Boiler Circulator Pump Not Working or Won’t Stop Running

Is anything asking it to run?

Read the relay’s lights. On a Taco SR501 switching relay, Taco says the green light should always be on, showing the power is connected and the fuse is good, and the red light comes on when a thermostat calls, sending power to that zone’s pump. If the red light does not come on, check the thermostat and its wiring. Argo’s ARM relays light “Circulator On” when the relay closes, and Taco’s Priority Zoning Circulator turns its green light on when the thermostat calls.

Zone valves call the pump. On Taco’s ZVC403 zone valve control, the system pump and boiler turn on when any zone valve opens, so a valve that never opens leaves the pump off.

The thermostat. If no light comes on with the thermostat turned up, see a thermostat not working. Grundfos’s own list for too little heat includes checking the thermostat’s setting and replacing its batteries.

It may be waiting on purpose. With Taco’s SR502 relay wired to a boiler control that has a low limit, Taco says the pump comes on only once the boiler is above that low limit. Bosch says that in summer mode, or warm weather shutdown, its Greenstar switches the heating pump off while hot water stays on.

Is it running, and does it have power?

Listen to it. On a Taco 007, Taco’s test is to put a screwdriver’s tip to the pump casing and the handle to your ear, have someone switch the boiler off and on, and listen for the pump to slow down and speed up. Taco says the pump is so quiet you would normally only hear a hum.

Read its light. Taco’s 007e shows an orange light when it is powered. With the light off and the pump not running, Taco’s causes are no power supply, a tripped breaker, which it says to check at the panel and reset if needed, a failed pump, or overheating. For overheating it says let the pump cool for some minutes, then try to restart it, and check the water and room temperatures are within its ratings. Grundfos gives the first three for an ALPHA2 whose light is off.

A breaker that trips again: see a breaker that keeps tripping. The boiler’s own switch and breaker are in a boiler not working.

A light on, but stuck

A Taco 007e with a red light. Taco says red means the pump is locked or sticking. Switch the power off and on to start its automatic release: it makes 100 attempts to restart over about 15 minutes, with a short white flash at each. If it still shows red after that, Taco’s next steps mean closing the valves, letting the pump cool and taking the motor off to clean the impeller. Taco says servicing this double insulated pump should be done by qualified service personnel. Taco’s table also gives too low a supply voltage for a red light, which a technician checks against the name plate.

A Grundfos ALPHA2. With its light on and the pump not running, Grundfos lists a rotor blocked by dirt, cleared by removing the impurities, a technician’s job, or a supply voltage that is too high.

Why pumps stick. Burnham’s Alpine control runs each pump for 20 seconds after it has sat unused for more than 7 days, to help stop the rotor seizing. In its manual summer mode, Grundfos’s ALPHA2 starts the pump for two minutes every 24 hours, to stop lime deposits blocking it.

Three speed pumps. On a Taco 007-IFC, a new motor or cartridge goes in with the pressure at 0 psi, the system at room temperature and the pump closed off by its valves or drained, Taco says. That is a technician’s job.

Air, a closed valve, or low pressure

Air. Taco’s 007e table gives a house that does not warm up, with the light on, as a system that may be air bound: vent it, and have the installer repeat the fill and purge steps. Taco lists air as the cause of loud water noise with a flashing white light, too. Grundfos says you cannot vent the system through its pump, and Weil-McLain’s gas boiler manual says bleed air through the vents on the radiators or baseboard: see a radiator not heating. Taco also asks whether the system was drained lately, which can leave an air block.

The valves. For an ALPHA2 running dry, Grundfos says make sure the system is filled and vented and open the isolating valves. For too little heat, it says make sure all valves are working, with lime deposits removed. A closed zone valve is in a zone valve not working.

The pressure. For a noisy ALPHA2, Grundfos lists too low an inlet pressure: raise it and check the air in the expansion tank, a technician’s job, as pressure leaks can lead to cavitation. Taco gives cavitation from low suction pressure as a cause of a noisy 007e. Weil-McLain’s gas boiler user manual says never add cold water to a hot boiler, as the shock can crack the heat exchanger. For low pressure, it says fill to the correct pressure and have a technician repair any leak at once. See boiler pressure.

The speed setting. Grundfos lists the pump set too low as a cause of too little heat, fixed by raising its speed. Ask the installer before changing it.

A noisy pump

Air in the pump. Grundfos says air in its ALPHA2 may cause noise that stops after a few minutes running. A short spell on speed III vents it faster, and once the noise stops Grundfos says set the pump back to its recommended setting.

The rest of Grundfos’s list. In the pump: too low an inlet pressure, or no water in it. In the system: air, too much flow, which means reducing the speed, or pipes growing as they warm, which means checking their brackets.

Taco’s list for the 007e. A noisy pump with its light on can mean cavitation from low suction pressure, or something caught in the impeller, which means taking the motor off to clean it: a technician’s job. More sounds are in a boiler making noise.

A pump that keeps running

Running on after the burner stops is normal. Weil-McLain’s EG and PEG manual says that when the water is hot enough the burner shuts off and the circulator keeps running until the thermostat is satisfied. Burnham’s MPO-IQ and Series 2E have a circulator overrun that runs on after the call ends to send out the boiler’s leftover heat, and Lochinvar’s Noble runs its boiler pump on for a factory set 30 seconds.

Short runs with no call. Burnham’s Alpine exercises an idle pump for 20 seconds after 7 days, and Caleffi’s ZSR multi-zone pump control, with exercise switched on, run each pump for 30 seconds after 72 hours. Rinnai says its I-Series Plus may run the pump to stop the boiler freezing.

Running all the time. If a Taco SR501’s red light stays on with the thermostat turned right down, Taco’s own reading of that light is power going to the pump, so have a technician check the thermostat, its wiring and the relay. A zone that keeps heating is in a radiator that won’t turn off.