Boiler Fault Finder

Thermostats – Liquid Expansion

Thermostats – liquid expansion / Vapour Pressure Type Liquid expansion / vapour pressure Reasons to affect a liquid / vapour expansion type thermostat: Contact points worn out, burnt out or pitted Loss in manufacturers calibration Phial incorrectly located into pocket The unit consists of a remote phial (located within or very near to heat exchanger) and a capillary tube that joins it to the bellows chamber. Within the phial is a volatile fluid having a rapid expansion rate. As the phial heats up, the fluid expands, in some cases changing to a vapour, and passes through the small capillary tube to fill the bellows. The bellows in turn, increases in size to take up this expansion and in so doing, breaks the electrical circuit and extinguishes the pilot and / or main burner. On cooling, the volatile fluid contracts and is forced back by the pressure of a spring acting on the bellows, through the capillary tube to the phial and remaking the electrical circuit once more to relight the pilot and / or main burner. To test a Liquid expansion / vapour pressure type thermostat: Liquid expansion / vapour pressure type thermostat – (thermostat calling) Electrically isolate boiler / system controls Remove E N L wires from thermostat terminals Set multimeter on ohms scale for resistance readings, across L N at thermostat terminals: Closed circuit of less than 1 ohm = ok No resistance/infinity = faulty Continuity = ok Liquid expansion / vapour pressure type thermostat – (thermostat satisfied) Electrically isolate boiler / system controls Remove E N L wires from thermostat terminals Set multimeter on ohms scale for resistance readings, across L N at thermostat terminals: Closed circuit of less than 1 ohm = faulty No resistance/infinity = ok No continuity = ok To test a Liquid expansion / vapour pressure type thermostat 240 V LIVE TEST Boiler thermostat on maximum setting (thermostat calling) Set multimeter to correct voltage scale and check for 240V AC between NC and N (neutral) Zero voltage = faulty Turn boiler thermostat to O min setting (satisfied) Set multimeter to correct voltage scale and check for 0V AC between NC and N (neutral) 240V AC = faulty The control thermostat may rest in a Normally Open (NO) position of this type. On some control thermostats, the manufacturer may need to be consulted in order to identify the relevant terminal connections for correct operation….

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