Automatic water feed valve (C) Daniel FriedmanFix an Air-Bound Hot Water Heating System
Procedure#1, Using Built-In Controls & Valves

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Air-bound heating baseboard or radiator repairs:

Here we explain how to remove un-wanted, air from noisy or air-bound hot water heating system pipes, radiators, convectors, and baseboards using the automatic water feed valveOn a heating boiler.

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.

If necessary there are additional methods used to remove air from air-bound hot water heating systems using two different service procedures to force air out of airbound pipes in a hot water heating system.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

How to Diagnose & Fix an Air-Bound Hot Water Heating System

Manual air bleed valve on a heating baseboard (C) Daniel FriedmanDiscussed here: How to diagnose cold heating baseboards or radiators, How to diagnose heating circulator pumps that won't stop running.

A Guide to Air Bleeder Valves on Heating Systems: Heating System Radiator, Baseboard, or Convector Air Bleeder Valve Troubleshooting & Repair Guide, Cold radiators: if your radiators won't get hot : how to check for an airbound radiator and other causes.

How to diagnose and fix heating system noises & air in hot water heating system pipes.

If some heating radiators or some sections of heating baseboards of your building are not getting hot, or if your building circulator pump runs continuously but heat is not being delivered to the heating zone served by that circulator, the discussion beginnin

at AIR BLEEDER VALVES (just bleed air out of the system) and continuing here (how to forcer air out of the system) can help diagnose and cure that problem.

If you cannot bleed out un-wanted air and if using the water feed valve method described here do not work, you'll need to


Readers should also see DIAGNOSE OIL HEST NOISES for diagnosis and repair of other heating system noises on both oil and gas fired heating equipment. This article series answers most questions about central heating system troubleshooting, inspection, diagnosis, and repairs. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.

If your heating system is not working properly,

see NO HEAT - BOILER or NO HEAT - FURNACE. This article series answers nearly all questions about Heating System Boiler Controls on central heating systems to aid in troubleshooting, inspection, diagnosis, and repairs. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.

Question: Why does my heating circulator keep on running?

Here’s the problem, I have a one pump three zone hot water baseboard heating system. The circulating pump keeps running even though there is not call for heat. This happens even in the summer, so I just shut the emergency switch off, but now, it’s getting cold and I want to turn the system on. One of the zones does not get heat, so I replaced the TACO zone valve head thinking that was the problem.

Well, the pump is still running, and no heat even when the thermostat is set to 90. The furnace does not fire up either. I plan on eliminating the thermostat to see if the furnace would turn on by touching the two wires together, but saw your email and decided to write to you first.

The other two zones do heat up when heat is called. This is an American Standard system that was installed new in 1960. The pump has been changed a few times over the years. Any suggestions? - anonymous, Union NJ


Your heating system baseboard, hot water piping, or one or more radiators may have become airbound if:

Your thermostat for the heating zone(s) involved is calling for heat, the boiler is hot, the circulator pump is running, but all or part of the heating zone served by that circulator pump is not getting hot. More than one problem could cause this symptom but below we list possible causes in order of probability:

  1. The heating pipe, baseboard or radiator has become air-bound: this means that a sufficient volume of air has become trapped in the radiator or hot water distribution piping or baseboard piping so that the circulator is unable to cause hot water to circulate through the heating distribution system.

    Keep in mind that most circulator pumps do not have a lot of "lift" or water pushing ability. To be able to move hot water around the heating piping loop the circulator pump depends on the piping being full of water, so that water "falling" on the return side of the heat piping loop reduces the "lift" needed to be provided by the circulator pump.

    If this is the problem your heating system is having, the article below explains how to cure the difficulty as well as how to prevent it from happening again.
  2. A check or control valve in the heating water piping system that should be open happens to be closed, such as a stuck heating zone valve or a flo-control valve that has been manually put into the "closed" position.


  3. A room heating thermostat may be stuck, damaged, or set too high, or its wires shorted -


    On occasion a heating zone control relay could also be stuck or damaged, but we don't encounter that problem often. If the circulator keeps running even when you are not calling for heat, such as in summer (the reader's example above) there could be the problem at the thermostat, zone valve switch, or circulator pump relay switch. Typically we inspect and eliminate each of these in turn.

    Watch out: Be careful to turn off electrical power before working on thermostats or other electrical components. In addition to shock hazards, shorting a wire can add to your troubleshooting woes by blowing a hidden fuse (such as on a control board), or damaging a component such as a transformer.
  4. The circulator pump itself could be damaged or defective. Try checking the problems above first. If none of those conditions apply, you may need to replace the circulator pump or pump assembly or motor.

  5. There could be some other problem we haven't thought of but that your experienced heating service technician may point out.

    CONTACT us to add that information here to help others.

In this article we explain how to locate, inspect, diagnose problems with, 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.

This article series is divided into these main sections:

Service Procedures: How to Fix an Air-Bound Hot Water Heating System

Here we describe the procedure that a heating service technician may use 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.

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 here.

If your heating system does include both automatic and manual air-bleeder valves it is possible that you can correct a noisy or airbound heating system yourself.

See AIR BLEEDER VALVES where we explain how to find and use these to bleed air out of various spots in a hot water heating system.

Airbound Heating System Relief Procedure #1 Using Water Feeder & Boiler Drain

LARGER VIEW of a heating boiler cad cell relay switchWe use this procedure to remove air blocking heating water flow through baseboards or radiators when there is no convenient air bleed valve already installed on the airbound section of heating baseboard or radiator.

Of course if your heating system already has air bleeder valves installed on high sections of baseboard or radiators (higher hydronic heat delivery baseboards or radiators are the more likelyi ones to become airbound as air naturally migrates upwards to highest piping in the heating system) you should try opening one or more of those first to see if you can bleed out the air.

See AIR BLEEDER VALVES for more detail.

The following procedure is the more simple of the two we describe for correcting an air-bound hot water (hydronic) heating system, and it avoids the need to use pumps or to install extra service drains that may have been omitted on the heating system.

While this is the easiest and simplest procedure to remove air from an airbound heating system, you might not want to use this method if

In these cases see Procedure #2


Here are the steps in airbound heat remedy #1

  1. Turn off the heating boiler, using the service switch. If necessary,

  2. Confirm that the heating system appears to be air-bound: heat is on and boiler temperature is up and the circulator pump is running; by touch the technician confirms that one or more sections of radiator, convector, or heating baseboard remain cold even though all radiator or convector or other circulating system valves are in the open position.
  3. Heating boiler drain (C) Daniel Friedman

    Connect a garden hose to the boiler drain
     and run the end of the hose outside or to a convenient indoor building drain. Open this drain.

    Our photo (left) shows a typical boiler drain valve - this one has been leaking, as you can see by the stain on the floor and the mineral deposits on the drain valve.

    All boiler drains have threads that accept a standard garden hose.

  4. LARGER VIEW of a heating boiler cad cell relay switchOpen the water feeder bypass: Locate and open the bypass or "over-ride" on the automatic water-feeder pressure-reducer on the heating boiler.

    On a residential heating boiler the automatic water-feeder/pressure reducing valve that automatically provides makeup water to the heating boiler if pressure drops below 12 psi. (12 psi is for typical U.S. / Canadian residential heating systems normal cold temperature starting pressure. U.K. and european heating systems should be pressurized to between 1 and 1.5 bar - cold.)

    Usually this valve has a lever that can be lifted to temporarily bypass the pressure-reducing function and feed water directly to the heating boiler at street water pressure (up to 70 psi).

    See WATER FEEDER VALVES, HYDRONIC BOILER for details about how this valve works.

    Lift the lever to feed high pressure water into the heating boiler. This should also force high pressure water through the heating distribution pipes, radiators, convectors, or baseboards, forcing air out of those components.

    Pressure in the boiler should not exceed 30 psi (otherwise the pressure/temperature relief valve will begin to spill water). Let the water "run" through the heating system, watching the hose output end for evidence that air along with water is spurting out of the hose.
  5. Close the water feeder bypass and quickly move to the next step - just below. This will stop forcing higher-pressure water into the boiler and the heating system piping.
  6. Close the boiler drain valve. This will stop water from leaving the boiler.
  7. Set the proper boiler cold water pressure: The automatic water feeder will put additional water into the boiler until it reaches its starting pressure.

    If the boiler pressure is below its normal level the automatic water feeder should correct this problem. Watch the boiler pressure fill up to its normal cold pressure setting - typically this is around 12 psi on a two story home.

    If the boiler pressure is too high and the boiler is cold, use the boiler drain to drop the boiler pressure to the proper starting level.

    See WATER FEEDER VALVES, HYDRONIC BOILER for a description of the typical pressures needed in residential hot water heating systems depending on the height of the highest radiator or baseboard above the heating boiler.
  8. Turn on the heating boiler and assure that the thermostat is calling for heat. When the system has reached normal operating temperature and pressure, check the radiators, convectors, or baseboard sections that were previously cold - they should now be warm.

    If the previously cold radiator (etc) still remains cold, either you have not removed enough air from the system or there is another problem causing loss of heat. In that case

  9. Check the boiler drain valve to be sure it's not leaking. In an emergency we screw a garden hose cap on the end of a leaky heating boiler drain.
  10. Monitor heating system operation: we never leave a property where we have worked on the heating system without first checking for leaks, inspecting for obvious safety hazards (such as a bad relief valve, blocked flue, improper oil or gas burner operation), and confirming that the heating system runs through it's on-off cycle normally.

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

Contact us if you have other suggestions for improving this procedure. We are pleased to give credit and links to contributing reviewers, authors, or critics.


Continue reading at AIR BOUND HEAT SYSTEM REPAIR by PUMP or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or 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.

Or see AIRBOUND HEAT SYSTEM REPAIR by WATER FEED VALVE FAQs - questions & answers posted originally at this page.

Or see these

Articles on Fixing Air-Bound Heatiing Systems

Suggested citation for this web page

AIRBOUND HEAT SYSTEM REPAIR by WATER FEED VALVE at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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