On Wednesday, February 12, 2014 11:27:00 AM UTC-5, Stormin Mormon wrote:
On 2/11/2014 6:52 PM, micky wrote:



All of these steps done when the house was cold again.



A) When the thermostat wires for heat are connected to the thermostat

screws (for heat) on the furnace control board, there are 24VAC between

them. That alone means that the thermostatic switch in the thermostat

is open, or the voltage drop would be very close to zero.

SM: Are you reading at the furnace? Between which two terminals?





B) But to be sure, I disconnected one of the wires to the themostat,

measured the voltage between that wire and the other wire/screw (not

disconnected), and it was zero of course. Measured the resistance it

was over 100,000 ohms. Confirmation.

SM: Sounds like thermostat was open connection.





OKay, so an open switch means the thermostat is not calling for heat,

even when it is cold in the house.



SM: So far, yes.





C) Then I jump the two screws on the furnace control board where the

thermostat is connected (the same two I've been working with above.)

Jumping them should call for heat but the furnace doesn't start!!



SM: Were the screws on the furnace lettered? Probably R and W?





A and B indicate a bad thermostat. C indicates a bad furnace.

Specifally, a bad furnace control board because other tests have

excluded everything else.



I just don't believe they both failed the same night.



BTW, the furnace control board only has 2 resistors, 1 ceramic disc

capacitor, 2 transistors, and 2 relays***.



Any suggestions or corrections?



SM: Might be time to call a service company.







***I have found in the "schematic" the black relay whose armature I've

pushed to start the furnace. It is labeled "limit switch" and it

interrupts the 110 volts to the whole control board and the motors and

ignition transformer.



SM: Usually there for a reason. I'm not familiar enough with your model

of furnace to know which limit switch is this, adn what it detects.


Since pressing that started the furnace and kept it going,
it would seem to me
that either some sensor is telling the control board to shut down
or the control board is bad. A faulty flame sensor would be
a prime culprit.

The other part, where he's saying the thermostat wires at the
furnace indicate an open, don't know what to say about that.
Operator error perhaps?