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HVACR114 – Electrical for Gas Heat

HVACR114 – Electrical for Gas Heat

Limits and switchesLimits and switches

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High Limit SwitchHigh Limit Switch

• Is a bi-metal switch that will open if the furnace exceeds a pre-determined temperature.

• Usually about 200 degrees.

• Some are manually reset, others are automatic reset

• Is a bi-metal switch that will open if the furnace exceeds a pre-determined temperature.

• Usually about 200 degrees.

• Some are manually reset, others are automatic reset

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Roll out switchesRoll out switches

• Looks very similar to the high limit switch.

• Is also a bi-metal switch that opens if the flame rolls out and touches the switch.

• Some are manually reset, others are automatic reset.

• Looks very similar to the high limit switch.

• Is also a bi-metal switch that opens if the flame rolls out and touches the switch.

• Some are manually reset, others are automatic reset.

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Fan Limit switchesFan Limit switches

• Fan limit switches do two things. It controls the fan and acts as a high limit switch.

• The fan limit switch automatically turns the fan on and off based on a pre-set temperature.

• This is to allow the heat exchanger to heat up before the fan comes on.

• In doing this, keeps cold air from being circulated through the duct.

• Fan limit switches do two things. It controls the fan and acts as a high limit switch.

• The fan limit switch automatically turns the fan on and off based on a pre-set temperature.

• This is to allow the heat exchanger to heat up before the fan comes on.

• In doing this, keeps cold air from being circulated through the duct.

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Fan limit switchFan limit switch

• This switch also has a delay in shutting the blower down.

• This allows the heat exchanger time to cool off and dissipate the furnace heat at the end of the cycle.

• This switch also has a delay in shutting the blower down.

• This allows the heat exchanger time to cool off and dissipate the furnace heat at the end of the cycle.

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Fan limit switchesFan limit switches

• The temperature sensing element of the switch, called a bimetal helix, is located in the air stream near the heat exchanger.

• When the furnace turns on and the air is heated, the bimetal expands and closes the contacts, turning on the fan.

• The temperature sensing element of the switch, called a bimetal helix, is located in the air stream near the heat exchanger.

• When the furnace turns on and the air is heated, the bimetal expands and closes the contacts, turning on the fan.

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Fan limit switchesFan limit switches

• The limit switch side of this device is a safety device.

• If the fan does not come on or if there is another problem causing the heat exchanger to overheat, the limit switch will open it contacts, which shuts power to the gas valve. Turning the system off.

• The limit switch side of this device is a safety device.

• If the fan does not come on or if there is another problem causing the heat exchanger to overheat, the limit switch will open it contacts, which shuts power to the gas valve. Turning the system off.

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Fan Limit setting knobs and there functions.Fan Limit setting knobs and there functions.

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Pictures of different types of limit switchesPictures of different types of limit switches

• They look different, but work the same.• They look different, but work the same.

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Sail switchesSail switches

• Looks exactly as its name. it has a sail at the end of a small shaft.

• It is usually installed in the flue pipe.

• It is used to sense the inducer motor operation.

• Commonly found on roof top units.

• Looks exactly as its name. it has a sail at the end of a small shaft.

• It is usually installed in the flue pipe.

• It is used to sense the inducer motor operation.

• Commonly found on roof top units.

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Sail switchSail switch

• On a call for heat, the inducer motor starts. Which has to prove a draft in the flue. The draft pushes the sail, which makes contact with switch.

• The switch closes sending power to the gas valve or ignition control, thus starting the heating cycle.

• They can also be found in duct work to prove enough airflow is being supplied. If not, it shuts the system down.

• On a call for heat, the inducer motor starts. Which has to prove a draft in the flue. The draft pushes the sail, which makes contact with switch.

• The switch closes sending power to the gas valve or ignition control, thus starting the heating cycle.

• They can also be found in duct work to prove enough airflow is being supplied. If not, it shuts the system down.

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