Intermittent on-off heating zone operation:

Heating zone circulator pump seems to run just intermittently - it starts and stops. What's wrong, how do I fix it? We start addressing this question by asking whether or not anything is wrong in the first place.

When should the heating zone circulator be running? Then if it seems to be acting oddly, why & how do we fix it?

This article series describes how to diagnose & fix circulator pump problems on hot water or hydronic heating systems.

Question: circulator pump runs intermittently

My furnace's circulator pump has been running in short bursts only lately, as opposed to full length cycles. It's a gas-fired hydronic system, a high efficient one w/ a Peerless Pinacle furnace. never experienced this before.Do you have any ideas? - mark4man 12/5/2012

(Dec 29, 2011) Richard VanMetter said:

When my thermostat calls for heat the gas comes on and the internal temperature of the furnace increases (approaching 250 degrees) but the circulator pump only runs intermittently (5 seconds on 30 seconds or more off). Shouldn't it run constantly once the internal temperature of the furnace is adequate?

(Nov 11, 2012) Anonymous said:

Hi i have an open fire with back boiler the pump is beside the back boiler and the thermostat is on
the boiler. my problem is the pump only comes on for about 30 seconds and thats it and it does nothing for the rads the pipe that the stat is on goes cold so i reckon the stat is on the wrong pipe,
please help. cheers

Phil said:

My circulator pump turns on then shuts off in 3-4 sec. could this be a switch?

Reply:

First I would check all of the electrical connections involved since a loose connector or a corroded connector such as a spade connector on a heater control can cause intermittent operation.

... the furnace draft fan began to pulse several times upon starting. After making sure the control module showed a steady green, ... I wiggled all the spade connectors to be sure there was good contact between the copper and aluminum. ... there only needs to be a little corrosion to cause problems. That seems to have done the trick. - M.F. 12/24/2014

Next I'd check for a bad circulator control relay or possibly a thermostat wire that is intermittently shorting together. Occasionally a bad thermostat can also cause intermittent on-off calls for heat that can lead to odd circulator behavior.

A second set of possibilities might be a circulator pump that is binding or failing.

See CIRCULATOR PUMP RELAYS & OTHER CONTROLS

and see THERMOSTATS, HEATING / COOLING for details.

A short on-off circulator cycle such as you describe sounds to me like a wiring (damaged wire), thermostat, or circulator relay problem.

Phil, I wonder if there is a bad relay in the circulator control, or an intermittent short of the thermostat wires. Try disconnecting the thermostat entirely to rule out that source. Then connect the red and white heat wires (or equivalent if yours are different) that calls for heat. Connected = heat on. Disconnected = heat off.

Watch out: other readers note: other reasons that a heating zone circulator may not run continuously include

• Normal operation: in some areas circulator pumps are wired to run continuously if the boiler has electrical power but in others the circulator only runs in response to a call for heat. In the latter case the boiler temperature turns the burner on and off.
• Improperly-set primary control - check your aquastat settings to be sure they are correct - see AQUASTAT HI LO DIFF SETTINGS
• Shorting thermostat wires: a room thermostat whose wires are shorted or intermittently shorting together may turn the circulator on and off
• Poor thermostat settings: on some systems an improperly set heat anticipator may cause the circulator to stutter on and off; this problem may also show up as "contact bounce" in the thermostat
• Damaged circulator pump relay: in the primary control or in additional circulator pump control relays a failing switching relay can casuse funny circulator pump on-off operation.
• Tankless coil snafu: if your heating boiler does not make use of a tankless coil to produce domestic hot water but the aquastat was installed with its factory settings that control may make the heating boiler run for no apparent reason - though usually only briefly. Some may confuse boiler-burner-on with circulator-on operation. If the thermostat is not calling for heat the circulator won't run (unless it's wired for "always on" operation).

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2017-03-26 by Anonymous

my boiler runs until my thermostat is up to temp but the circulator switches on/off at times.

My understanding after replacing the circulator pump/L8124A/C aquastat/boiler temp/pressure gage.....the boiler runs until the high temp is made and then should shut off and the circulator pimp should run until the thermostat is up to temp. Why does my V-73 boiler burnham run until the thermostat temp is made and my circulator is on/off all the time.

On 2015-10-22 by eddie g

Rcd trips when timer kicks in for heating

On 2014-12-16 by (mod) -

glad to help, and your feedback helps us.

On 2014-12-16 by Bruce

Thank you again for the help. This is a great site with LOTS of helpful information. I'm a recent transplant to New England and so my first experience with a Hydronic heat system. This site has been invaluable in helping me learn about how the system works and what to look for when it does not! Very informative!

On 2014-12-16 by (mod) - try a replacement fuse

Dunno but it should be trivial to just try a replacement fuse and to check that its contacts are sound. I'd do that.

No power TO the pump is a helpful fact - one just follows the power circuit backwards from that point with a VOM until finding where there is a disconnection or switch or relay that's off.

Intermittent problems are always difficult to find unless we can perform inspections and tests during the event.

On 2014-12-16 by Bruce

I'm wondering if it may be as simple as a fuse going bad? There is a 5amp fuse, looks like a slo blo, that in in the circuit for that pump. Do fuses ever develop intermittent problems? No harm in swapping it out I guess?

Dan, thanks again. I'm sure the pump is OK, just installed it new and does the exact thing the old one was doing. If I had thought to test with a voltmeter during the malfunction, I would not have changed the pump. I've tried pounding on it , and the old one, it does not make any difference because when it is not working, there is no power going to it

. The thermostat sends the heat signal with no problem, and the fan comes on as it should.

I have it narrowed down to a problem between the relay and the pump I think so I guess that only leaves the circuit board? I do not see any burn marks or damage on it.

Thank you for your suggestions. I guess we will try getting the HVAC guys back out now that I have a better idea of what is happening. Bruce

On 2014-12-1 by (mod) - double checking the thermostat wires for continuity

Bruce

It might be worth double checking the thermostat wires for continuity - even if the end connections are tight there could be damage enroute.

I'm beginning to suspect a failing circ motor that sticks.

1. If when we know the circ should be running and it's not, will it start if you rap it?

2. You may be able to get current draw specs from Taco for this pump model, then check the amps draw to see if it's high - which can indicate a failing bearing. OF course other electrical failures such as a winding that opens in a motor are harder to detect.

Some electric motor diagnostics are at

https://inspectapedia.com/electric/Electric_Motor_Diagnosis.php

Or if you have other circulators in the home you could try swapping motors. Taco motors may be changeable without any plumbing work.

On 2014-12-16 by Bruce

Dan, forgot to say that it has also had the malfunction happen with both relays, and both relays work fine when in water heater circuit, so I'm pretty sure the relays are working. They have clear covers and I see no signs of burnt contacts or anything.

Thank you Dan. for the fast reply! I know the wiring to the pump from the relay panel is not loose since I just replaced the pump and was very careful with the connections. I've checked the screw terminals at the relay board also to make sure they are tight.

I've tested the relay by swapping it out with the other one on the board that goes to the hot water heater and the hot water heater and the recirc pump both work with either relay. I've checked the connections from the thermostat wires to make sure they are tight (although they are just wirenuts). Bruce

On 2014-12-15 by (mod) -

Sounds like a loose wire or bad connection in either the power wiring or the thermostat wiring.

That is, if the relay is really good. Swapping the relay OUT to another control and seeing it work there would be diagnostic but may not be practical.

On 2014-12-15 by Bruce

We have a Burham Hydronic heat system, natural gas fired. Random and intermittent failure of the Taco recirc. pump starting when the thermostat calls for heat.

I have replaced the pump but that was not the problem. I find now that when it is not working, no power is being sent to the pump.

If I turn the thermostat off for 2-3 hours then back on, it works again. We have had several expensive service calls but of course it works perfectly for them and no problems are found.

I have tested the relay by swapping it with the one for the hot water and the relays seem to be working fine. The thermostat seems fine too and the fan in the air handler comes on when the thermostat calls for heat, just sometimes the pump does not come on to send hot water up to the air handler. Any ideas would be greatly appreciated. Bruce

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Continue reading at CIRCULATOR OPERATION CHECKS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• CIRCULATOR DIAGNOSTICS
• COLD HEATING RADIATOR REPAIR (hot water / hydronic heat)

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CIRCULATOR PUMP RUNS INTERMITTENTLY at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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