our furnace fan runs all the time

It doesn't switch off when the furnace does and keeps blowing cool air

I've replaced the filter and it's still running

The thermostat (RTH6400D) isn't set up to run the fan. It has only two wires connected, W and R

The furnace is a Beckett(by Honeywell) R7184B

Thank you


  • Can you post some photos of how your furnace is wired and/or a wiring diagram of your furnace? – ThreePhaseEel Jan 17 '16 at 23:27

If this problem just started (the furance hasn't always operated like this), then it's likely an open high limit switch.

If it's a manually resettable limit, you can try resetting it yourself. If it's an automatic reset device, you'll probably have to replace it.


It could be that your control is set to "Auto" instead of "Heat". Try changing the switch to "Heat" mode, adjust the thermostat set-point to slightly higher than the current room temperature & wait a minute or so for the furnace to turn on. After the set-point is reached, the fan should run a bit longer (less than 2 minutes) to blow the remaining heated air out of the heat exchanger & then shut off. If that suggestion does not work, then your system may not be wired correctly or you may have a defective control system.

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