LARGER VIEW of a heating boiler circulator pump setHeating System Circulator Pumps & Circulator Relays

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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.

This article series answers most questions about Heating System Boiler Controls on central heating systems to aid in troubleshooting, inspection, diagnosis, and repairs.

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.

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Heating Circulator Pumps: how are circulators used to control heating zones & how do we Diagnose & Fix Circulator Pump Troubles?

B&G Red Circulator Pump (C) Daniel FriedmanHeating circulator pumps or "zone circulators" are used to force hot water from the heating boiler through radianting devices such as hot water baseboards or radiators. The circulator is switched on as needed or in some designs may be wired to run continuously.

Our photo at left of a red B&G heating circulator pump shows equipment more than twenty years old and still spinning along nicely.

Proper installation, protection from leaks, and lubrication at annual service can give a long circulator life. Poor maintenance or improper installation can give less happy results.

[Click to enlarge any image]

After a brief introduction we describe what goes wrong (or how to get things to go right with heating zone circulator pumps. We also link to related articles for circulator choices, installation, troubleshooting, repair or replacement. Also see Checks for Circulator Operation.

Article Series Contents

How Hot Water Heating Circulators Work: One vs Multiple Circulators

Multiple heating zones with individual circulators (C) Daniel Friedman

Hot water may be circulated throughout multiple zones using a single circulator pump and individual zone flow control valves, or each heating zone may be built with its own individual circulator pump.

Either approach to individual heating zone control can work just fine - using zone valves or using individual circulators.

Our photo above shows a single circulator system (no zone valves are in the photo - this may be a single-zone heat system) while at left our photograph shows a three-zone heating system with three B&G circulators in a home in Two Harbors, MN.

You'll notice that one of the circulators has been replaced with a newer Bell & Gossett Circulator SLC-30.

See MULTIPLE HEATING ZONE CONTROL for as much argument as you can stand about multiple circulator pumps versus multiple zone valves for heating zone control.

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

Leaky circulator pump (C) Daniel Friedman

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.

[Click to enlarge any image]

The circulator relay is an electrical switch which, in response to a request for heat from a thermostat, turns on the circulator pump. See RELAY CONTROL SWITCHES for details about how the circulator pump relay operates.

Single Circulator Heating Systems, One or Multiple Heating Zones

Some heating systems use a single circulator to move hot water through the building's heating devices (convectors, radiators or baseboards). 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.

Also see ZONE VALVES, HEATING - home.

Even a single zone heating system might use two circulator pumps. In this case the second circulator is being used to control a separate indirect-fired water heating system to provide domestic hot water for washing and bathing.


Multiple Circulator Heating Systems, Multiple Heating Zones

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.

Details are at CIRCULATOR PUMP RELAYS & OTHER CONTROLS. Also see RELAY CONTROL SWITCHES for details about how circulator relay controls work.

Mixed Multiple Circulator Heating Systems Plus Zone Valves

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. We often see this arrangement when the building owner/manager wants to sub-divide an existing heating zone into multiple zones of control.

Circulator pump leak at mounting flange (C) Daniel Friedman

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.

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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 (above left) shows leaks at the circulator pump mounting flanges. Notice those rust stains on the bottom circulator mounting flange. [(CIRCULATOR PUMP LEAKS)

Circulator pump mount (C) Daniel Friedman

Reader Question: radiator exploded after circulator pump repair

(Jan 30, 2014) Heather said:

I have a boiler for heat in my home with two circulator pumps that supply two different zones (upstairs and downstairs). My circulator pump for the first floor went bad and was replaced, fixing the problem. The boiler itself had no issues.

An hour later, a radiator upstairs (different zone supplied by the other circulator pump) exploded and a 6 inch piece of metal was cast from the radiator causing water to flood the house. This happened in a different zone connected to the other pump from the same boiler.

Could the exploded radiator in the other zone have been affected by the replacement of the circulator pump to the other zone? I cant understand how this coincidentally happened when there seemed to be no problems with the supply to the zone on the second floor where it happened. Any feedback is appreciated.

This question was originally posted at HEATING SYSTEMS



This sounds horrible - and peculiar. Forensic investigators start by disbelieving coincidences.

But it's not clear why a radiator would explode under any circumstances. Really "explode" ? We're talking about hot water heat, at normal pressures under 30 PSI.

Watch out: if your heating system pressures were abnormally high (over 30 psi) then either your system was missing a critical safety device - a Temperature/pressure relief valve at the boiler - or the valve was installed but was jammed, or subverted, or not working.

IF that is is the case this is a VERY DANGEROUS condition as an exploding boiler can cause a BLEVE explosion of tremendous force. (Search InspectAPedia for Bleve explosion to read Details).

In short, a normal hot water heating circulator pump does not have great pumping power; if there was an overpressure problem in the system it seems more likely it came from another component. Naturally everbody involved in working on your system will be scared to admit fault. Focus on a thorough inspection of the heating boiler starting with

- the location, type and condition of pressure/temperature relief valve
- the normal operating pressure and temperature of the boiler

If on your own, before the heating company shows up, if you see high pressure readings on the boiler gauge (30 psi or above) or if you see water spilling out of a relief valve I would SHUT OFF THE SYSTEM immediately and would get the heck out of there.


The PSI was not high before the repair. It was around 12. After the repair and the flooding upstairs, it is running around 18 psi now. I did look at the system this morning.

The only problem I see is that a pressure reducing valve has a slow drip from the bottom. The valve for the intake had been turned off last night to stem the flooding and then turned back on after the flow to that radiator was capped so I don't know if that affected the valve since it is about 10 inches away from the cutoff.


12 PSI cold is normal for a typical 2 story home.

BTW I would not want to confuse an air bound system with one that is at too low pressure.

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

Honeywell room thermostat

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

Question: circulator pump cost


Question: circulator mounting position vs system pressure


How to Hook Up Multiple Circulators or Zones

We moved this discussion to a new article: please see MULTIPLE HEATING ZONE CONTROL

Question: circulator pump won't start

We moved this discussion to a new article: please see CIRCULATOR WONT RUN

Question: circulator pump runs intermittently

We moved this discussion to a new article: please see CIRCULATOR PUMP RUNS INTERMITTENTLY

Weak Heat Questions

We have moved this discussion to a new article: please see WEAK HEAT, WEAK CIRCULATOR, TOO COOL

Question: Circulator Pump Noises, heating system noises

We have moved this discussion to a new article: please see CIRCULATOR NOISES.

Also see HEATING SYSTEM NOISE DIAGNOSIS for a more broad range of heating noise diagnosis & repair procedures.

Question: circulator pump runs even when we are not asking for heat



Continue reading at BYPASS a BAD CIRCULATOR or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.



Or see CIRCULATOR PUMP REPAIR FAQs for diagnostic questions & answers about heating circulator pumps & circulator relays.

Or see HOT WATER CIRCULATOR PUMP SPEEDS DELIVERY using a hot water circulator on domestic hot water supply to fixtures

Or see these

Hot Water (Hydronic) Heating Boiler Circulator Articles

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CIRCULATOR PUMPS & RELAYS at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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