problem with vintage oil furnace starting

[trader4]

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

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.

Seems unlikely.

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.

[Stormin Mormon]

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

SM: Sounds like thermostat was open connection.

SM: So far, yes.

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

SM: Might be time to call a service company.

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

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

SM: Can't see it from here.

[Stormin Mormon]

SM: If you have no volts at R and W or R and C at the furnace, you may have a bad fuse or bad transformer.

SM: What's with that? BEhind furniture?

SM: If 24 VAC from R to W, then the transformer is good. And the low voltage fuse.

[trader4]

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?

[micky]

Yes. R & W.

Yes.

Not yet. Probably not ever. I have the house's original thermostat, and I have a replacement control panel, from the same model furnace. One of those, or both, will make it work. But I just can't believe they both failed in the same night.

I'm trying to think of something that could account for the apparent failure of both the stat and the control panel, but I can't.

They should use different names.

Let me hold it up to the screen. Better?

For now, I"ll run the furnace manually 2 or 3 times a day for a couple hours a day, or whatever it takes I was able to get the house to 79, then turn off the furnace for at least 10 hours and it was still 64. So I can sleep all night. I need time to think and I'll figure out what's wrong in a day or two.

[micky]

Definitely the right wires.

FWIW, the handles on two of my toilets failed within 3 months of each other, after 34 years. I have to check the third toilet, that I don't use. But that's still not the same night.

I measured it with no load. You're right, I should measure again with a load.

A lot of the places I'd like to take measurements from are not available. The circuit board is in a metal box, a half inch about the metal bottom.

I have insulation piercing alligator clips, for testing. Maybe I'll try those. (In high school I had to use an ice pick, but it was only 12 volts.)

Not sure whether to just change control boxes or to try to debug this one.

I'm going to go eat lunch. Seems like a reasonable compromise.

[trader4]

Maybe we should recap where you're at.

A - You say with the thermostat wires disconnected at the furnace, you can connect a meter to those wires going back to the thermostat and measure an open circuit, even though it's cold and the thermostat should be calling for heat. That's across the red and white wires, which were connected to the R and W terminals, before you removed them. Have you verified this? It's repeatable? You're sure the meter is on the ohm or continuity setting? How many wires go to the thermostat? 3? heat, fan, power? If you're

100% sure about this, then there is a problem with the thermostat or the wiring going to it.

It's also a little weird that you said something about not being able to get to the thermostat without moving furniture? That limits the ability to diagnose.

B - You say that shorting the R and W terminals, the furnace won't start.

C - You can get the furnace to fire up and run by holding down that relay button. It's not clear, is that with the thermostat wires disconnected and the terminals open? From the description it sounds like that is some kind of limit relay, but it seems odd to me that it would make the furnace start up by pushing it, if there is no call for heat. Any relay, etc I've seen that was marked as "limit" could cut it off if something wasn't right, but would not make it fire if there was no call for heat.

Have you looked at the flame sensor? Not dirty? With a furnace like that I'd have spare nozzle, flame sensor, filter, etc, ie the common cheap things that can go wrong.

Also, with years of experience with that furnace, I would think you'd know what gets tripped by what, at least to some extent. For example, if it starts up, no flame is detected, it shuts down. What do you have to push to reset it?

[micky]

Very good idea. I've been ignoring the flame sensor, except to note how it's connected on the schematic.

I can disconnect one lead and test if it's bad or not. It conducts when light is sensed, I red. Cadmium sulfite "The resistance value of an LDR becomes smaller, as the LDR is more and more exposed. The material is usually cadmium sulfide, the dark resistance is 1.10 M ohm resistor while the light resistance is about 75-300 ohm. LDR's have a relatively slow response time." I"m sure disconnecting will still work, though the Carrier diagram gives no explanation for the "flame detection electronic network." It's just a "black box". Even t hough it's made of clear plastic, I still have no idea how it works.

They're only $14 on Amazon, with lots of brackets, plus $5 shipping, but I have a spare one fo them too.

Me? Never.

[makolber]

google is your friend and a picture is worth 1000 words

Mark

[Stormin Mormon]

SM: That rules out the 24vac transformer gone bad.

SM: That is reasonable.

SM: Sometimes,one fails, and then "takes out" the other.

SM: Cascade failure.

SM: I agree.

SM: No, I've got the wrong glasses on.

SM: I suspect you will. I'd be looking for safety devices, and limit switches which may have been activated.

[Stormin Mormon]

I'm suspecting a safety switch of some kind. Either tripped with (or without ) cause.

[Stormin Mormon]

A couple Sundays ago, at church we had a furnace go down. Cold in that section of the building. It had given trouble several times, and this time they called in an outside company to look at it. Sure enough, one of the safety switches was just a bit out of place, and wasn't getting activated like it ought have.

[micky]

Will do.

BTW, in the original thermostat, a round Honeywell that was so popular for so long in the 60's and 70's (not the one I'm using now,) there were some square sine waves circuit traces on the little round circuit board, and I see something on the heating cooling wiring schematic that seems to correspond.

On the schematic they are labeled HTG ANT and CLG ANT.

Do you all know anythihng about these?

[clare]

Heating anticipator and cooling anticipator

[Stormin Mormon]

I've got a pretty good guess. I've had to set the old rounds, and yes, pretty sure I know.

[micky]

Ah. I thought they were heating antenna and cooling antenna. Your answer makes a lot more sense.

P&M because it's been two days and the group is busy.