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Honeywell Aquastat 8124

Honeywell aquastat on Peerless tankless system, heat and h/w. Settings (high/low/diff) at 200/170/20. It is my understanding that at 170 and 20 diff, boiler should fire at 160 and will heat water to 180. The problem that we are having is when domestic hot water is run, the boiler temp is dropping to 130s before the boiler fires. It should fire at 160. The capillary thermo is fully inserted to the well stop. This is a new aquastat. Any ideas on why the aqaustat is not firing at 160?
Robert O'Brien


  • STEVEusaPA
    STEVEusaPA Member Posts: 6,506
    I'd guess my first question is why did you replace the 8124 with another 8124 and not a more modern aquastat?
    Second, you have to confirm the temperature settings on the aquastat are matching the temperature gauge.
    The bulb may be fully immersed, but is it making good, full contact?
    Turn the diff down to 10° see if it gets you closer.
    You can do a quick check of the low limit by seeing during a call for heat if the circulator is turning on/off at that low limit setting.
  • nunesman
    nunesman Member Posts: 5
    Why the 8124? No good reason other than it was what was on there and has been working for 10 years. I’m a novice at this.

    Confirm temps match. I had several cycles I watched where the low set turned off at the expected temp. Not sure how else to do this.

    Bulb contact. The fit is snug and I made sure it bottomed out, ie fully inserted. The was careful in handing the capillary tubes. I do plan to make sure the well is good and clean and use thermo paste.

    Regarding Diff setting, it is my understanding that the boiler should fire 10 degrees below the low set regardless of Diff setting, and the Diff setting tells the boiler when to stop firing from the point it starts to fire. If my low is set at 170, the boiler fires at 160, and the stops firing at 180. This is what I read in a thread on this forum.

    From what I have seen watching the temp gauge, when the change in water temp is slower, for example a zone is calling for heat, the boiler seems to fire when expected. But when the temp drop is faster, for example someone is taking a shower, the temp gauge on the boiler goes down to 130s before the aquastat calls. I expected some lag, but this seems excessive. The shower is going from hot to barely warm.

    I’ve contemplated draining the exchanger down enough to remove the well to see if the actual water contact surface has some weird corrosion or other issue. Grabbing at straws.

    Again, novice here. I appreciate the feedback.
  • mattmia2
    mattmia2 Member Posts: 9,632
    Do you have thermal past in the well now? it makes a big difference. Even at that, it could take the aquastat a minute or 2 to respond to the temp it is exposed to if the temp falls particularly quickly.
  • nunesman
    nunesman Member Posts: 5
    No paste yet. Will try tomorrow. Thank you for the feedback.
    HVACNUT Member Posts: 5,831
    If you're going top drop the pressure and check the well, order a Hydrostat 3250 Plus (with well) and return the L8124.
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,832
    The actual temperatures of the boiler gauge and the aquastat may not be precise. The only way to know for sure is more expensive professional testing equipment that is calibrated before and tested after your boiler temperatures are measured. But this is not rocket science. Most of us in the trade use a digital meter with a temperature probe that may get calibrated every other year if at all. The real bottom line is ... how is it working? Do you have enough hot water? Do you run out of hot water during a shower? If so raise the low limit temp by 10 degrees and see if that helps. I recommend putting the settings at 160° low 180° high 10° dif to start with. If you need more hot water then raise the low and high by 10° if you have enough DHW then you can tweak lower temps in an attempt to reduce the fuel bill.

    At 10 years old, has your coil been flushed with a cleaning agent? do you have a mixing or tempering valve? Could those 2 items be in need of service?

    Don't get hung up on the numbers on the gauges and aquastat. in many cases, they are measuring in 2 different places and the actual calibration may be plus or minus 10° off from the factory. (NOT ROCKET SCIENCE)

    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics