problem with vintage oil furnace starting

[micky]

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.

Question 1: When the thermostat is set to auto, as it always is, the air circulation fan goes on about a minute after the furnace starts. In a furnace of this age, is that because of the Fan and Limit Control???? As opposed to something on the small control board, which has only about

15 resistors, capacitors, transistors, including 2 relays.

Question 2: Does the Fan and Limit Control keep the furnace running once it reaches fan termperature (and until the thermostat says for it to stop.)? I would think the answer is No, and that the control board alone starts the furnace and keeps it running, until the thermostat says no.

To try to get the furnace going, I pressed the reset button, but it didn't need to be reset. I checked the circuit breaker; measured the voltage on the secondary of the power transformer, and removed the control box cover, just in case anything cried out, "I'm broken".

Besides the relay with the reset button, there is another relay next to it, about 3/4" cubed. I pressed the armature down and the furnace started. Let go after 3 seconds and it stopped.

Thought about checking out the thermostat, but didn't know what color wires were used for heat. Thought about replacing the whole control circuit board, but figured I'd better test the thermostat first.

To get some heat until I could learn wire colors and maybe replace the thermostat, thought I'd jam a piece of wood between the relay armature and something above it. Didn't find the right length of wood, but even a shorter piece was heavy enough to hold down the armature, just leaning against the cabinet.

That started the furnace and immediately I thought, "Will the house fan start? Can't run the furnace for long without the fan" Ran for about 3 minutes but didn't hear the fan start, so I removed the piece of wood. Furnace is still running!!!. Does the Fan and Limit Control on a 34 yo Carrier keep the furnace running once it gets hot enough but the thermostat hasn't said Stop yet?? I thought not, but the furnace is still running. If the answer really is NO, something has changed since the furnace didn't run all night.

I had to leave the furnace room to find an air vent, and indeed, the fan is blowing.

I'm 99% sure it will turn off when the house is warm, but not at all sure it will restart when it should.

Any suggestions, other than getting a new furnace?

[philoÂÂ]

I'm sure it's fixable but really

why don't you get a high efficiency gas furnace?

When I got rid of my oil burner and went to gas I saved so much money it was well worth it.

It paid for itself within two years.

[Bob_Villa]

On Tuesday, February 11, 2014 7:58:44 AM UTC-6, philo  wrote: philo, the key here is:

[Stormin Mormon]

SM: Night, colder, furnace works harder.

SM: Can be either.

SM: No, typically not.

SM: Amazing, what you can find with a good look see.

SM: Blower fan, or burner assembly started?

SM: No sense throwing parts at it.

SM: Started the furnace. You mean the burner gun, or the air handler blower? I guess if the fan didn't start, which fan was that? The air handler fan?

Does the Fan and Limit Control on a 34 yo

SM: Typically the fan limit does two things. Turns the air handler fan on when the fire box is hot. turns the burner off, if the firebox is too hot.

I thought not, but the furnace is

SM: Call a repair and service company. Hard to diagnose from here.

[trader4]

In the old ones I've seen, yes. Temp switch in the plenum turns on the fan when it gets hot, shuts it off when it gets cool.

I'd say that's kind of right. The limit switch does form part of the circuit that keeps it running though. If the temp gets too high, it shuts it down.

Red is power, white is heat, yellow is AC, green is fan. That's the usual convention. So for heat, connect red to white.

Thought about replacing the whole control

Without knowing exactly what that relay button bypasses, I sure wouldn't jamb it closed with a stick.

Find a schematic that shows the controls, how they are wired, and proceed with the debugging. Or call for service.

[David L. Martel]

Micky,

Do you have a circuit diagram? Have you checked the relay that is no longer closing? Is there power to the relay? Is the coil of the relay fried?

Dave M. ce?

[Stormin Mormon]

Since you forced the burner on, but the air handler blower didn't come on, makes me wonder if the fan limit switch is open, and reading high temp overlimit?

[philoÂÂ]

He can do it today

[micky]

No, it did come on. See my two line paragraph above. I didn't *hear* it come on when I was standing right next to the furnace. Too much noise there.

[micky]

Only the oil pump and the fan on the same motor started, and the ignition. That's all that normally starts. Later, the big air fan turns on.

I hate that term "air handler". What did they call it before air conditioning? But yeah, that's what I was referring to but it did start. I just didn't hear it.

I guess I thought about the second but not about the first, until today.

Thanks.

[micky]

I'd forgotten that the owners manual included a both a wiring schematic and a wiring diagram, they call them. The second is laid out more like in real life, with things that are physically close being close on the diagram.

The diagram is hard for me to understand because I'm used to pure electronics diagrams. Or maybe it's because they updated things before ending this model. I got this off the web, but I need to find the paper manual that came with the furnace, which will match the furnace exactly.

(For example, the online disagram shows things running off the center tap of the secondary, but when I had to replace the transformer 30 years ago, nothing was connected to the center tap. The online manual shows a solid state switch, intead of two mechanical relays. It shows no reset button (which is mechanical, not electric exactly, but would still be indicated if they were still using one.

(It shows that the air fan is on high for AC and one of three setting for heat, but mine uses the same speed for AC and heat, I'm 99% sure.

(The online manual seems to indicate one resistor, marked 1K, but it also shows a set of contact poitns or a capacitor marked 1K2 !! 1K and

1K2 might be the missing relay, but I really need to find my paper owners manual.

I havent' checked that. Good point. Probably not.

I doubt it because the armature stayed down after I held it down for a couple minutes. And the furnace turned off when the second floor temp was 66, so after another test, I pushed the armature down again, (maybe on the right side instead of the left and probably farther than the first time, when it was just 3 seconds) and this time I heard it click, like the armature hitting the coil, and it stayed down from the beginning. So I think that's the coil holding it in.

(Hmm. The furnace just stopped. Upstairs temp is just 70 degrees (It's usually warmer than the first floor. ))

The test I made was jumping the thermostat connections at the furnace and that didn't start it up. I think that means the thermostat is okay, Or at least there is a problem with the control board, but to be sure I want to disconnect the stat and measure the resistance between those wires when the house is warm and when it is cold. Can't do it when the thermostat's connected because there are 24 volts between those wires/screws.

If it's not the thermostat, it's the control board, and I have a spare one from the identical burner a neighbor replaced. (Well I think it's identical. When I take the cover off the control board, it might match the online wiring diagram.)

Since the current control board has only 2 resistors, one capacitor, 2 transistors, and 2 relays, maybe I could fix it, but exchanging it is easier. Maybe I'll try to fix the old one then.

[micky]

Makes sense now. Where else would it be? But I just figured it was a mystery box.

When I counted, my imagination was way off. There were only 2 resistors, 1 capacitor, 2 transistors, and 2 relays. ....

Thanks. That helps.

Well, I was standing right there, and I took the stick off after 3 minutes, to keep the firebox from overheating, but (it stayed on and) when I went to a quieter room, I could hear the "air handler" blowing and next I found a a vent within reach where I could feel the air blowing.

It didn't.

The manual I found online has been good for specs, but the schematic has too many discrepancies from my furnace, even though it's the same model number. The one online is probably the final schematic and mine wasn't. I see that in tvs too. I'll look for my paper copy that actually came with the furnace.

[micky]

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.

[clare]

A relay coil can fail in such a way as to be able to hold a contact closed, but not be able to PULL it closed. Check the relay coil. Also check the control transformer voltage. If it is supposed to be 24 and it is only 16, the reays are not going to function properly. Also check the wires for high resistance.

[Stormin Mormon]

Ah, that makes more sense. Thanks.

[micky]

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.)

[David L. Martel]

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.

[micky]

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.

[micky]

Will do.

[Stormin Mormon]

My pleasure. BTW, did you get it repaired?