On Tuesday, February 11, 2014 6:52:50 PM UTC-5, micky wrote:
On Tue, 11 Feb 2014 07:56:18 -0500, micky

wrote:



I have an oil furnace, forced air, It's a Carrier 58HV085, 34 years

old, and works well, until last night after I fell asleep.



(Please, no one tell me to get a new furnace. I've heard that before.

When I can, I will.)



I woke up early and the 2nd floor temp was 60 instead of 68.



With your help, I've made a lot of progress on the furnace, but I have a

paradox, I think.



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.



Assuming you're on the right wires, I agree.




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.


Again, if you're indeed on the wires that go to the
thermostat, then either there is a break in the wiring
or the thermostat is bad.




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

even when it is cold in the house.



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!!



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.



Seems unlikely.





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

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



Any suggestions or corrections?


You have 24V at the two contacts at the furnace that go
to the thermostat. Should be the red and white wires.
When you connect them together and the furnace doesn't start,
did you measure the output of the transformer? 24V there?

You could have a bad connection there in the furnace wiring.
If a connection was high resistance, you could see 24V
at the terminals, but when you connect them together,
enough current would not flow to close the relay.
Imagine the circuit being correctly working, but then
a 10K resistor is put in series (simulating the bad connection)
You'd still see 24V open circuit, but if you connect
the terminals not enough current will flow to close the
relay. Could also be some bad component too, eg the
relay coil, presenting a higher resistance than should
be there.

The other part, about the thermostat and/or wires going
to it, no more ideas on what's going on there.




***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. 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.



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.