Here’s what I learned long ago on the care and feeding of triple-acting aquastats.

"It’s just a switch!” I said it over and over as the aquastat kept me stumped. Just what was going on with this job?

First things first: I was green with service and really didn’t have a strong grip on how an aquastat worked. Sure, I’d taken enough electrical courses to understand how my tester worked, but I must have been out the day they taught us the concept of a triple aquastat. That was a day I’ll never forget. Now let me share what I learned with you.

As I said, an aquastat is nothing more than a switch. There are two operational concepts with an aquastat, both initiated by the temperature of the heat transfer fluid within a boiler or hydronic system. Each concept uses trade jargon taken from the electrical trade.

Two common terms are “makes” and “breaks.” The terms refer to the action of the switch - “makes” describes a switch that closes to complete a circuit and “breaks” describes a switch that opens to stop the flow of electricity.

Aquastats are designed as individual controls for the operation of specific devices. Examples of these devices are circulators, burners and motorized valves. When more than one aquastat is applied to one device, the application is referred to as “nonintegrated.” The aquastat that had me baffled is considered an “integrated” control as it performs more than one function while using one temperature sensor.

A boiler control for our basic circulated hot water system will require controls for the burner and the circulator. The burner circuit needs a controller to shut the burner off when high temperatures are attained, commonly referred to as a “high limit.”

The high limit on a residential system by design was intended to be applied as a safety rather than an operating control. In theory, if the system is designed perfectly, the burner should operate continuously during a call for heat and maintain design temperature as the circulator and system shed energy into the structure.

Starting Back Up

Now that you know how it shuts off, the question becomes what makes it come back on? That’s the differential. The limit shuts off when the aquastat senses the set point. As the temperature of the heat transfer fluid drops to a predetermined point, the switch closes again, starting the burner to maintain transfer fluid temperature.

Most aquastats that break on rise have a fixed differential, typically of 10-15 degrees. Some versions are available with variable differentials that allow the contractor to design in a delay, allowing the switch to close by a wider temperature margin.

Temperature sensing has come a long way since the first aquastat. Early versions had a bimetal sensor that tipped a mercury-filled switch. Contacts within a glass vile and the mercury would complete or open the circuit, depending on if the control was designed to make or break.

That version was cumbersome at best and was replaced by a copper sensor connected to a bellows-actuated switch by a capillary tube, then filled with fast-acting fluids. The sensor can be installed surrounded by the heat transfer fluid or surface-mounted against the system piping. As the fast-acting fluid expanded or contracted, the bellows would react and open or close a switch, again depending on the design and application of the aquastat.

This design facilitated multiple actions and the production of integrated controls. Now a single sensor can act as the “brains” for multiple devices, such as the burner and the circulator. A new style that uses a thermistor has gained favor with the recent advent of solid-state and microprocessor controls.

Applying An Aquastat

OK, let’s apply an aquastat. Here’s an example using nonintegrated aquastats. Envision an installation that has an oil burner, tankless coil, a zone relay and a circulator for one zone. On a call for heat, the thermostat closes T-T on the zone relay. The relay closes two sets of contacts: one for the burner circuit and one for the circulator.

Let’s describe the burner circuit first. The burner starts when the relay closes the contacts. A high-limit aquastat is wired in series with the primary control for boiler protection. If the boiler water temperature reaches the set point of the high-limit aquastat, the burner is shut off, coming back on if the circuit is still energized and the water temperature drops to the fixed differential setting.

Another aquastat is wired into the burner circuit to maintain temperature in the boiler. When a tankless coil is present, there must be sufficient boiler water temperature to meet the energy demand for domestic hot water. This aquastat is in series with the high limit and makes on temperature fall. It has a differential that breaks on rise, somewhere around 20 degrees above set point. Far from simple, though nothing more than another boiler/burner control.

Controlling The Circulator

Now let’s control the circulator. It would seem simple enough to start the circulator and allow it to circulate heat transfer fluid through the system. The burner is running, what could be a problem? But remember that tankless coil? What happens if the operation of the circulator causes a dramatic drop in boiler fluid temperature? There might not be a complaint on the heating side, but the occupant of the shower is going to be disappointed when the tankless coil can’t produce hot water.

That’s where a “reverse-acting aquastat” saves the day. This aquastat is wired in series with the circulator and breaks on temperature drop, stopping the circulator and directing all of the burner’s efforts toward maintaining boiler temperature. When the heat transfer fluid within the boiler is “hot” enough, the reverse-acting aquastat’s differential makes and energizes the circulator again.

Boy, that’s a lot of controls for what should be a simple installation. Three aquastats, a zone relay and what looks like a mile of wiring. The term “nonintegrated” finally takes on meaning.


My nemesis that day, however, was an integrated control, commonly referred to as a “triple-acting aquastat.” It replaced all of the aquastats, the controller/relay and simplified the wiring. Within the aquastat is a step-down transformer to power the relay coils, a circulator relay, a burner relay, a very imposing block with a differential, high-limit and low-limit dials and contacts controlled by the actions of the sensor.

There are terminals for a thermostat, a terminal providing power for the burner, another for the circulator, indicated by B1 and C1, respectively, and commons for the burner and circulator, indicated by B2 and C2. Terminals L1 and L2 are for providing power to the control.

On a call for heat, the thermostat closes T-T and energizes the burner and circulator relays and starts both devices. For this example, the low limit is set at 160 degrees F and the high limit is set at 200 degrees F.

I mentioned the differential dial. There are two differentials: one fixed and one variable. The fixed differential is on the high-limit contact, again typically 10 degrees, and closes on temperature drop from the high limit. With the high limit set at 200 degrees F, the burner circuit will open at 200 degrees F and close once the temperature drops to 190 degrees F, allowing the burner to start again if a call for heat has not been satisfied.

The low limit has a variable differential. It is set using the dial and is “subtractive,” with settings that work differently than one might think. For this example, consider that the low limit is set at 160 degrees F. The burner will start at 150 degrees F, with a fixed differential that energizes the burner 10 degrees below the low-limit setting.

Now that third dial comes into play. If the dial on the differential is set at 20 degrees, the burner will operate until the heat transfer fluid reaches 170 degrees F, 20 degrees above set point less the fixed differential setting.

That’s where the confusion starts and the term “subtractive” falls into play. Boiler temperature is maintained between the range of the low limit, less fixed differential, then to a maximum of the fixed differential setting plus the variable differential setting.

If a call for heat and a hot water demand should occur simultaneously, the circulator will be controlled by the fixed differential, shutting off the circulator if heat transfer fluid temperature falls below the low-limit setting. If the installation has multiple zones, use an aquastat that has ZC-ZR contacts. When those contacts are properly wired, the system will continue a sequence that follows the reverse-acting operation.

That’s how our boiler controls have evolved and operate. Safety is paramount. Testing a control requires a little common sense, combined with the knowledge of what the control is designed to do.

Remember that anytime thermostat contacts are closed (T-T), the burner and circulator relays are energized. If the heat transfer fluid drops below the low limit, the circulator relay will open (no voltage reading at C1-C2). If the aquastat does not have a low limit, both B1 and C1 will be energized with the high limit opening the burner (B1) circuit.

Just don’t do what I did as a green mechanic. Review the instructions before that fateful day you’re faced with an aquastat issue.