Guide to Heating System Circulator Pumps & Circulator Relays - Home Page
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Circulator Pumps: Hot Water Heating System Circulator Troubleshooting & Repair Guide
Circulator pump relay switches & controls on heating systems: Troubleshooting & Repair Guide
How to tell if a heating circulator pump is working
Why do some heating zones work and others not, why does heat come out of zones where thermostats are not calling for heat?
How to fix an air-bound heating system and blocked circulator pump
How to cure circulator pump noises
Where should a circulator pump be installed on a heating boiler? On the inlet or on the outlet side of the boiler?
Use of air exit tanks and circulator pumps on heating boilers
Questions & answers about installing, inspecting, & troubleshooting hot water heating system circulators

Hot water heating circulators or circulator pumps: install, troubleshoot, repair advice: this article series discusses Circulator Pumps: how to find, inspect, diagnose, and repair problems with Hot Water Heating System Circulator Pumps or circulator pump relay switches and controls. Also you will see that This website answers most questions about Heating System Boiler Controls on central heating systems to aid in troubleshooting, inspection, diagnosis, and repairs.

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Which is better: heating control using zone valves or heating control using individual circulators?

Also see additional circulator diagnostic checks at Checks for Circulator Operation. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.

Either approach to individual heating zone control can work just fine - using zone valves or using individual circulators. Some heating contractors prefer using individual circulator pumps, one per zone, perhaps because they recall the history of unreliable zone valves which tended to clog or jam in some models by some manufacturers. The "zone valve" fellows retort that modern heating zone valves are as reliable or more reliable than [some models of] heating circulator pumps and that a zone valve costs a lot less.

Adding zone control to an existing heating system may be less costly by adding a zone valve than by adding a circulator pump.

What goes wrong with hot water heating circulator pumps and circulator switches?

The heating system circulator pump, such as the trio of pumps shown at the top of this page, is used to move hot water from the heating boiler out through one or more loops of piping in a building, through heating devices such as radiators, heating baseboards, or convector units, then through return piping back to the heating boiler. When the water temperature drops to a pre-set level the heating boiler will re-heat the water.

The circulator relay is an electrical switch which, in response to a request for heat from a thermostat, turns on the circulator pump.

Some heating systems use a single circulator to move hot water through the building's heating devices. In a one-circulator system, the building may still divide its heat into various zones or sub-areas of individual heat control, by using either individual radiators in rooms or perhaps by using electrically controlled zone valves which open and close flow of hot water through sub-loops in the building heating piping.

Some heating systems use multiple circulators to provide heat to individual building areas or "zones". In this case each heating zone will have its own thermostat which, acting as a low-voltage "heat on-off switch" will turn on individual circulator pumps when heat is desired in that zone.

Less common are mixed heating zone systems in which multiple circulators are used but one or more of the circulators feeds a heating water pipe which is subsequently divided into additional sub-zones of heat control, each sub-zone being controlled by a zone valve.

Before assuming the heating system is not working when the thermostat is calling for heat, feel the radiators or baseboards to see if they are getting hot. On a call for heat at the thermostat, if the heating boiler is already hot (above the lower limit or cut-in temperature) then the circulator should turn on and move hot water to the baseboards or radiators.

Only after the boiler temperature drops below the cut-in temperature will the heating boiler turn on to re-heat the water. The connection between the thermostat and the heating boiler turning on or off is indirect. On most heating systems the thermostat turns the circulator pump on or off and the temperature of the heating water turns the heating boiler on or off.

Our photo (left) shows severe leaks at the circulator pump mounting flanges.

The circulator pump may not be running at all. The result can be no heat delivered to the occupied space, or heat may be delivered but only very slowly, as hot water is rising through the heating system by convection rather than being forced (more quickly) by the circulator pump assembly.

Because some circulator pumps are very quiet and may have no visibly moving parts (Taco® for example), below we explain how to determine whether or not the circulator is working at all.
The circulator pump may be running but heat does not enter some building areas - part of the system may be air-bound. We introduce this topic below and explain how to correct an air bound heating system at Air-bound boilers & radiators
The circulator pump may be leaky - often leaks occur at the circulator pump mounting flanges - a leak we describe in detail at Leaks at Circulators. (See our photo, above left).
The circulator pump motor may run but the pump may not be moving water - this less common fault occurs if the circulator pump vanes or other internal parts have become damaged. The motor will run but the circulator does not pump. If the circulator motor runs but there is no temperature change on the heating system piping on the inlet and outlet side of the pump then this could be the problem.

The circulator pump is noisy - this can be due to a variety of problems. Start by making sure that your system does not contain excess air since this is a very common hydronic heating complaint. See these three articles on purging air from hot water heat systems: Air Bleeder Valves and Air-bound boilers & radiators and Air Scoops Purgers Separators.

A bearing could be failing in the circulator pump; in this case the pump will need repair or replacement. If you are mounting a new circulator pump assembly in an existing bracket,for example when just the pump impeller and housing are being replaced, make certain that the pump is properly mounted and supported in position.

We found an entire series of circulator pump failures at the same mounting point (photo, above-left) because every replacement had been mounted askew, placing stress on the Bell & Gossett circulator pump shaft.

For some circulator pump models, particularly if the circulator pump is used in a multiple-pump installation, the pump's internal check valve could be a noise source. Tac o, a manufacturer of circulator pumps and other heating system equipment, says that circulator noise can be due to the built in check valve on a radiant heating system and that the fix is to remove the check valve from the pump.

Question: I recently installed a Radiant Mixing Block to heat 2 zones of radiant floor. When the block injection pump injects at the 20–40% range, it makes a growling noise that sounds similar to dry bearing. Any suggestions?

Answer: The check valve in the inlet connect from the boiler supply line is making the noise. To prove a point, put an ice cube on the radiant supply sensor which should make the injection pump speed up. If the noise goes away when the pump speeds up, then it is the check.

To eliminate the noise, break the hot water supply connection and unscrew the adaptor fitting from the block manifold. Look inside that connection and you should see the check and a snap ring holding it in place. Remove the snap ring and then the check. Reinstall the adaptor fitting and re-splice the copper tubing.

This noise problem has occurred once in a while and we are developing a new check that should overcome it. At this time, this is our only solution and I have not heard of any problem operating without the check. -- Taco FAQ 01/15/2009
High Mineral-Content Water & Circulator Pump Problems - in a closed hydronic system, because the same water remains in the system forever except when emptied for service (or if there is a leak), the mineral content of the original water loaded into the heating system is not normally a problem.

In open pumping systems where new water is constantly introduced, mineral content in the water can clog and damage circulator pump internal moving parts. Here is a diagnostic example from Taco.

It is very unusual for a circulator to fail, but there are 4 basic modes of failure:
1. Oxygen in the system oxides the rotor laminations. The corrosion build up acts like a brake and stalls the rotor of the pump.
2. A weakness in the motor winding.
3. A loose wire or connection that needs to be repaired.
4. If there are many minerals in the water, they plate out on the rotor and eventually stall it. [DF - note that this will be a problem on open systems but not on a closed system that does not take in new water]
If the problem is a stalled circulator pump rotor, Taco has a replacement cartridge. If the motor is bad, you'll need a new pump. If you drain your system and put in fresh water, or if you have a leak where new water is being added all the time, oxygen and minerals will be continually added to the system. Whatever the reason, it is unusual for a 00 pump to fail.
Circulator pump location - where to put the pump, on the supply side or return side of the boiler? We were taught that circulator pumps may have a longer life and run cooler if placed at the return side of the boiler where the returning heating water is cooler than it was on first leaving the supply side of the boiler. But at least some manufacturers say it doesn't matter. Here is Taco's position on circulator pump position:

All of the "00" pumps can be used on the supply side or the return. It is preferable to put a circulator on the output of a boiler, but the expansion tank should be between the boiler and the pump section. Most of our "00" iron pumps have a max water temperature of 230ºF. The max for the bronze is 220ºF. 160ºF is cold. No matter what the water temperature is, the motor housing will be hot. There will be no change in performance or life. - Taco FAQ
Circulator pump is not capable of pumping against the head pressure of water in the heating system: each manufacturer provides a table of their circulator pump models and the head pressure against which the pump can work, at various flow rates. Many residential heating circulator pumps do not have a large "lift" capacity and depend on the heating piping to be full of water without any air blockages in order to function satisfactorily. Increasing the "cold" pressure of the water in the heating system may be necessary, especially in buildings taller than two stories. An alternative is to connect an additional circulator pump in series with the first one.

The gallons per minute (GPM) that a pump puts out are dependent on the head produced by the pump. The system resistance is the controlling factor in determining the flow produced by a pump. As the flow increases in a piping system, the resistance or pressure drop rises at the rate of the square of the ratio of the flows.

For example, if the resistance of the system is 2 ft. at 3 GPM, at (6 gpm/3 GPM)² x 2 ft.= 8 ft. Using this equation we can generate a system resistance curve and plot it on the pump curve. The point where the system resistance curve and the pump curve intersect is the maximum flow that the pump will produce in that piping system.

If any [circulator] pump is put in series with another pump, the heads are additive at the same flow. To increase the flow, the [circulator] pumps have to be put in parallel. With pumps in parallel, the flows are additive at the same head. When [circulator] pumps are put in series, install a nipple between 2 flanges. Bolt the discharge of one [circulator] pump to one flange, and the suction of the other [circulator] pump to the other flange." - Taco FAQs

If the boiler runs but the heating baseboard or radiators in an area do not get hot, the problem could be

The thermostat - where multiple thermostats and heating zones are installed, the boiler may be running but the thermostat for a given zone may not be calling for heat; or the boiler may be running to make domestic hot water and even a single zone may not be calling for heat. See THERMOSTATS.
The zone valve - may be stuck closed, if your system is using zone valves. See Zone Valves
The heating system may be air-bound - part of the system piping, baseboards, convectors, or radiators may be air-bound. We introduce this topic below and explain how to correct an air bound heating system at Air-bound boilers & radiators
The circulator pump itself may have stopped working - some common circulator pump brands shown at this website in our diagnostic articles include Bell & Gossett™, Grundfoss™, and Taco™ - these are discussed below.

If the heating boiler itself if does not turn on in a response to a call for heat see NO HEAT - BOILER.

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Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

  Circulator Pumps & Relays
    Circulator Pumps vs. Zone Valves
    Circulator Pump Trouble List
    Checks for Circulator Operation
    Single Zone Heat Tests
    Multi-Zone One Circulator Tests
    Multi-Zone Multi-Circulators Tests
    Individual Zone Tests
    Un-wanted Heat in Some Zones
    Leaks at Circulators
    Circulator Motor or Bearing Failures
    Where to Mount Circulator Pumps
    Other Circulator Controls

"Installation and Operation Instructions, Raytherm Residential Boilers," courtesy of Raypak®, and technical advisor Wayne Hoffman, personal communication 5/11/2009. Mr. Hoffman is a technical advisor with more than 30 years experience in the heating field. Raypak is a Rheem company that provides hydronic heating boilers for residential, pool, and commercial use. Raypak can be contacted at 866-583-0664 for technical support or for assistance in selecting the proper heating equipment for a specific application. Rheem Corporation is a manufacturer of water heaters and heating equipment including Ruud heating and cooling products.
Bell & Gossett Air Separators and other heating system components, 8200 N. Austin Ave., Morton Grove IL 60053, USA - Tel 847 966-3700 Fax 847 965-8379 . Air separators are discussed by B&G at http://www.bellgossett.com/homeowners/BG-airseparator.asp
Taco air scoops and other heating system components & controls: Taco Inc., 1160 Cranston Street, Cranston, RI 02920 / (401) 942-8000 / Fax (401) 942-2360 Taco (Canada) Ltd., 6180 Ordan Drive, Mississauga, Ontario L5T 2B3 / (905) 564-9422 / Fax (905) 564-9436 www.taco-hvac.com
Taco also has some helpful FAQs that are useful in diagnosing circulator problems - see http://www.taco-hvac.com/en/faq.html#
Sparco air purgers, air scoops, and other heating system components & controls:

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.

The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 2010, $69.00 U.S., is available from Carson Dunlop. The Home Reference Book is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. InspectAPedia.com ^® author/editor Daniel Friedman is a contributing author. Field inspection worksheets are included at the back of the volume.

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.)
Links to our list of additional information on heating system inspection, repair, maintenance
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Which is better: heating control using zone valves or heating control using individual circulators?

What goes wrong with hot water heating circulator pumps and circulator switches?

If the boiler runs but the heating baseboard or radiators in an area do not get hot, the problem could be

Questions & Answers regarding this article

Ask a Question or Search InspectAPedia

Technical Reviewers & References

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