air bleeder valves to remove unwanted air in a hot water heating system in order to correct noisy gurgling pipes or to correct the loss of heat due to an air-bound radiator is described below.

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.

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

Step 2: Find the Air-Bound Hot Water Radiator:

If you have not already done so, once the heating system is up to operating temperature and pressure, check each radiator 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.

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).

If one radiator 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.

Step 3: Bleed Out Excess Air From the Hot Water Radiator:

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.  In a pinch, you may be able to open and close the valve using needle-nose pliers that have a point fine enough to reach into and grasp the square end of the valve control.

A radiator air bleeder valve key can be readily purchased.  This little key tool 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.  You’ll find these at local hardware stores.  Air bleeder valves are also sold by local plumbing suppliers and by building supply stores such as Lowes and Home Depot, and also from online plumbing suppliers.

Caution: if no air comes out of the air bleeder valve, just water, then the radiator served by that valve is not air-bound.  If nothing comes out of the air bleeder (no air and no water), the system could still be air-bound, or it may just not be hot enough yet.

Step 4: Close the Air Bleeder Valve on the Hot Water Radiator:

When water begins to come out of the valve, close it. You then should feel heating entering the pipes and radiator.  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 air-bound location, or there is a separate problem with the heating system, in which case Proudfoot Plumbing Heating & Air would be more than happy to assist you! At Proudfoot Services, we specialize in helping homeowners protect their investment in these types of systems.  A great way of protecting your investment in these systems is with a maintenance plan for Hot Water Radiators and the boiler that feeds them – see our maintenance page for more information.

Image Credits – Adobe Stock

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