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Honeywell AQ475 misbehaving (?)

I recently installed a Honeywell AQ475A outdoor reset control on my hydronic system, and I'm puzzled by its behavior: it fails to turn the boiler off until the supply temperature has risen at least ten degrees F above the "boiler max temperature" setting. The warmer the outdoor temperature, the greater the disparity between the AQ475 setting, and the actual shutdown temp: three weeks ago, when we had outdoor temperatures in the upper 40s, the AQ475 allowed the supply temperature to rise to nearly 170 despite a boiler max temp setting of 140. Last night and this morning, with outdoor temps in the single digits, the disparity is pretty consistently ten to fifteen degrees.

Honeywell tech support has been kind enough to provide me with the temperature/resistance data for the sensors, which has enabled me to confirm that both sensors, and their wiring, are good.

So what's going on with this control? Do I have a defective unit, or what?

One thing I wondered about that might explain the larger disparities between set and actual temperatures during warmer weather: Once the AQ475 makes the T-T output connection, is there a certain minimum length of time that it holds that closed before breaking it, regardless of temperature? That would certainly explain the inconsistencies...

System consists of:

Lennox GWB8 ng boiler w/ Honeywell L8148A aquastat

approx 350' of Burnham BaseRay 7" baseboard emitters

Seven zones total:

-- four zones on one B&G series H circulator, with Honeywell and W-R zone valves

-- one zone on each of three B&G series 100 circulators, with no zone valves

Honeywell T87 thermostats throughout

Circulators are controlled by individual switching relays, not by the AQ475 or the L8148A.


  • Larry (from OSHA)
    Larry (from OSHA) Member Posts: 699
    I'll take a stab at this


    I am not in the business, but I do have one on my system. They do work pretty good, but don't expect exact temp control to what you have it set to. There may be issues in your installation that affects the performance of this control.

    What is the supply sensor attached to? If on black pipe, the response will be slower that if on copper. If you can put it in a well, that would be best. The control seems to think more slowly than you would expect and can lead to temp overshoot if only one or a few zones are calling.

    The control does not hold the tt contacts closed longer than the call. It will run the circulator for ten seconds after the call to purge, but that's it.

    Hope this helps. If you have pics or can be more specific, perhaps more info can come your way.

  • Doug Miller
    Doug Miller Member Posts: 4

    Thanks for the response. Sorry, should've specified more. Supply sensor is surface-mounted on copper, about 30 inches downstream from the boiler outlet, strapped to the pipe, insulated and taped per instructions in the product data sheet. See photo at


    Resistance measurements made at various times and a variety of temperatures, using a digital multimeter, consistently show the sensor's indicated temperature within 0.2 degrees Celsius of the temperature shown by the digital thermometer whose probe can be seen in the photo.

    I'd been wondering if a well might improve it, but after seeing how closely the sensor resistance tracks the temperature the thermometer indicates, I'm not sure I'd actually see much improvement.

    You're right about the control not holding the TT contacts closed after the call for heat ends. I've observed that several times, and it opens the TT contacts instantly. However... given Honeywell's description, it seems to me that it should be opening TT a lot sooner than it does when the supply temperature rises. Just makes me wonder if there's a minimum duration for holding TT closed, something like "three minutes or until the call for heat ends, whichever comes first".

    *** OOPS *** meant to say "run for at least three minutes unless the call for heat ends first". Don't know if that's what it actually does, but that would explain most of the erratic behavior.
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