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how does a tripple aquqstat work and what is the differential fo

carol_3 Member Posts: 397
yeh, the "triple" is high limit (for safety), low limit (for water temp) and circulator (circulator won't function if the water isn't hot enough.


  • cph_3
    cph_3 Member Posts: 5
    triple aquastat

    how does the trile aquqstat work and what is the differential for
  • will smith_4
    will smith_4 Member Posts: 259

    Not 100% sure which aquastat you're dealing with, but I'm guessing it might be an outdoor reset control. If so, you'd have a temp sensor mounted outside as well as one measuring the boiler water temp. The two setpoint dials on the control give you the ability to change the temp of the water as the outdoor temp changes; as it gets colder, you may have the temp at 190-200,whereas if it's warmer out, you may lower it to 180 or so. The differential is the difference between the "on-off" setpoints, and can be additive or subtractive-depending on the control.
    Unless you have one which just serves as the low limit, operator, and high limit. One would make sure the boiler maintains a low end temp, one sets the high range/might enable a pump, and one might serve as the high temp safety. Post the model/manufacturer if you can.
  • Ken D.
    Ken D. Member Posts: 836

    It is probably a hi-lo-operating control. There are millions of boilers with them. The high limit keeps the boiler water from exceeding the set temperature. The low limit keeps the water at a minumum temp. The operating control will not allow the circulator to come on until the minimum temp. (low limit) is reached. That is the circ. will turn off if the temp. goes below the low limit setting. The differential is the temp. swing between on and off cycles.
  • I e-mailed CPH

    a complete breakdown of the Triple Aquastat Relays and how they work. Hopefully it will help him.
  • cph_3
    cph_3 Member Posts: 5
    Tim McElwain

    Tim could you please send me the information on the aqua stats again. I most have erased them. thank you.
  • Here it is

    I have posted it here for you CPH. It is not mine originally but my good friend George Lanthier, I am sure he will not mind me sharing it with you.


    One of the most common types of controller used in hydronic systems is the triple aquastat. Although the control can be purchased by itself, it is normally used as the heart of the hydronlc system controller. It allows for the control of the high limit, low limit and circulator functions by a single aquastat control.

    This basic hydronic system controller is sold by Honeywell as the L8124, and by White-Rogers as the 8F43A. It consists of the triple acting aquastat and a built in switching relay.

    This is where all of the temperature control for the system, except for room comfort settings, is determined. Let's take a look at how it works.
    First of all let' s look at the easiest part, the high limit, and then the low limit-reverse.

    The high limit is adjusted using the knob marked 'HI'. It is marked and adjustable from 130° F to 240° F, and has a fixed differential of 10° F. So, if it were set at 200° F, it would shut the burner off and interrupt power from the B 1 terminal at 200° F and turn the power on again at 190 °F. The terminals marked Black and Yellow relate to this switch. The low-limit-reverse is adjusted using the knob marked 'LO'. It is marked and adjustable from 110 °F to 220° F, and has an adjustable differential of between 10° F and 25° F. The differential is set using the knob marked 'DIFF'. Now here is where it gets a little complicated in the Honeywell control.

    If you had the LO set at 180° F with a 10° F differential or DIFF, it would shut the burner off for low limit at 180° F and turn it back on again at 170° F to maintain hot water. The circulator could run between these two temperatures.

    Not to hard to understand so far but, although many think that this is the 'normal' setting for any control with this setup they are normally wrong. These settings can lead to short cycling of the burner and the circulator. With some heating systems like the series-loop, the last room or space on the loop may never get hot enough, because the last piece of radiation may not get hot enough before the pump is shut off by the reverse.

    Another danger is the burner running off the high limit control all winter long. Why, because the heat never gets out of the boiler fast enough. Why dangerous? Because the high limit is a safety control, not an operating control. Now you know why safety and code commercial people demand two high limits. Makes you wonder though, doesn't it?

    Now let's take a more difficult example to understand and why everybody just sets the DIFF for 10° F. We will say that the LO was still set at 180° F, but the DIFF was set at 25° F. The burner would come on at 170°F to maintain hot water but it would shut off at 195°F because of the differential or DIFF setting of 25°F. The circulator could run for heat between 170°F and 195°F, or higher if the burner input can exceed the circulator output in BTU's. This type of operation is common with overfired boilers, under radiated houses, and in zoned systems.

    The terminals affected by these settings are C 1, ZC and ZR. The terminals marked Blue, White and Red relate to this switch. Now for another example, and to try to get that domestic hot water temperature back to around an average of 175°F. Now the LO is set at 175°F, with the DIFF set at 20°F. The burner would come on at 165°F to maintain hot water but it would shut off at 185°F because of the differential or DIFF setting of 20°F. The circulator could run for heat between 165°F and 185°F. One more, okay?

    The LO is set at 170°F, with the DIFF set at 15°F. The burner would come on at 160°F to maintain hot water but it would shut off at 175°F because of the differential or DIFF setting of 15°F. The circulator could run for heat between 160°F and 175°F. Now, are you starting to see the pattern?

    Let's lay down a few ground rules about these triple acting aquastats.

    1.The reverse always opens at 10F below the LO setpoint.

    2.The low limit always opens at the reverse open, plus the differential.

    3.The DIFF setting never has an affect on the HI setting.

    4.The LO setting has no affect on the HI setting and vice-versa.

    5.The operation of any Honeywell control that incorporates the 'white block' aquastat, operate the same. These include the following controls: L4081, L6081, L8124, L8151, R8182

    Taken from WIRING & Oil Burning Equipment, George Lanthier, Copyright 1997-2000, Firedragon Publications. This material may not be used for profit except with the express written permission of the author and publisher.

    Thank you George for the use of your explanation.

  • cph_3
    cph_3 Member Posts: 5
    Tim McElwain

    Tim, Thank you for your help. Cliff
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