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.


But the wiring "diagram" on the same page for my
model shows another limit switch in the fan and limit control, a
separate box that extends iiuc into the firebox. No wonder I'm
confused.

SM: Furnace usually have several things that turn it off, and one that
turns it on. That's safety for you.



The second relay might not be a relay exactly. It works with the
cadmium photo cell to turn the furnace off if there is no flame in the
firebox, like if the furnace runs out of oil. And maybe if the firebox
gets too hot. I don't know. I've never had that particular problem.

SM: Can't see it from here.