17 Gas Heat – Control

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The control of a central gas heating system is fairly simple. An extra-low-voltage, 120 to 24 V transformer provides the power for a control circuit, while a dedicated 120 V low-voltage circuit provides power to the blower fan. Extra-low-voltage conductors connect the furnace to the room thermostat.

A circuit diagram, described in the surrounding text.
Control circuit for gas-fired furnace

When the room temperature gets sufficiently low, the room thermostat clicks shut, and the completed circuit opens the main gas valve. The inrushing gas is then ignited and begins to heat the air in the heat exchanger.

It is important that the heat exchanger isolate the products of combustion from the heated air that is to be circulated through the house. Once the temperature in the heat exchanger reaches a sufficient level, a second thermostat inside the furnace housing clicks shut and engages the blower motor.

When the heated air causes the temperature in the house to rise sufficiently, the main room thermostat contacts will open, de-energizing the main gas valve, thus cutting off the gas supply and extinguishing the burner flame.

Though the main gas valve has now been closed and the flame extinguished, the heat exchanger is still very hot, and so the blower motor continues to run, dissipating and distributing any residual heat. This will continue until the blower motor thermostat senses a sufficient drop in plenum chamber temperature, and opens its contacts.

This residual heat would normally cause overshoot of the desired room temperature, and so the low-voltage thermostat may incorporate a series connected, anticipator resistor, which creates a small amount of heat causing the thermostat to open before the set temperature is reached.  A few minutes after the thermostat opens, the blower control thermostat opens, de-energizing the blower motor, and the room  reaches the desired temperature.

The furnace will also contain a high temperature safety cut-out switch that will disconnect all gas to the heating elements if the temperature rises above a pre-set safety threshold. This can happen if the blower fan fails to engage and drive the heated air through the house, and thus draw cool air into the heat exchanger.



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