Ask a service engineer what kills capillary thermostats and they rarely blame the thermostat itself. Kinked capillaries, bulbs hanging loose in their wells, tubes pinched behind panels — installation faults that drift the calibration or destroy the unit before it has switched a load a hundred times. The thermostat, its capillary and its bulb are one sealed, liquid-filled system, calibrated as a unit at the factory. This guide covers how to measure, place and route that system so the control performs to its specification.
Measure the capillary length on the real route
Capillary length is not the straight-line distance between bulb and control — it is the length along the path the tube will actually take, around bends and through grommets, plus slack. Lay a flexible tape or a piece of wire along the intended route: from the bulb seat, out of the tank or oven cavity, along the harness path, to the control’s mounting position. Add roughly a hand’s width of service slack, so the control can be withdrawn from the panel for replacement without straining the tube, and order that length — we configure capillary lengths to order on the AG16 and AG32 series. Too short is worse than too long: a taut capillary transmits every vibration and panel flex straight into the bulb seal.
Seat the bulb where the temperature is real
The bulb is the only part that senses. It must sit fully in the medium — the oil, the water jacket — or fully inserted in its thermowell, in the flow of the heated medium, away from direct contact with the heating element. Three rules cover most applications:
- Fully inserted — a bulb half out of its well reads a blend of process and ambient temperature, and the control will overshoot
- Firm contact — clamp the bulb or use the well’s spring or gland so metal meets metal; a loose bulb lags, and the control hunts
- Right diameter — the bulb diameter must match the well; an undersize bulb needs the correct gland or packing, never random shims
On fryers, the bulb belongs in the oil stream below the crumb zone, not resting on the tank floor where sediment insulates it. On ovens, it belongs in the cooking chamber’s air stream, clear of the elements and the fan wash.
Route the capillary like a sealed tube — because it is one
- Never kink, fold or crush it. A sharp bend restricts the fill path; pinched flat, the tube is scrap and the calibration is gone
- Bend wide. Form bends gradually by hand over a generous radius — think of a saucer, not a coin edge
- Protect every pass-through. Where the tube crosses sheet metal, fit a grommet; vibration will saw an unprotected tube through in months
- Keep clear of heat and motion. Route away from burner flames, element surfaces and fan blades — the capillary fill responds to temperature along its whole length, so a section baked by a flue falsifies the reading
- Coil the surplus properly. Excess length is coiled in wide, flat loops, tied loosely in a thermally calm area — never stuffed behind the control or wrapped around hot parts
- Never cut it. The capillary cannot be shortened or repaired in the field; a cut tube means a new thermostat
Clamp without crushing
Support the tube at intervals along long runs with clips or ties snug enough to stop chafing, loose enough not to dent the tube wall. At the bulb, use the manufacturer’s gland, spring or bracket. At the control body, leave the service loop outside the panel, so the unit can be pulled forward for testing without disturbing the bulb.
Commissioning checks
- Heat the appliance to its working temperature and compare a calibrated probe thermometer against the dial, allowing for the control’s differential
- Watch two or three full cycles: overshoot or short-cycling usually points back to bulb seating, not to the control
- Trip-test the safety limiter independently — it is a separate device and must be proven separately
- Re-check after the first week of service: panels settle, clips relax, and a chafe point shows itself early
Installed this way, a capillary thermostat is fit-and-forget for years. Installed badly, the best control in the world will drift, hunt or fail early — and the failure will be charged to the thermostat. For how the sensing system works, see How to Choose a Capillary Thermostat; when readings go wrong in service, work through Capillary Thermostat Troubleshooting.
Capillary Thermostat Troubleshooting · Capillary Control Thermostats