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Triple Acting Aquastat

This is one of the subjects we get a lot of questions on in seminars and personally one of my favorites to teach. If you peruse the Internet you will see questions on the triple acting aquastat control show up over and over again, and it’s no wonder when you find out how it really works. In a recent seminar it was described as ‘the most confusing control we work on” by one of the participants, he’s right!

The basic hydronic system controller has been sold by Honeywell as the L8124, the L8151, the L7124 and the new L7224. 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 for oil-fired systems, they are sold by Honeywell as the R8182. The L8124 and R8182 share the same aquastat assembly and are the subject of this article.

A drawing of the exterior of the traditional triple acting switch is shown in Figure 1, with an internal schematic for the switch shown in Figure 2. 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 130º F to 240º F, and has a fixed differential of 10º F. So, if it is set at 200º F, it will shut the burner off and interrupt power from the B1 terminal shown in Figure 1 at 200º F and turn the power on again at 190º F. The terminals in Figure 2 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 110º F to 220º F, and has an adjustable differential of between 10º F and 25º 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 have the LO set at 180º F with a 10º F differential or DIFF, it will shut the burner off for low limit at 180º F and turn it back on again at 170º F to maintain hot water. The circulator is able to run between these two temperatures. Not too hard to understand so far, but although most of us think that these are the ‘normal’ settings for any of these controls, with this setup they are probably 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 control.

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 everyone just sets the DIFF for 10º F. We will say that the LO is still set at 180º F, but the DIFF is set at 25º F. The burner will come on at 170º F to maintain hot water, but it will shut off at 195º F because of the differential or DIFF setting of 25º F. The circulator can run for heat between 170º F and 195º F, or higher if the burner input can exceed the circulator output in Btus. 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 in Figure 1. The terminals in Figure 2 marked blue, white and red relate to this switch.

For another example, and to try to get the domestic hot water temperature back to around an average of 175º F let’s set the LO at 175º F, with the DIFF set at 20º F. The burner will come on at 165º F to maintain hot water, but it will shut off at 185º F because of the differential or DIFF setting of 20º F. The circulator can run for heat between 165º F and 185º F.

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

Let’s lay down a few ground rules about the L8124 style triple acting aquastat.

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