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Trane XR95 ~2.2V at Thermostat

Woke up this morning to a cold house and a thermostat message saying “no power to the Red wire.” Measured R and C using a multimeter and it reads ~2.2 Volts. Took the covers off the furnace, taped the cover-switch down and read the R and C voltage at the board and it reads 24v.

The furnace control board has 2x Blue wires, 2x Green Wires, 2x Red Wires and 1x White wire coming from it. Tracing the wires FROM the control board:

White = To "THERMOSTAT" / Red wire #1 = To "BLUE BOX" / Red wire #2 = To "DAMPER" / Blue wire #1 = To "THERMOSTAT" / Blue wire #2 = To "DAMPER" / Green Wire #1 = To "DAMPER" / Green Wire #2 = to "THERMOSTAT"

BLUE BOX: A Blue-colored box on the floor with tubing coming out of it. Looks like some sort of pump. The White wire coming from this box is twisted to the RED wire going to the THERMOSTAT. So this Blue box appears in-line of the RED Thermostat wire.

DAMPER: This damper has an electric motor and a manual control lever that has 0-50-100 written on three places on the duct to where the lever is pointed. In-line wired between the duct motor and the furnace is what looks like a day/time rotary-type timer box. I assume this is some kind of whole-house ventilation or make-up air system.

THERMOSTAT: Nest White Thermostat

There is continuity from the thermostat wires in the house to where they run into the furnace, so the wiring appears to be good.

I’m thinking the power issue is stemming from that blue box/pump on the floor. I don’t have central AC, but live in a fairly wet environment and this state has heavy regulations on what needs to be installed in houses, such as pumps/make-up air dampers/whole-house ventilation/etc. I lived in another house nearby that was newer and did not have one of these.

My question: Would it be safe to disconnect this blue box/pump (whatever it’s for) and wire the RED thermostat wire directly to the furnace board effectively bypassing it?

Thanks!

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  • Bad transformer?
    – Huesmann
    Nov 20, 2023 at 12:55

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The box on the floor is likely a condensate pump. Temporarily, yes, you could bypass it to see whether the furnace runs again.

The Trane XR95 is a condensing furnace. This means that it extracts so much heat from the exhaust stream that the water vapor in the exhaust actually condenses to liquid. This liquid drains back through the exhaust pipe into the furnace, where it accumulates in a collector box and then flows out toward the floor. If a floor drain is available the condensate can/should be directed into that drain. When there's no drain available, a condensate pump removes the liquid away to a place where a drain is available.

There is a float switch inside the condensate pump. It is wired so that if the pump reservoir fills with liquid, which means the pump is failing to evacuate the liquid, then the float switch opens and interrupts operation of the furnace (or air conditioner, if applicable). This is done to avoid property damage that would result from an overflow of the condensate.

You likely need to inspect the pump and its discharge line to find out whether there's an obstruction or simply a failed pump.

A furnace can produce a fair amount of condensate -- maybe even quarts per day. If your condensate pump has failed, you could temporarily capture the condensate in a bucket or other container, with the understanding that it's up to you to see that the liquid is carried away and dumped before it overflows.

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