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
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
Our site offers impartial, unbiased advice without conflicts of interest.
We will block advertisements which we discover or readers inform us are associated with bad business practices,
false-advertising, or junk science. Our contact info is at
InspectAPedia.com/appointment.htm.
Here we discuss where the circulating pump can or should be mounted on a hot water or hydronic heating boiler. 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. The heating boiler piping diagram at page top, courtesy of Raypak® shows the heating system circulator pump assembly (red in our sketch) on the return or inlet side of the heating boiler.
Where to Mount the Circulator Pump on a Heating Boiler
We often receive the question "Where should the heating boiler circulator pump be mounted: on the supply or boiler outlet side (the hot side) or on the return or boiler inlet side (the cool side) of the heating piping loop?"
Read the instructions: The short answer is, mount the circulator and arrange all other piping and controls according to the boiler manufacturer's installation and operating instructions and you won't go wrong. A sample boiler installation and operation manual that is particularly clear and easy to follow is provided by Raypac® but of course you should obtain and follow the instructions from the maker of your particular boiler maker.
On the boiler return or inlet side: most but not all residential heating boilers and probably most commercial heating boilers mount the circulator pump on the return or inlet side of the heating boiler. This location is a traditional one preferred by some old-school boiler installers and service people, dating from a time when a boiler came from the manufacturer without the circulator pre-installed on piping attached to the unit.
Some heating service technicians (including the web publisher) opine that the cooler heating water temperature on the return side of the boiler means that the circulator pump and its bearings may have a longer life than if installed on the hot side. Typically heating water returning to a boiler after circulating through a building is about 20 degF. cooler than water leaving the boiler. The heating boiler piping diagram at page top, courtesy of Raypak® shows the heating system circulator pump assembly (red in our sketch) on the return or inlet side of the heating boiler. By piping convention the triangle (which we surrounded in red for emphasis) points down, showing the direction of water flow.
On the boiler supply or outlet side: as we explain just below at Air Exit Tanks, in some heating boiler installations that use an air exit tank to combine the Expansion Tank function with the Air Bleeder Valve or air purge valve function on a heating system, the air exit tank is mounted on the header piping on the supply side of the boiler and the installer may prefer to place the circulator pump below that tank to improve the air purge function of the system. Air exit tanks are not common on residential heating systems.
It does not matter which side of the boiler gets the circulator pump: ultimately, from a functional viewpoint, it does not matter one bit which side of the heating boiler has the circulator pump mounted, supply side or return side. That is because in a hydronic heating system the circulator is pushing water around a closed loop: the boiler and boiler piping are filled with water and there is (or should be) no air in the system. (If a hot water system piping or boiler contains more than a few bubbles of air the system is likely to become Air-bound and heat will be lost or missing in all or part of the building.
The circulator pump is only a water mover, not a water lifter. The hydronic heating system is relying on the starting water pressure in the heating system to provide enough lift to get hot heating water into baseboards, convectors, or radiators in the upper floors of the building. Typically the cold starting pressure in a heating system is 12 psi, provided by the Water Feeder Valve on the boiler. On tall buildings higher pressures are needed - we provide a chart of pressure reducer/water feeder valve settings at Water Feeder Valves, Hydronic Boiler.
Critique, contributions wanted: Contact Us to suggest corrections or additions to articles at this website, and if you wish, to receive online listing and credit as a contributor. Particular thanks are due to the many experts and also consumers who read and critique technical articles at InspectAPedia.com.
Additional technical contributors & reference sources for this article are listed below.
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.
"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.
Other heating system controls related to hot water circulator pumps - three alternative ways to control hot water heat circulation
Individual High Limit and Low Limit relay switches on heating boilers: these may be provided on older heating boilers. See Limit Switches, Boilers
Zone valves on heating boilers - heating water piping in a building, particularly where hot water baseboard heat is used, may be divided into separate heating zones (different floors, or different areas on a single floor) to
permit more detailed control of heat distribution in a building.
The control of heating water through these different heating zones may be accomplished
by use of zone valves (one per heating zone or area or "loop" of heating piping) which in turn are connected to individual thermostats. When the thermostat
calls for heat in a particular building area, the thermostat switch causes the zone valve to open, to permit hot water to flow through that zone.
When the zone valve is fully open, an "end switch" inside the valve tells the heating system's circulator to begin operating, causing hot water to flow
through the zone. Typically heating systems using zone valves will have two or more zone valves (usually but not always located close to the heating
boiler) and a single circulator pump (usually located on the return end of the hot water piping close to the heating boiler).
Circulator relays on heating boilers - an older and by some heating service technicians, a preferred method to control the distribution of heat to individual
building areas uses a individual circulator pump to force heating water through each individual heating zone piping. This system too is usually found
on hot water baseboard heating systems.
Two or more circulator relay switches, one relay for each circulator pump, will be installed to turn on and off
each heating water circulator if more than one heating circulator is installed. Individual low-voltage thermostats located in the living area will respond to
a call for heat by switching on the circulator relay which in turns on the (120V) circulator pump.
Air exit tanks on heating boilers and circulator pump mounting location - some heating boilers use an air exit tank mounted on the hot water header above the heating boiler to combine the function of an Expansion Tank with the Air Bleeder Valve or air purge valve to rid the hydronic (hot water) heating system of un-wanted air. Air in the heating system will often rise to a high loop of radiator or heating baseboard piping where it will block the flow of hot water, leading to an air-bound heating system and no heat in some or all of the heating system.
The air exit tank captures and vents unwanted air from its location above the heating boiler on the boiler outlet or supply side. When an air exit tank is used instead of a simpler expansion tank at a heating boiler, the boiler manufacturer may specify that the circulator pump be mounted on the boiler outlet side to improve the function of this device. Thanks to Wayne Hoffman, technical advisor for Raypak, a Rheem company that provides hydronic heating equipment, for this detail.
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Our recommended books about building design, inspection, and repair, and about indoor environment testing, diagnosis, and cleanup are at the InspectAPedia Bookstore.
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 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.)
InspectAPedia® Home & Site Map - Building & Environmental Inspection, Testing, Diagnosis, Repair, & Problem Prevention Advice: In-depth research & advice on diagnosing, testing, correcting, & preventing building defects & indoor environmental hazards. Unbiased information, no conflicts of interest.
The Mold Information Center: What to Do About Mold in Buildings, When and How to Inspect for Mold, Clean Up Mold, or Avoid Mold Problems
Environmental Inspection, Testing, & Diagnosis On-Site IAQ, Gas, Air Testing, Mold Investigation, Sick Building Diagnosis, Lab Services, & Remediation Plan Preparation - indoor air quality testing, problem source determination, supporting lab work, written remediation plan addressing removal of environmental and other hazards and prevention of their recurrence.
InspectAPedia ® Home & Site Map 
Mobile View
InspectAPedia® Home & Site Map
