An aquastat is the temperature-sensing control that tells a gas boiler when to fire and, on many combination systems, when to prioritize domestic hot water.
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An aquastat is a temperature control device on a gas boiler that senses the water temperature in the system and signals the burner to fire or stop firing to maintain the set range, and on many residential boilers it also manages priority between space heating and an indirect domestic hot water tank. It typically fails by drifting out of calibration, sticking due to mineral buildup or corrosion around its sensing well, or through a worn switch contact, producing symptoms like inconsistent temperature, short cycling, or a boiler that runs constantly without properly satisfying either heat or hot water demand.
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An aquastat is essentially a thermostat for the boiler's own water, distinct from the room thermostat that calls for heat throughout the house, and it typically has a high limit setting that prevents the boiler from overheating and, on many models, a differential setting that determines how far the temperature can drop before the burner fires again. On combination systems that also supply an indirect-fired domestic hot water tank from the boiler's own hot water, the aquastat, or a related priority control, often governs which demand, space heating or hot water, gets the boiler's output first when both are calling at the same time.
| Symptom | Likely aquastat issue | Category |
|---|---|---|
| Boiler short cycles more than it used to | Aquastat differential drifted or contacts wearing | Calibration / wear |
| Domestic hot water inconsistent even though heating is fine | Priority control or aquastat not switching correctly | Priority switching |
| Boiler runs continuously without ever satisfying either demand | Aquastat stuck or reading incorrectly | Sensor / switch failure |
| High limit seems to trip at the wrong temperature | Aquastat sensing well fouled or miscalibrated | Sensing accuracy |
Most aquastats sense temperature through a probe inserted into a well in the boiler's piping, and if mineral deposits, corrosion, or trapped air affect the contact between that well and the surrounding water, the aquastat can read a temperature that does not accurately reflect what the water is actually doing. This produces symptoms that look confusing at first glance, such as a boiler that seems to run longer or shorter than its settings would suggest, because the control itself is working from bad information rather than the burner or pump having a mechanical fault.
It is worth distinguishing the aquastat from the room thermostat and from the boiler's own gas control valve, since homeowners sometimes use these terms interchangeably. The room thermostat tells the system a zone wants heat; the aquastat governs the boiler's own water temperature and, often, hot water priority; the gas control valve on the burner itself responds to signals from these controls to actually modulate gas flow. A fault in any one of the three can produce overlapping symptoms, like inconsistent heat or hot water, which is exactly why a technician traces the actual signal path rather than assuming which control is at fault from the symptom alone.
Noting whether the issue seems tied to space heating, hot water, or both happening at once (which points toward priority switching specifically) gives a technician a useful starting point. Watching how long the burner runs during a typical cycle compared to what felt normal previously is also useful information. The aquastat itself, along with its sensing well and wiring, sits directly in the boiler's control system and pressurized piping, so testing and replacement should be left to a licensed technician rather than attempted as a homeowner repair.
Safety note
The aquastat includes a high-limit safety function that helps prevent the boiler from overheating, and a malfunctioning aquastat can affect that protection. Have any suspected aquastat fault diagnosed by a licensed technician rather than left unaddressed. If you smell gas near the boiler at any point, leave the home and call FortisBC at 1-800-663-9911.
No, the room thermostat calls for heat in the house, while the aquastat governs the boiler's own water temperature and, on many systems, priority between heating and domestic hot water.
Yes, on systems where the aquastat manages priority for an indirect domestic hot water tank, a fault there can affect hot water specifically while space heating continues to work normally.
A technician measures actual boiler temperature against the aquastat's readings and switching behaviour to determine whether adjustment or replacement is the appropriate fix.
It involves working directly on the boiler's electrical controls and pressurized piping, so it should be left to a licensed technician.
It can affect the high-limit protection that helps prevent overheating, so it should be diagnosed and corrected by a technician rather than left unaddressed.
That can point toward an aquastat sensing inaccurately due to a fouled sensing well or worn contacts, among other possible causes a technician can test for.
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