2 stage heater-blower staying on low


Old 12-12-11, 05:24 PM
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2 stage heater-blower staying on low

Hi, I have a Robert Shaw 9615 tstat hooked to a rheem 2 stage gas furnace. This winter the high stage doesn't kick in or if it does only for about 15-30 seconds. the tstat clicks as if is sigfnaling the high stage, but nothing happens. New batteries. Any thoughts?
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Old 12-12-11, 05:35 PM
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You may have a W1 and W2 connection at the furnace that allows the thermostat to switch on 1st stage or second stage heating.

Use an AC voltmeter to determine what is being turned on. It may be that the thermostat is switching on the second stage heat, or it may not. Or the thermostat may be switching on the 2nd stage heat but the furnace isn't making that change.

Measure to see what is actually happening.

And listen carefully if the second stage is being turned on to see if the inducer motor is switching to a higher speed and the gas burners switching to higher input.

The pressure switch probably needs to register a switch in pressures as well --- that needs to be checked as well.

Welcome to the wonderful world of troubleshooting 2 stage furnaces.
Old 12-13-11, 02:07 AM
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Ensure that the W2 wire is connected to the W2 terminal at the t-stat and furnace control board.

With the power to the furnace off, jumper terminals W2 and R at the t-stat.

With the power off at the furnace, check for continuity (ohms/resistance multi-meter setting) between terminals R and W2.

If the circuit is open, the wire is broken.

If it's closed, turn the power to the furnace back on and jumper R and W1 at the t-stat.

Once the blower comes on, jumper R and W2 (leave R and W1 jumpered); if the inducer and and blower motor don't speed up, there's a internal problem with the furnace and you should call a technician.

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