Boiler Primary Control (Aquastat) Testing

Published: July 21, 2026 — CANRO Heating & Cooling — Greater Vancouver, BC

How the Aquastat Works

The aquastat is a temperature sensing and control device that regulates boiler water temperature. It has three main functions: high-limit (stops the boiler if water gets too hot), low-limit (maintains minimum water temperature), and circulator control (turns the pump on and off based on temperature).

Testing the Aquastat

With the boiler running, use a thermometer to measure the actual water temperature at the boiler. Compare to the aquastat setting. If the boiler shuts off before reaching the set temperature or runs past it significantly, the aquastat may be faulty. Test the high-limit by turning the setting down — the boiler should shut off within a few degrees.

Replacement Cost

Aquastat replacement costs $300-600 including the device and labour. The boiler must be drained partially for replacement. If your boiler is over 15 years old and the aquastat fails, consider boiler replacement — the aquastat is a sign of overall aging.

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Safety Reminder

If you smell gas, suspect carbon monoxide or believe there is an immediate danger, leave the property and contact emergency services or the appropriate gas emergency authority. Do not remain inside — exit the building immediately and call for help from outside.