Aquastats

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What Are Thermostats and Aquastats?

Thermostats and aquastats are temperature control devices used in heating systems. While thermostats regulate air temperature, aquastats control water temperature in boilers and water heaters.

Understanding Aquastat Controls

An aquastat control monitors water temperature and activates the burner when heat is needed. It ensures the system maintains safe and efficient operation.

Aquastat for Boiler Systems

An aquastat for boiler units helps manage both high and low temperature limits, preventing overheating and optimizing performance in hydronic heating systems.

Aquastat for Water Heater

Installing an aquastat for water heater setups allows for precise temperature control, helping reduce energy waste while delivering consistent hot water.

Honeywell Aquastat Options

Honeywell aquastat models are known for reliability and accuracy. They are commonly used in residential and commercial systems for trusted water temperature control.

When to Replace an Aquastat

If your system isn’t heating properly or runs continuously, the aquastat may be malfunctioning. Replacing it with a compatible unit can restore performance and safety.

Frequently Asked Questions

  • Single‑function (high-limit only) – primarily serves as a safety cutoff.
  • Dual-function – includes both high and low limits for maintaining minimal temperature and safety.
  • Triple-function – adds circulator/pump control to the above.

There are also strap-on models (mount to pipes) and advanced outdoor-reset aquastats that adjust boiler temperature based on outside conditions.

Typical settings:

  • High limit ~180 °F (82 °C)
  • Low limit ~160 °F (71 °C)
  • Differential (DIFF): controls the gap between on/off switching, typically around 10–20 °F.

Always turn off power before opening the aquastat cover and use manufacturer guidelines during adjustments.

The aquastat's low limit might be set too high, forcing the boiler to maintain temperature even when no heating is needed. Adjusting or disabling the low limit can convert it to "cold-start" mode.

  • A thermostat controls air temperature and calls for heat.
  • An aquastat controls boiler water temperature and safety.
  • A pressuretrol regulates steam pressure in steam boilers and works alongside the thermostat and aquastat.

Replace it if:

  • The boiler cycles off/on at incorrect temps.
  • The differential adjustment stops working.
  • Safety resets trip or don't reset properly.
  • It’s over 10–15 years old or shows noticeable wear.

Inspect annually for:

  • Correct high/low temperature cutoffs.
  • Proper differential function.
  • No corrosion or failed switch action.

Yes, but caution is essential. You must:

  • Turn off power and water.
  • Match bulb type (immersion, strap-on, remote), capillary length, switch type, and differential range to the original unit.

Professional installation is recommended for complex setups, such as outdoor-reset models.