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Triple acting aqaustat

Could someone please explain how triple acting aquastat operates? What is the differential and how does it affect the operation of a tankless h/w coil. What is the optimum setting for tankless coils, assuming 180 h/l, 140 l/w
and differential?? Thank you for your help.


  • From

    Dave, this was posted on my web site By Firedragon on Sunday, February 27, 2000. I'm sure it'll answer most if not all of your question.

    One of the most common types of controller used in hydronic systems is the triple acting aquastat. Although the control can be purchased by itself, it is normally used as the heart of the hydronic 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. The triple acting aquastat, a switching relay and a primary control make up the most elaborate of the
    system controllers. They are sold by White-Rodgers as the 6C92 and by Honeywell as the R8182.

    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 130F to 240F, and has a fixed differential of 10F. So, if it was set at 200F, it would shut the burner off and interrupt power from the B1 terminal at 200F and turn the power on again at 190F. 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 110F to 220F, and has an adjustable differential of between 10F and 25F. 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 180F with a 10F differential or DIFF, it would shut the burner off for low limit at 180F and turn it back on again at 170F 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 10F. We will say that the LO was still set at 180F, but the DIFF was set at
    25F. The burner would come on at 170F to maintain hot water but, it would shut off at 195F because of the differential or DIFF setting of 25F. The circulator could run for heat between 170F and 195F, 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 C1, 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 175F.
    Now the LO is set at 175F, with the DIFF set at 20F. The burner would come on at 165F to maintain hot water but, it would shut off at 185F because of the differential or DIFF setting of 20F. The circulator could run for heat between 165F and 185F. One more, okay?

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

    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

    This information was 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. It was later posted on Oil Tech Talk by Mr. Lanthier.

    Your friend in the industry,
    Alan R. Mercurio

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