Testing electricals of furnace?

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Old 12-15-08, 11:27 AM
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Testing electricals of furnace?

Our old furnace quit yet again yesterday. We have had several people look at it over the last couple months, replace parts, but it still keeps quitting.
1. It has power. I can turn the fan motor on
2. Pushing the reset button starts it, but will soon shut off
3. Its getting oil, since hot water is fine, and it runs off same.
4. Thermostat is working correctly

We have had the burner replaced 2 years ago. And the nozzle replaced a couple months ago. There is plenty of fuel in the tank, and the last guy to come look at it charged us $120 to press the reset button.. ugh.

The thermostat wont start it. Taking it apart and connecting the terminals together does nothing. The fan works fine in manual override. The reset button starts the furnace, but it quits on its own after about 10 seconds.

Beyond that, I'm clueless. Its my dads house, and the furnace is an old Ford 112R. I've had good luck fixing wall mounted propane units in my last apartment, but the components were easy to see and work with. With this one, I'm kinda out of my range of ideas.

Lately, all we get from service calls to different companies is a huge bill and nothing else. 3 so far, and none of them have been able to get it working reliably for more than a week.

In the past week, I've had to go press reset more and more often, but it would start up. The last 2 days, it won't light anymore.

I'm more of an electrician than HVAC, so I'm leaning towards testing the components, but I have no idea what I should be testing, or results I should be getting. Hopefully someone can point me in the right direction, as heating this house with 3 space heaters isn't working very well.
 
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Old 12-15-08, 03:55 PM
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Originally Posted by centralnydude View Post
Lately, all we get from service calls to different companies is a huge bill and nothing else. 3 so far, and none of them have been able to get it working reliably for more than a week.
But they did get it working for about a week, when, before they came, it would go out more frequently? If so, then they must have done something right? What did each of them do? Did they all replace nozzles. Could they all be failing due to impurities in the system? Is tank sludged on bottom? Was filter and screen cleaned?

With oil burners, you have to understand the entire system before just choosing to pick at one aspect. You have to have the proper grade fuel and nozzle to go with it. Clean line, clean filter, clean screen, good pump, good electrodes gapped and positioned properly, a proper operating combustion blower, proper drafting, perhaps a properly functioning cad flame sensor, and a sophisticated array of control boxes relays, etc., that have to all do their job.

And if you want to test that electrical stuff, you may want to make yourself a map as to what goes to what, and what causes what to happen in the electrical circuity, to understand that if you get say some reading HERE, but not THERE, that that could be an important find.

I had to do that very thing on a conversion burner, from oil to gas, but basically works same way, and has the resettable Honeywell control, aquastat, 24 volt transformers (and you will have a very high voltage secondary transformer), relays, reset, motor, pump, coupler, etc. And after basically mapping out/analyzing the system, I was able to discover where I lost my power (took me a while though, as these types of systems are not really my usual line of work). In my case it happend to be in a thermosdisc relay that jointly sends power to both the gas valve and the fresh air burner blower -which it would not do, every 17th(number to illustrate sporadic behavior) run time.
 

Last edited by ecman51; 12-15-08 at 04:05 PM. Reason: Typos
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Old 12-15-08, 05:11 PM
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Well, as far as getting it working... I think it was just chance. The last guy came out on a saturday afternoon. Took the cover off, looked at some stuff, opened the inspection doors, pressed the reset button, and up and running it went. So, it was basically a $120 call to have him press the reset. Since it worked, he left. I think theres something more to it. Its done the same for me. Sometimes the reset worked, others it doesn't. Now resetting does nothing. From what my father has told me, is that the last time, they replaced the air filter, before that was the nozzle. Apparently the ignition or coil was replaced 2 years ago. The motor itself is about 10 years old.

It's behavior is just plain sporadic until now. I pulled apart the electrical side of things as far as I dared. I cleaned the flame sensor. No change. I jumpered the sensor terminals to see if that was it, no change. Its not getting a flame for some reason. It is definitely getting oil. Thats not the problem. Hence why I'm trying to understand the electrical side.
I work mainly on automotive systems, I'm a 12V junkie. I'm trying to equate what I'm looking at to how a motor fires in a car.
Wish this site would allow uploading images, it would probably help people.
It has a Carlin motor, honeywell control, and a Carlin (what appears to be the coil, correct? Mounted atop the motor, and has 2 springs protruding underneath with a light sensor mounted on the underside to the rear of those springs. )

I have searched for wiring diagrams, but come up short. There are 2 wires connected to a box on the inside labeled 24v, both white in color appearing to be from an external source, 4 wires connected to the honeywell, 2 black and red, from external source, and two yellow ones connected into the box. I see a spark in the relay after it has been reset and shuts down. So its getting some power where it should be. But not where it needs to be. There is some sort of humidifier control connected into the white wires that terminate at the box labeled 24v.

I assume, the terminals should see 24v? or are they converting something else to 24v? This box is on top of the main power input circuit.

At this point, I'm equating it to a motor that wont run. Its getting gas, getting air, but no ignition. Pull the spark plug, weak/no spark indicates bad coil, ohm the coil, and check wire resistance. But this is a bit more complicated.....
 
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Old 12-15-08, 06:18 PM
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Oil Burner

I'm obviously guessing at this point but what you describe sounds like an igition problem. If you know how to remove the nozzle assembly, do so. Remove the nozzle & flush the assembly with carb or brake cleaner. Obtain & install a new nozzle of the same capacity, spray angle, & spray pattern. All this is stamped on the flats of the nozzle. Also install a new fuel filter & pump screen if applicable. Now lets get to the igniton system: The electrodes should be removed & cleaned. Re-install & set the electrodes to manufacture's spec. Without knowing what make & model the burner is, I'll have to give you some generic settings. Electrode tips 1/16" ahead of the nozzle face, 5/16" above the nozzle center, & 5/32" apart. Reassemble the burner, purge the air from the fuel system & try it. If you get fuel & it fails to light, we can go further.
 
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Old 12-15-08, 07:14 PM
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I'm pretty sure it is an ignition problem. I don't think the electrodes are the source. They appear in good shape and in close proximity to your specs.
FYI, the burner is a Carlin model 100CRD.

I'm thinking the relay could be the problem. The burner will run as long as I like if I jumper the CAD sensor circuit.
I also believe I am not seeing 24v output from the transformer. There is an in line humidifier in this system, wired into the 24v. And the motor turning the barrel never turns at all when the system should be functioning.
How and where do I test to see if the 24v is present where it should be? I know I can short the wires at the thermostat to take that out of the equation. But seeing as how we have had to reset it so much lately, wouldn't it be reasonable to believe that whatever component is responsible for sending the signal is faulty? Intermittently before, completely now?
If the relay and transformer are working correctly, shouldn't there be 24v present at the terminals at the thermostat?
 
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Old 12-15-08, 07:23 PM
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24 Volts

I don't think you have a 24v problem but maybe a 10-20 kv problem. Anyway, you should get 24 volts AC when measured at the primary control's thermostat terminals provided the control is not locked out.
 
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Old 12-15-08, 08:10 PM
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Nope, zip, zero volts at the terminals.
Tested with everything off. Zero.
Reset the burner, jumpered the CAD terminals, put the fan on manual, and it reads .24ish.
So, if I should be getting 24v on that output, then whatever is providing the 24v, (transformer?) is likely at least a suspect...
Shouldn't it be 24v constantly though?

Maybe it is the coil (or ignition box or whatever its called) that isn't firing.
Or... what else am I missing?
We were supposed to get a call today from our fuel supplier (aka the furnace repair guys), but they never did.
Ultimately, someone thats not me, will probably end up fixing this thing. I just want to know as much as I can when we can get someone here. I don't want them to just throw parts at it, hit reset and get lucky.
I'm the same way with my cars. If I can fix it myself, I will. If I have to let someone else fix it, then I figure out all I can ahead of time, so theres no surprises.
 

Last edited by centralnydude; 12-15-08 at 08:30 PM.
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Old 12-16-08, 02:31 PM
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Finally got someone to look at it today. Replaced the coil. Wasn't putting out any spark. The one that was on it wasn't that old, but I guess it had been dying for some time. Which makes sense in the context of the problems we've had with it lately. It was failing, running intermittently, and now it died completely.
So in the end, I was pretty close. Not bad for learning how a oil furnace basically works in a couple days.

Thanks for the advice! Its nice to have heat again! Beer 4U2
 
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Old 12-16-08, 02:35 PM
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Ignition

If you want to spark test the ignion transformer, I have to warn you these things can bite & bite hard.
 
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Old 12-16-08, 02:46 PM
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Grady,

He is refering to his secondary 10-20k (coil) that went out? If so, why was he not getting 24 volt at stat? What would that have to do with no 24 volts at stat?
 
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Old 12-16-08, 03:04 PM
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No 24

When the primary locks out, it shuts off the 24 too.
 
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Old 12-16-08, 03:39 PM
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By "primary" - you mean he lost power even to the primary side and it was not just the fact that he lost the secondary high voltage coil that sends all that ramped up power to the electrodes? Nice to understand all this, in case one gets call to a place with a similar problem, and no power at stat.

Is this common to lose power like that on the primary side, which naturally would cause the secondary side to shut down? Some wire gets burned off in there or bad contacts?
 
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Old 12-16-08, 03:50 PM
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"Primary"

Sorry EC, my oil burnerish is showing. By "primary" I meant the primary control. If there is an ignition failure, the control locks out after it's alloted safety time. When it does so, since it is the source of the 24 volts, it locks out the 24 volts.
 
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Old 12-16-08, 04:00 PM
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Thanks Grady. I, and others with some knowledge, and voltmeter in hand, could likely discover that then.
 
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