Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

DHW circulator constantly running with aquastat

I have a Taco bronze circulator on my recirc line that seems to run all of the time. It is hardwired to a Honeywell aquastat (strapped on to the pipe). How can I tell if the aquastat is bad? Is there an alternative to this? I was thinking of switching to a Taco Smart Plug but I've read mixed reviews on it.


  • Larry Weingarten
    Larry Weingarten Member Posts: 3,141
    Hi, I'd start by turning the aquastat up and down over its full range and see if the pump ever stops running. If it does, you could just play with it a bit. Turn it down so the pump just goes off, then wait for the lines to cool a bit and see if it turns the pump back on. Let us know what you find. :)

    Yours, Larry
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 6,736
    edited February 2021
    I like Larry's advice. The numbers on the aquastat are a guide. not an exact measurement of temperature. Think of it like this. If the water temperature in the tank will be somewhere between the set point of the aquastat (thermostat) setting and the differential of the aquastat. So it will fluctuate between about 118° cut-in temp and 123° limit cut-out temp while on standby. It may drop below the cut in temp during use, but we are talking about the re-circ pump that keeps the hot pipes charged with hot water during stand-by.

    If the water temp is set at this approx 120° temperature and the control that operates the re-circ pump is set at say 125°, the re-circ pump will never shut off. This is because the source is set at a lower temperature than the Re-circ pump control. You cant get 125° from a 120° tank of water. It ain't gonna happen!

    In my experience, the user of hot water wants instant gratification for short usage of hot water, for instance washing hands 20 seconds after waiting for a minute and a half (or longer) for the hot water to get there. In this case, if the water was 100° to 105°, the user would not need to wait for hot water. The user would be satisfied with 100° water, the re-circ pump would not need to operate constantly, and the goal of not wasting water would be accomplished.

    So try Larry's idea of turning the re-circ pump's temperature control to a lower setting to see if you get the pump to stop. Then if the lower temperature water is acceptable, in a few days I would lower the temperature again a few degrees. Continue to lower the temperature 2° every few days until there will come the point where the water temperature in the hot pipes is unacceptably cool. At that point, turn the temperature adjustment back up to the previous acceptable temperature and leave it there.

    The numbers on the gauges, controls, and thermometers are a guide, not a 100% absolutely accurate measurement. These devices are calibrated to a fairly loose tolerance. One device may read 100 and the other device may read 106 at the same place. It's not "rocket surgery."

    Yours truly,

    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • BigEHead
    BigEHead Member Posts: 28
    Thank you guys. It was set to 120 and turned off when I hit 100. I will turn it down and see if it works for us.
  • SteveSan
    SteveSan Member Posts: 215
    The Smart Plug has two modes of operation. Smart mode where it learns usage ( must be more than a 5 minute event ) like a hot shower, dishwasher cycle, or hot cycle for washing machine. It won't record if you just simply wash your hands for a few seconds. Then it has a pulse mode where it will be on for 5 minutes and off for ten minutes. Is the aquastat on a copper pipe and is it insulated?
  • BigEHead
    BigEHead Member Posts: 28
    Yes it is on the copper pipe but it is not insulated.
  • hot_rod
    hot_rod Member Posts: 20,790
    Insulate as much if the piping you can get your hands on, that will lower operating costs and reduce the gpm you need to circulate, longer off cycle on the circulator
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • BigEHead
    BigEHead Member Posts: 28
    Thanks.  There isnt much exposed in the mechanical room but I will insulate as I can.