Thermostat - bi-metallic (snap-action)

Air Conditioning and Heat Pumps · Evaporative Coolers · printed page 225 (PDF page 226)

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Snap Action Snap Action – Air Conditioning

Text in the illustration

the central hub of the bi-metallic coil is attached to the thermostat temperature setting dial - turning the dial down (counterclockwise) means that it will have to get even cooler before the magnet activates the circuit
bi-metallic coil
when the coil warms up, it wants to straighten out - this pulls the magnet away from the contact switch
magnet
electrical circuit to gas valve or oil burner
circuit open
as the bi-metallic coil cools, the magnet moves toward the contact switch - pulling up the bottom armature - this closes the circuit and starts the furnace
circuit closed

Generated explanation

Two side-by-side diagrams show the open and closed states of a snap-action bi-metallic thermostat. On the left, warming causes the spiral coil to straighten, moving its attached magnet away from the contact switch and leaving the burner control circuit open. On the right, cooling moves the magnet toward the switch, pulling up the bottom armature to close the circuit and start the furnace. The central coil hub connects to the temperature setting dial; the annotation explains that turning the dial counterclockwise lowers the temperature required to activate the circuit.

Components

bi-metallic coil central coil hub thermostat temperature setting dial magnet contact switch bottom armature electrical circuit to gas valve or oil burner

Materials

bi-metallic strip

Related illustrations

0763

Thermostat - bi-metallic (snap-action)

· p. 124

The illustration compares the furnace-off and furnace-on states of a bi-metallic snap-action thermostat. A spiral bi-metallic coil has a central hub attached to the temperature-setting dial and an outer arm carrying a ma