problem with vintage oil furnace starting

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On Tue, 11 Feb 2014 19:13:52 -0500, Stormin Mormon

And by the way, the stuff I had for salting sidewalks that turned into a glob was calcium chloride, like you thought.
Very different from my 50 pound bag of CaCl2, which never turned into a glob (though I had it less than a year, not 10 or 15 like this) , and which had a big puddle of water at the bottom and on the other side of the separator,( not like this which had no water, but was in a capped container, but otoh, has cracks in the container from age. I should tape them.)
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wrote:

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.
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.
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.
BTW, the furnace control board only has 2 resistors, 1 ceramic disc capacitor, 2 transistors, and 2 relays***.
Any suggestions or corrections?
***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.
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micky,
\> All of these steps done when the house was cold again.

Yes, the thermostat is open. What is the setting of the thermostat? Should it be open or closed? Set the temp to 50 and it should be open, unless your house is really cold. Set it to 85 and it should be closed.

What? You have 24v across the wires when they are connected to the thermostat but not when they are disconnected from the thermostat? This makes no sense to me.
Measured the resistance it

You've lost me. What are you confirming? Are you measuring the resistance across the thermostat? Is the thermostat open or closed? Set the thermostat really hot (80 or 90). Be sure that the stat is set for heat That should close the switch. You should get a very low resistance

Yes, if the thermostat is open it is not calling for heat. Look you aren't being very clear. Should the thermostat be calling for heat? Please tell us the temp setting for the thermostat and confirm that the stat is set for heat.

Is there 24v across these wires before they are connected?
Dave M.
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On Tue, 11 Feb 2014 20:14:03 -0500, "David L. Martel"

I'm sorry. The furnace, not the thermostat. No voltage between the thermostat wires when one wire is disconnected from the *furnace*, not the thermostat, I'm standing at the furnace and disconnecting the wire there. . The thermostat itself is where it's always been, but it is almost inaccessible right now, since I hurt my back.

That the thermostat switch is open.

Yes. One wire, red still screwed to the furnace control board and one wire, white, unscrewed from it. Plus I checked which screws get the thermostat wires, and those are it.

It should have been closed then because the house was cold at the time, maybe 58 degrees when the stat is set at 68 and normally keeps the house at 68.
But when I measure the resistance between the two wires from the thermostat, it is over 100,000 ohms. It's not closed when it should be closed.

Well I can't reset the thermostat until I get access. Requires some heavy lifting. Probably need a friend.

Set at 68 and set to call for heat. I havent changed that in years. (The AC is broken and during the late-spring, summer, early fall I just turn off the power to the furnace, so it doesn't start on a cold night.)

There was a couple minutes earlier, when I did test 1, And I measure it earlier too and also got 24v. So a jumper from one screw to another should start the furnace. But it doesn't start.
Sorry I was confusing.

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On 2/11/2014 8:35 PM, micky wrote:

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.
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Christopher A. Young
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On Tuesday, February 11, 2014 6:52:50 PM UTC-5, micky wrote:

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.

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On Wed, 12 Feb 2014 07:37:30 -0800 (PST), " snipped-for-privacy@optonline.net"

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.

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google is your friend and a picture is worth 1000 words
http://www.electrical-online.com/thermostat-wiring-explained/
Mark
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On 2/12/2014 12:12 PM, micky wrote:

I'm suspecting a safety switch of some kind. Either tripped with (or without ) cause.
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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.
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On 2/11/2014 6:52 PM, micky wrote:

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.

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On Wednesday, February 12, 2014 11:27:00 AM UTC-5, Stormin Mormon wrote:

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?
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On Wed, 12 Feb 2014 08:44:35 -0800 (PST), " snipped-for-privacy@optonline.net"

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.
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On Wed, 12 Feb 2014 11:27:00 -0500, Stormin Mormon

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.
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On Wednesday, February 12, 2014 12:02:07 PM UTC-5, micky wrote:

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

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

SM: I agree.

devices, and limit switches which may have been activated.
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Christopher A. Young
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