Guide to Fixing an Air-Bound Hot Water Heating System - Procedure#1, Using Built-In Controls & Valves
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Service Procedures to force air out of an air-bound hot water heating system
How to diagnose cold heating baseboards or radiators
How to diagnose heating circulator pumps that won't stop running
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
Questions & answers about fixing an air-bound hot water heating system

Airbound 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 valve on 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.

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Diagnosing an Air-Bound Hot Water Heating System

If heating radiators or 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 here can help diagnose and cure that problem.

Readers should also see Diagnose Oil Heat Noises for diagnosis and repair of other heating system noises on both oil and gas fired heating equipment. This website 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 you don't know what kind of heat your building uses, we explain how to figure out the answer at HEATING SYSTEM TYPES. 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

Reply:

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:

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.
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 (see ZONE VALVES) or a flo-control valve that has been manually put into the "closed" position (see CHECK VALVES, HEATING SYSTEM).
A room heating thermostat may be stuck, damaged, or set too high - see THERMOSTATS. 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.
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.
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 is divided into these main sections:

Our discussion of radiators or baseboards that do not get hot when they should includes these key sections:

Air Bleeder Valves - a Guide to Air Bleeder Valves for Hot Water Heating Systems: Radiators, Baseboards, Convectors - how to find and use manual and automatic air bleeders to fix noisy gurgling heating pipes or an airbound heating system. Air purges for steam heating systems are discussed separately at Steam Vents.
Air Scoops, Air Separators, Air Purgers: Since on many hot water heating systems a key air bleeding or air vent point is at the air scoop or air separator closer to the boiler, see Air Scoops Purgers Separators.
Airbound Heating System Relief Procedure #1 Using Water Feeder & Boiler Drain - how to use built-in controls and valves to force air out an air-bound heating system whose radiators, convectors, or baseboards are not warming up
Airbound Heating System Relief Procedure #2 Using a Pony Pump - how to use a portable pump, short sections of garden hose, and heating system valves to force air out an air-bound heating system whose radiators, convectors, or baseboards are not warming up.
WATER FEEDER Valves, Hydronic - a defective automatic water feeder valve on a hot water heating boiler can result in too little starting water pressure in the system - radiators on upper floors may fail to receive heat. On a steam heating boiler a defective automatic water feed valve can cause the heating system to shut down completely or can lead to boiler damage or even unsafe conditions.

At BOILER PRESSURE & TEMPERATURE SETTINGS we discuss the pressures needed for hydronic or hot water heating systems - a taller building needs higher starting (cold) pressure in the heating boiler. At WATER FEEDER Valves, Hydronic we discuss problems with water feeder valves that also control water pressure in the boiler and we explain how to adjust the water feeder valve and thus boiler water pressure. If your heating system uses a steam boiler, see WATER FEEDER Valves, Steam.

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 and then How to Open Manual Air Bleeder Valves.

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

We 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 (see 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.

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

The boiler is very hot and it is a cast iron unit that could be cracked by a sudden surge of incoming very cold water
The heating system has been filled with antifreeze.
The heating boiler drain (located at or near the bottom of the boiler) is in poor condition and may not be opened and then closed reliably.

In these cases see Procedure #2 at Air-bound heating systems part-2.

Here are the steps in airbound heat remedy #1

Turn off the heating boiler, using the service switch. If necessary, see ELECTRICAL POWER SWITCH FOR HEAT.
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.
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.
Open 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.
Close the water feeder bypass and quickly move to step 6 just below. This will stop forcing higher-pressure water into the boiler and the heating system piping.
Close the boiler drain valve. This will stop water from leaving the boiler.
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.
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 see HEATING LOSS DIAGNOSIS-BOILERS.
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.
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?

If you open a manual air bleeder valve on a hot water heating system and air hisses out, there was air that needed removal. If only water comes out, that device was not the one that is air bound.
If the heating boiler is already running and hot, quite quickly, in a minute or three, the radiator or convector that was air bound will get hot to the touch. Feel first at the pipes that enter the radiator, convector or heating baseboard since that's where hot water will begin entering the previously air-bound device.

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.

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BOILERS, HEATING
BOILER OPERATION DETAILS
BOILER CONTROLS & SWITCHES
Air Bleeder Valves
Air-bound boilers & radiators
Air-bound heating systems part-2

Thanks to reader Les Luka for pointing out that we needed to clarify step 5-6 in the air bleed procedure, 08/03/2010
Thanks to (anonymous) reader for discussing circulator run-on problems, October 2010

For details about the setting, re-setting, or function of the controls and switches commonly found on hot air heating systems see these articles:

Hot Air Heating Furnace Basic Operating Steps
Guide to Heating System Components: see BOILER CONTROLS & SWITCHES and FURNACE CONTROLS & SWITCHES and STEAM HEATING SYSTEMS or if appropriate for your building, RADIANT HEAT. If your building uses a heat pump, see AIR CONDITIONING & Heat Pump SYSTEMS
Air Bleeder Valves for Hot Water Heating Systems: Radiators, Baseboards, Convectors
Aquastats Heating Boiler Primary Controls: A guide to Aquastats for control of hot water heating boilers and tankless coils
CAD CELL RELAY SWITCH on oil fired furnaces or boilers as flame sensors & safety devices
Check Valves Guide to check valves and backflow preventers on hydronic hot water heating systems gas or oil fired
Draft Hoods on gas fired heating equipment, function and safety
Draft Regulators & barometric dampers on oil fired heating equipment
Expansion Tanks Guide to pressure control and expansion tanks on hydronic hot water heating systems gas or oil fired
Fan Limit Switch on hot air furnace heating systems gas or oil fired
Flue Gas Spill Switch on gas fired heating equipment sense combustion and protect from blocked flues
Gas Piping, Valves, Controls including gas regulator assembly and other controls
MIXING / ANTI-SCALD VALVES: Guide to mixing valves on hot water heating systems and radiant heat systems
Noises: see Diagnose Oil Heat Noises
Power Switches to turn on or off heaters for furnaces & boilers, oil & gas fired, heat pumps or electric furnaces or boilers
RELIEF VALVES - TP VALVES: A guide to temperature and pressure relief valves & safety controls on hot water and steam heating systems
Stack Relay Switches Explained on oil fired furnaces or boilers as flame sensors & safety devices
Steam Vents provides details about air vents or steam vents on steam heating systems and diagnoses hissing sounds and failure of a steam radiator to get hot.
Thermostats & Heat Controls for furnaces & boilers, oil & gas fired, heat pumps or electric furnaces or boilers
Water Feed Valves: A guide to water-feeding/pressure-reducing valves on hot water and steam heating systems
Zone Valves: A guide to zone valves for heating zone control on hot water heating systems

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
Carbon Dioxide Gas Toxicity
Carbon Monoxide Gas Toxicity, exposure limits, poisoning symptoms, and inspecting buildings for CO hazards
DUST CONTAMINATION FROM HVAC? An Investigation of Indoor Dust Debris Blamed on a Heating/Cooling System Reveals Carpet Dust
Fuel Oil & Oil Heating Magazine, 3621 Hill Rd., Parsippany, NJ 07054, 973-331-9545
Goodman Furnace High Temperature Plastic Vent HTPV safety recall US CPSC notice
Home Heating System Should Be Checked [for proper venting and for CO Carbon Monoxide Hazards - DJF]
Inspection Procedures for Oil-Fired Heating Systems Detailed step by step approaches for inspecting complex systems]
Lennox Pulse Furnace Safety Inspection/Warranty Program: Carbon Monoxide Warning
Oil Tanks - The Oil Storage Tank Information Website: Buried or Above Ground Oil Tank Inspection, Testing, Cleanup, Abandonment of Oil Tanks
Oil Tanks Above Ground, UL Standards, guidance for home owners, buyers, and inspectors
Plastic Heating Vent Pipe & Other Heating Safety Recall Notices
Weil McLain Model GV Gas Boiler/gas valve CPSC recall/repair
Domestic and Commercial Oil Burners, Charles H. Burkhardt, McGraw Hill Book Company, New York 3rd Ed 1969.
National Fuel Gas Code (Z223.1) $16.00 and National Fuel Gas Code Handbook (Z223.2) $47.00 American Gas Association (A.G.A.), 1515 Wilson Boulevard, Arlington, VA 22209 also available from National Fire Protection Association, Batterymarch Park, Quincy, MA 02269. Fundamentals of Gas Appliance Venting and Ventilation, 1985, American Gas Association Laboratories, Engineering Services Department. American Gas Association, 1515 Wilson Boulevard, Arlington, VA 22209. Catalog #XHO585. Reprinted 1989.
The Steam Book, 1984, Training and Education Department, Fluid Handling Division, ITT [probably out of print, possibly available from several home inspection supply companies] Fuel Oil and Oil Heat Magazine, October 1990, offers an update,
Principles of Steam Heating, $13.25 includes postage. Fuel oil & Oil Heat Magazine, 389 Passaic Ave., Fairfield, NJ 07004.
The Lost Art of Steam Heating, Dan Holohan, 516-579-3046 FAX
Principles of Steam Heating, Dan Holohan, technical editor of Fuel Oil and Oil Heat magazine, 389 Passaic Ave., Fairfield, NJ 07004 ($12.+1.25 postage/handling).
"Residential Steam Heating Systems", Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
"Residential Hydronic (circulating hot water) Heating Systems", Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
"Warm Air Heating Systems". Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
Heating, Ventilating, and Air Conditioning Volume I, Heating Fundamentals,
Boilers, Boiler Conversions, James E. Brumbaugh, ISBN 0-672-23389-4 (v. 1) Volume II, Oil, Gas, and Coal Burners, Controls, Ducts, Piping, Valves, James E. Brumbaugh, ISBN 0-672-23390-7 (v. 2) Volume III, Radiant Heating, Water Heaters, Ventilation, Air Conditioning, Heat Pumps, Air Cleaners, James E. Brumbaugh, ISBN 0-672-23383-5 (v. 3) or ISBN 0-672-23380-0 (set) Special Sales Director, Macmillan Publishing Co., 866 Third Ave., New York, NY 10022. Macmillan Publishing Co., NY
Installation Guide for Residential Hydronic Heating Systems
Installation Guide #200, The Hydronics Institute, 35 Russo Place, Berkeley Heights, NJ 07922
The ABC's of Retention Head Oil Burners, National Association of Oil Heat Service Managers, TM 115, National Old Timers' Association of the Energy Industry, PO Box 168, Mineola, NY 11501. (Excellent tips on spotting problems on oil-fired heating equipment. Booklet.)
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Guide to Fixing an Air-Bound Hot Water Heating System - Procedure#1, Using Built-In Controls & ValvesInspectAPedia® -

Diagnosing an Air-Bound Hot Water Heating System

Question: Why does my heating circulator keep on running?

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

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

Here are the steps in airbound heat remedy #1

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

Questions & Answers regarding this article

Ask a Question or Search InspectAPedia

Technical Reviewers & References

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

Guide to Fixing an Air-Bound Hot Water Heating System - Procedure#1, Using Built-In Controls & Valves
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