Aquastats

Aquastats are temperature-control components used in HVAC, oil heating, and hydronic heating applications to manage operating limits and circulator operation. HVAC technicians, facility maintenance teams, and heating professionals use them for service, replacement, and control installations where temperature-based switching, high-limit protection, low-limit control, or remote sensing is required.

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Helpful Buying Guide

Aquastats: Temperature Control for Hydronic and Oil Heating Systems

Aquastats are the temperature-sensing switches that keep boilers, circulators, and oil heating systems operating safely within defined limits. Whether you need a high-limit cutoff, a low-limit hold, or a combination control for a full hydronic zone, this guide walks through how to identify the right aquastat for your installation.

Visual Guide

Popular examples in this category

High-Limit Oil Aquastat Relay, 120VHigh-Limit Oil Aquastat Relay, 120VWBB394547High-Limit Manual-Reset Aquastat ControllerHigh-Limit Manual-Reset Aquastat ControllerWBB393840Remote-Bulb High/Low-Limit Aquastat Controller, 100-240°FRemote-Bulb High/Low-Limit Aquastat Controller, 100-240°FWBB394055Aquastat Immersion Well, 3/4 in. Pipe Connection, 3/8 in. Insertion Diameter, 1-1/2 in. DeepAquastat Immersion Well, 3/4 in. Pipe Connection, 3/8 in. Insertion Diameter, 1-1/2 in. DeepWBB895407
What These Products Are For

Where Aquastats Are Used

Aquastats protect and regulate boilers, oil burners, and hydronic heating systems by switching on or off at set temperatures. They prevent overheating, maintain minimum water temperatures, and control circulator pumps—making them essential for safe, efficient operation in residential and commercial heating systems.

How To Choose

Selecting the Right Aquastat

Start with the required control function—high-limit, low-limit, combination, or circulator control. Then confirm the temperature range, differential setting, voltage, and mounting style. For immersion-type units, match the well depth, insertion diameter, and pipe connection to your existing boiler tapping.

Common Applications

Typical Aquastat Applications

HVAC technicians use aquastats for boiler high-limit protection, oil-system relay control, and hydronic circulator switching. Facility maintenance teams rely on them for replacement and service work. Combination models handle both high- and low-limit functions in a single control, simplifying hydronic heating installations.

Additional Information

Installation and Sensing Details

Aquastats mount vertically or strap on to piping depending on the model. Remote-bulb versions use capillary tubes ranging from 39 to 66 inches for flexible sensor placement. Immersion-well models require matching the well depth and pipe connection size to the boiler fitting for accurate, reliable temperature sensing.

39–66 in.Remote Bulb Capillary Range

Remote-bulb aquastats are available with capillary tube lengths from 39 to 66 inches, giving installers flexibility in sensor placement relative to the control head.

2°F–30°FAvailable Differential Range

Differentials span from fixed 2°F increments up to adjustable 30°F ranges, letting you match the control's cycling behavior to the system's heating load and efficiency requirements.

120VStandard Oil-System Relay Voltage

Many aquastats designed for oil heating relay applications operate at 120V—always verify voltage before installation to avoid control damage or unsafe wiring.

5 FunctionsControl Function Types Available

Aquastats are available in five primary control functions: high-limit, high/low-limit, manual-reset high-limit, circulator control, and combination hydronic heating control.

Comparison Guide

Aquastat Types at a Glance: Which Control Fits Your Job?

Use this table to match the control function to the application before comparing individual models.

Control NeedBest Aquastat TypeWhy It Matters
Prevent boiler overheatingHigh-limit aquastatCuts burner power when water temperature exceeds the set point, protecting the heat exchanger and system components
Maintain minimum boiler water temperatureLow-limit aquastatHolds the boiler above a set minimum to prevent condensation, cold starts, and inefficient cycling
Manage both high and low limits in one unitHigh/low combination aquastatReduces wiring complexity and control head count in hydronic systems that need both functions
Require manual intervention after a safety tripManual-reset high-limit aquastatPrevents automatic restart after a high-temperature fault—required in some safety-critical or code-specific installations
Switch a circulator pump by water temperatureCirculator control aquastatTurns the pump on or off based on boiler water temperature rather than a thermostat call, improving zone efficiency
Planning Guide

Plan the right fit before choosing

Planning Guide

Step 1: Define the Control Function You Need

Before comparing models, identify exactly what the aquastat must do in your system. This determines the product category and narrows your options significantly.

  • High-limit only: shuts off the burner when water exceeds the set point—common in oil heating systems
  • Low-limit only: holds the boiler at a minimum temperature to prevent cold starts and condensation
  • High/low combination: manages both limits in one unit, typical for hydronic heating controls
  • Manual-reset high-limit: requires a physical reset after a high-temperature trip, used where safety lockout is required
  • Circulator control: switches the pump on and off based on water temperature rather than controlling the burner
Planning Guide

Step 2: Confirm Temperature Range, Differential, and Voltage

Once the control function is clear, match the aquastat's operating specifications to your system's requirements. Mismatched differentials or voltage ratings are a common source of installation problems.

  • Verify the aquastat's temperature range covers your system's normal operating and limit set points
  • Choose a fixed differential (2°F, 5°F, 8°F, or 10°F) or an adjustable differential (typically 5°F to 30°F) based on how tightly the system needs to cycle
  • Confirm voltage compatibility—120V is standard for oil-system relay applications; check your control circuit before ordering
  • Break-on-rise action means the switch opens as temperature increases—confirm this matches your wiring diagram
Planning Guide

Step 3: Match Mounting Style and Sensing Element to the Installation

Physical compatibility with the boiler or piping is just as important as the electrical and temperature specifications. Measure before ordering.

  • Strap-on models clamp to the outside of a pipe—verify the pipe diameter is within the clamp's range
  • Vertical-mount models attach directly to the boiler body or a tapping
  • Remote-bulb models place the sensing element away from the control head—measure the required capillary length (available in 39 to 66 inch options) before selecting
  • For immersion-well installations, record the pipe connection size, insertion diameter, and required well depth to ensure the well fits the boiler tapping and the bulb seats correctly
FAQ

Common questions before buying

What is the difference between a high-limit and a combination aquastat?

A high-limit aquastat performs one function: it shuts off the burner when water temperature exceeds the set point. A combination aquastat handles both high-limit and low-limit control in a single unit, and some models also include circulator switching—making them the standard choice for full hydronic heating system control.

How do I know which differential setting to use?

The differential determines how far the temperature must drop below the set point before the control restarts the burner. Tighter differentials (2°F–5°F) reduce temperature swings but increase cycling frequency. Wider differentials (10°F–30°F) allow more swing but reduce short-cycling. Match the differential to your system's load characteristics and the manufacturer's recommendations for the boiler.

Can I use a strap-on aquastat instead of an immersion-well type?

Strap-on models sense temperature through the pipe wall and are easier to install without draining the system, but they are generally less accurate than immersion types. Immersion-well aquastats place the sensing bulb directly in the water, providing more precise control. Use the type specified for your system or control application.

What capillary length do I need for a remote-bulb aquastat?

Measure the distance from where the sensing bulb will be located on the pipe or boiler to where the control head will be mounted. Remote-bulb aquastats are available with capillary lengths from 39 to 66 inches. Choose a length that reaches comfortably without sharp bends or kinks in the capillary tube, which can affect sensing accuracy.

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