• Sign In
  • Create Account
  • Sign Out
  • My Account
  • NEWS
  • PRODUCTS
  • CONTRACTORS
  • ENGINEERS
  • RADIANT & HYDRONICS
  • INSIGHTS
  • MEDIA
  • RESOURCES
  • EMAGAZINE
  • SIGN UP!
cart
facebook instagram twitter linkedin youtube
  • CONTRACTORS
  • BATH & KITCHEN PRO
  • BUSINESS MANAGEMENT
  • HIGH EFFICIENCY HOMES
  • TECHNOLOGY
  • WATER TREATMENT
  • PMC COLUMNS
  • PMC COLUMNS
  • Dave Yates: Contractor’s Corner
  • John Siegenthaler: Hydronics Workshop
  • Kenny Chapman: The Blue Collar Coach
  • Matt Michel: Service Plumbing Pros
  • Scott Secor: Heating Perceptions
  • ENGINEERS
  • CONTINUING EDUCATION
  • DECARBONIZATION | ELECTRIFICATION
  • FIRE PROTECTION
  • GEOTHERMAL | SOLAR THERMAL
  • PIPING | PLUMBING | PVF
  • PME COLUMNS
  • PME COLUMNS
  • Christoph Lohr: Strategic Plumbing Insights
  • David Dexter: Plumbing Talking Points
  • James Dipping: Engineer Viewpoints
  • John Seigenthaler: Renewable Heating Design
  • Lowell Manalo: Plumbing Essentials
  • Misty Guard: Guard on Compliance
  • RADIANT & HYDRONICS
  • RADIANT COMFORT REPORT
  • THE GLITCH & THE FIX
  • INSIGHTS
  • CODES
  • GREEN PLUMBING & MECHANICAL
  • PROJECT PROFILES
  • COLUMNS
  • SPONSOR INSIGHTS
  • COLUMNS
  • Codes Corner
  • Natalie Forster: Editorial Opinion
  • Guest Editorial
  • MEDIA
  • PODCASTS
  • VIDEOS
  • WEBINARS
  • RESOURCES
  • INDUSTRY CALENDAR
  • DIRECTORIES
  • EBOOKS
  • PM BOOKSTORE
  • CE CENTER
  • MARKET RESEARCH
  • CLASSIFIEDS
  • EMAGAZINE
  • EMAGAZINE
  • ARCHIVE ISSUES
  • CONTACT
  • ADVERTISE
  • PME EMAGAZINE ARCHIVES
search
cart
facebook instagram twitter linkedin youtube
  • Sign In
  • Create Account
  • Sign Out
  • My Account
  • NEWS
  • PRODUCTS
  • CONTRACTORS
    • BATH & KITCHEN PRO
    • BUSINESS MANAGEMENT
    • HIGH EFFICIENCY HOMES
    • TECHNOLOGY
    • WATER TREATMENT
    • PMC COLUMNS
      • Dave Yates: Contractor’s Corner
      • John Siegenthaler: Hydronics Workshop
      • Kenny Chapman: The Blue Collar Coach
      • Matt Michel: Service Plumbing Pros
      • Scott Secor: Heating Perceptions
  • ENGINEERS
    • CONTINUING EDUCATION
    • DECARBONIZATION | ELECTRIFICATION
    • FIRE PROTECTION
    • GEOTHERMAL | SOLAR THERMAL
    • PIPING | PLUMBING | PVF
    • PME COLUMNS
      • Christoph Lohr: Strategic Plumbing Insights
      • David Dexter: Plumbing Talking Points
      • James Dipping: Engineer Viewpoints
      • John Seigenthaler: Renewable Heating Design
      • Lowell Manalo: Plumbing Essentials
      • Misty Guard: Guard on Compliance
  • RADIANT & HYDRONICS
    • RADIANT COMFORT REPORT
    • THE GLITCH & THE FIX
  • INSIGHTS
    • CODES
    • GREEN PLUMBING & MECHANICAL
    • PROJECT PROFILES
    • COLUMNS
      • Codes Corner
      • Natalie Forster: Editorial Opinion
      • Guest Editorial
    • SPONSOR INSIGHTS
  • MEDIA
    • PODCASTS
    • VIDEOS
    • WEBINARS
  • RESOURCES
    • INDUSTRY CALENDAR
    • DIRECTORIES
    • EBOOKS
    • PM BOOKSTORE
    • CE CENTER
    • MARKET RESEARCH
    • CLASSIFIEDS
  • EMAGAZINE
    • EMAGAZINE
    • ARCHIVE ISSUES
    • CONTACT
    • ADVERTISE
    • PME EMAGAZINE ARCHIVES
  • SIGN UP!
Columns

Nothing But A Switch?
Jim Todd

By Jim Todd
October 1, 2007
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.



Triple-Acting

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.

Share This Story

Looking for a reprint of this article?
From high-res PDFs to custom plaques, order your copy today!

Jim Todd is a noted vocational technical educator in Waterbury, Conn., with more than 20 years' experience instructing plumbing and HVAC apprentices. He is a contributing editor for NORA and co-authored a text on air conditioning. You can reach Jim at mrheat@optonline.net.

Recommended Content

JOIN TODAY
to unlock your recommendations.

Already have an account? Sign In

  • Worker using the Milwaukee Tool SWITCH PACK drain cleaner

    Pipeline profits: Drain cleaning, pipe inspection create opportunities

    Drain cleaning and inspection services offer lucrative...
    Green Plumbing and Mechanical
    By: Nicole Krawcke
  • Uponor employee, Arturo Moreno

    The reinvestment in American manufacturing and training

    Plumbing & Mechanical Chief Editor Nicole Krawcke and...
    Plumbing News
    By: Nicole Krawcke and Natalie Forster
  • March 2024 Women in Plumbing hero image of woman engineer overlayed by circle of hexagon shapes with numbers from 1 to 10

    Celebrating 10 Influential Women in the Plumbing Industry

    Celebrating Women's History Month and Women in...
    Plumbing News
    By: Nicole Krawcke
Subscribe For Free!
  • eNewsletters
  • Online Registration
  • Subscription Customer Service
  • eMagazine
  • Manage My Preferences

IPEX celebrates grand opening of new Florida distribution center

IPEX celebrates grand opening of new Florida distribution center

NIBCO Press Solutions

NIBCO Press Solutions

AI can boost efficiency and profitability for plumbing, HVAC contractors

AI can boost efficiency and profitability for plumbing, HVAC contractors

Bell & Gossett Illustrates Path to Net-zero at AHR Expo

Bell & Gossett Illustrates Path to Net-zero at AHR Expo

More Videos

Sponsored Content

Sponsored Content is a special paid section where industry companies provide high quality, objective, non-commercial content around topics of interest to the Plumbing & Mechanical audience. All Sponsored Content is supplied by the advertising company and any opinions expressed in this article are those of the author and not necessarily reflect the views of Plumbing & Mechanical or its parent company, BNP Media. Interested in participating in our Sponsored Content section? Contact your local rep!

close
  • J.J. Keller CMV vehicles on road
    Sponsored byJ. J. Keller & Associates, Inc.

    The dash cam game-changer for small business safety

Popular Stories

Underfloor heating installation with drain sewer hole in bathroom close up on water floor heating.

Using hydronics to leverage time-of-use electrical rates

The interior of a government building.

President Trump signs executive order promoting skilled trades and apprenticeships

Watts Nexa mobile image

Behind the Wall: Where smart plumbing gets smarter

PMCE Home-X April 29 Free Webinar: From Legacy to Leadership: Preparing Your Home Services Business for the Next Generation

Events

November 13, 2024

Future Proofing MEP: Navigating the 2026 High Efficiency Water Heating Standards

Join our deep dive into DOE’s new standards so you can future-proof your MEP practice.

EARN: 0.1 ASPE CEU; 1 AIA LU/HSW; 0.1 IACET CEU*; 1 PDH

View All Submit An Event

Poll

Will business be up or down in 2025?

Do you anticipate business in 2025 to be up or down in comparison to 2024?
View Results Poll Archive

Products

The Water Came To A Stop

The Water Came To A Stop

See More Products

Download the FREE Water Conservation, Quality & Safety eBook: Plumbing Trends Increasing Safe Water Availability

×

Keep your content unclogged with our newsletters!

Stay in the know on the latest plumbing & piping industry trends.

JOIN TODAY!
  • RESOURCES
    • Advertise
    • Contact Us
    • Directories
    • Store
    • Want More
    • Supply House Times
  • SIGN UP TODAY
    • Create Account
    • eMagazine
    • eNewsletter
    • Customer Service
    • Manage Preferences
  • SERVICES
    • Marketing Services
    • Reprints
    • Market Research
    • List Rental
    • Survey/Respondent Access
  • STAY CONNECTED
    • LinkedIn
    • Facebook
    • Instagram
    • YouTube
    • X (Twitter)
  • PRIVACY
    • PRIVACY POLICY
    • TERMS & CONDITIONS
    • DO NOT SELL MY PERSONAL INFORMATION
    • PRIVACY REQUEST
    • ACCESSIBILITY

Copyright ©2025. All Rights Reserved BNP Media.

Design, CMS, Hosting & Web Development :: ePublishing

search
cart
facebook instagram twitter linkedin youtube
  • Sign In
  • Create Account
  • Sign Out
  • My Account
  • NEWS
  • PRODUCTS
  • CONTRACTORS
    • BATH & KITCHEN PRO
    • BUSINESS MANAGEMENT
    • HIGH EFFICIENCY HOMES
    • TECHNOLOGY
    • WATER TREATMENT
    • PMC COLUMNS
      • Dave Yates: Contractor’s Corner
      • John Siegenthaler: Hydronics Workshop
      • Kenny Chapman: The Blue Collar Coach
      • Matt Michel: Service Plumbing Pros
      • Scott Secor: Heating Perceptions
  • ENGINEERS
    • CONTINUING EDUCATION
    • DECARBONIZATION | ELECTRIFICATION
    • FIRE PROTECTION
    • GEOTHERMAL | SOLAR THERMAL
    • PIPING | PLUMBING | PVF
    • PME COLUMNS
      • Christoph Lohr: Strategic Plumbing Insights
      • David Dexter: Plumbing Talking Points
      • James Dipping: Engineer Viewpoints
      • John Seigenthaler: Renewable Heating Design
      • Lowell Manalo: Plumbing Essentials
      • Misty Guard: Guard on Compliance
  • RADIANT & HYDRONICS
    • RADIANT COMFORT REPORT
    • THE GLITCH & THE FIX
  • INSIGHTS
    • CODES
    • GREEN PLUMBING & MECHANICAL
    • PROJECT PROFILES
    • COLUMNS
      • Codes Corner
      • Natalie Forster: Editorial Opinion
      • Guest Editorial
    • SPONSOR INSIGHTS
  • MEDIA
    • PODCASTS
    • VIDEOS
    • WEBINARS
  • RESOURCES
    • INDUSTRY CALENDAR
    • DIRECTORIES
    • EBOOKS
    • PM BOOKSTORE
    • CE CENTER
    • MARKET RESEARCH
    • CLASSIFIEDS
  • EMAGAZINE
    • EMAGAZINE
    • ARCHIVE ISSUES
    • CONTACT
    • ADVERTISE
    • PME EMAGAZINE ARCHIVES
  • SIGN UP!