One day my parent's furnace and thermostat stopped working. The thermostat is a smart thermostat that depends on 24volts to the R and C wire.

We measured voltage between the R and C wire and was getting roughly 3-5 volts. Measured by using ACvoltage setting on multimeter and probing R and C terminals on circuit board.

We measured voltage of the transformer wires on the low voltage side and measured about 27 volts. Probe the transformer red and blue wire from transformer at the connector to circuit board. We unplugged the connector first, then probed the two wires.

Note: we made sure that the safety switch of the furnace door is pressed in prior to taking the measurements in all cases.

What could be the issue? Why isn't the voltage getting to my R and C terminals on the circuit board? Could the board be faulty? Is there anything else I should check before I go out to buy a new board? Could there be a faulty switch telling the circuit board not to send 24voltage to R terminal?

Can I potentially connect the wires directly from the transformer directly to the R and C wire, essentially bypassing the circuit board? What am I risking if I do this?

  • What make and model is this furnace? – ThreePhaseEel Mar 24 '18 at 18:28
  • Hello, and welcome to Stack Exchange. Could this just be a broken connection between the transformer and your thermostat? (If not, why?) – Daniel Griscom Mar 24 '18 at 19:50
  • Is this an electronic control board or just a narrow RCYWG connection board? – user83956 Mar 25 '18 at 2:06

Just wanted to come back to report solution. It turned out one of the high limit switches was faulty. It was the one that was mounted to the top of the frame of the furnace. HH18HA166

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