Novice needs help diagnosing problem with no heat

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Old 02-11-11, 10:59 PM
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Novice needs help diagnosing problem with no heat

Old furnace, would not run. Exhaust blower was frozen up. Ordered one, put it in today, now exhaust blower and normal fan both run. But no heat.
Control board is ST9120G 2008, with HQ1008773HW below that. Other unit in upper section is Honeywell S8600M.

I did not measure any voltage on the 24volt terminals on the S8600M. Nor on the microswitch that is activated by the exhaust fan. Nor on the leads on the control board that feed the S8600M. Unplugged the connector from the Control board to the S8600M and measured no output on the leads that feed the 24 volts on the S8600M (through the microswitch).

Should I see 24v at these pins on the control board when the thermostat is calling for heat? Could anything else be the issue? Any suggestions on where to get a new control board if that it it?
 
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Old 02-12-11, 12:03 AM
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How are you measuring the voltage?

Do you have 24 VAC on the R terminal? On the W terminal when the thermostat is calling for heat?
 
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Old 02-12-11, 05:14 AM
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Thank you so much for the quick reply. The furnace is in the home of two of my cousins who are 67 and 72 and who have basically no money. So I bought the exhaust fan and put it in to try and help them. I was pretty disappointed when it didn't fix the problem. The old one WAS frozen up, so this is progress, but still no hear.

I am using small digital voltmeter. For the 8600 measurement, I just turned on the power to the furnace and set the thermostat for heat and ran it up. Then measured on the terminals labeled 24v and 24v Gnd. Nothing. Also measured both sides of the microswitch that is set by the exhaust fan.

On the control board, with the wires to the 8600 in place, put my voltmeter leads in the locations on the plug with the wires that go to the 24v terminals on the 8600, pushed the switch in that is supposed to be cloased when the door panel is on, turned on the furnace power, ran the thermostat up. Nothing.

Then I repeated the last paragraph with the plug removed and my voltmeter leads in the places where those 24v leads go.

I never measured anything on the terminals to which the thermostat is connected. I assumed that since the exhaust fan came on, and after some time the circulating fan came on, the thermostat must be properly calling for heat. Was I wrong? Could there be a malfunction in the thermostat that lets the fans come on but does not send 24v to the 8600 to start the gas?

I suppose the 8600 could be bad, but until I see 34v on the two terminals labeled 24v and 24v gnd, I guess I assumed the source of the 24v, the control board, had to be the problem.

The control board had two dip switches that control the delay. It is set for 90 seconds. So I made sure that I held the call for heat for at least 2 minutes.
 
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Old 02-12-11, 07:07 AM
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first, are you getting spark from the s8600m? if no, check for 24v between 24v and 24v(gnd) after the exhaust blower has come up to speed. if no, check for 24v across the pressure switch terminals if yes, then you need to remove the tube that is hooked to the exhaust blower/ pressure switch and check for blockages or cracks in the hose. you can try to suck on the hose and see if this causes the spark module to operate, all this is telling you is if the switch in the pressure switch is good. You really need a manometer to check for proper operation of the press. switch.

Are you sure you have the pressure switch tube connected to the correct port on the exhaust fan? that it was connected to on the old fan?
 
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Old 02-12-11, 08:11 AM
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No spark. No voltage across the 24v - 24V gnd terminals. No voltage across the switch. No voltage to either side of the switch, as measured to the 24V gnd.

All of the above are in the top area. I removed the cover to the lower area (which of course stops everything because of the door switch). I put the voltmeter leads on the terminals that feed the 24 v circuit in the top. (one lead goes to a terminal on the 8600 directly, the other goes through the microswitch). Then I pressed the door switch in by hand, and held it in for over 2 minutes, with the exhaust fan running. No voltage on the two wires.

Then I unplugged the connector from the top (where I had been sticking my voltmeter probes into where the two wires went into the connector). Now, with the connector unplugged, I put my voltmeter probes on the male pins sticking up that would have gone into the connector to feed the 24 volt circuit. Pressed the door switch in, held it 2 minutes, fan running, no sign of 24 volts.

Seems to me that the control board is not sending the 24v to the 8600. But does that mean the board is bad, could there be something else causing it? I have mentally excluded the thermostat since the exhaust fan is coming on, since I assume that only happens when the thermostat calls for heating. And the circulating fan also comes on.

Any help will be greatly appreciated.
 
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Old 02-12-11, 09:30 AM
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This link may help. Have you checked the fuse on the control board?


http://customer.honeywell.com/techli.../69-2252EF.pdf
 
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Old 02-12-11, 10:25 AM
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Thanks for the link. That had a lot of information. It's for a replacement for the board we have (the ST9120G2008), but still very useful.

No, I have not checked the fuse on the board. Didn't know there was one until today. The new board appears to have two, and one is in the area where the connections are for the 8600 control. I wonder if the old board has the same and perhaps there is a fuse gone in it. I'll have to drive back down there and check it.

If I could be sure that the rest of the furnace worked, I'd buy the replacement board and just put it in. I found it for less than $100 on ebay. Any suggestions if I have to do this?
 
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Old 02-12-11, 09:10 PM
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i believe that you are not checking where asked. Best to call a pro for diag, I hate getting clients that only want diag, but in your case I think this sounds best and then either purchase the repair from the PRO or go online to do it yourself cheaper.
 
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Old 02-12-11, 09:26 PM
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I don't think I understand your saying that I am not checking where asked. Here are your questions as I understand them:

Are you getting spark: I had said already that I was not

Do I have 24 volts on the 24v and 24V gnd: I had already said, and I repeated, that I am not getting 24 volts anywhere, not on those two terminals, not on the microswitch, not on the terminals on the control panel that feed those points

I apologize for not answering the question about whether or not I had the tube connected as before. Yes. As there is only one port on the fan which can allow air, it was really the only choice, and it is in exactly the same place on the new one as on the old one. And I will add that I did suck on the tube and could hear the microswitch click to close and open.

So my question would be, given no 24v source from the control board, does that alone indicate that the board is bad, or are there other things to check.

goldstar has suggested a fuse, and I will check that as soon as I can get down there, probably Tuesday. Using the diagrams that were on the link provided, I see two fuses on the board, but it is a replacement board. I am assuming and hoping that there are fuses on the original which I can check.

I would appreciate anyone's suggestions of other things to check. From not seeing 24 volts coming from the control board, I am focusing my efforts there, knowing of course that even it it put out the 24 volts, there could be other problems downstream.

Thank you for any help. And if I have failed to provide an answer to a question or check something that you have asked, I will be more than glad to try and remedy that.
 
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Old 02-12-11, 09:42 PM
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did you suck on the pressure switch hose? Yes, check the fuse, but most likely if the fuse is blown then the exhaust blower would not be coming on. check on control board for 24v between r and c. check 6 wire plug at board and ensure it is seated well, follow these wires to the any other connectors and ensure properly seated.
 
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Old 02-13-11, 05:53 AM
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Thanks for the reply. And yes, I did suck on the tube. See last sentence in fourth paragraph on my last post.

I will check the fuse. From a schematic that someone referenced, I had been thinking there were two fuses. BUt after looking, I see only one. And if it is in the 110v line, then it is probably good.

I will check the R and C leads. But I suspect they will show 24 volts since the unit does respond to changing the thermostat settings. Will let you know.

Thanks again
 
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Old 02-13-11, 08:00 AM
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most likely there is not one on the 110V line. if it is on the control board then it is 24v. check your limits as well. check for 24v across the limits, you should get 0 normally. if you find 24v across the limits then that limit is open.
 
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Old 02-13-11, 10:00 PM
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Thanks. I will report back after getting down to their house to check on it. But I wanted to let you know that I checked the resistance across the limits with my ohmmeter while I had the 6 wire plug disconnected from the control board and measured zero resistance. So I believe they are both ok. And while I had the connector removed, I did try to put it on and off several times so the connectors would have a chance to clean themselves up. I can use contact cleaner when I go back just to be sure, but I believe it has a good connection there too.
 
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Old 02-14-11, 04:46 AM
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to check the limit by resistance, you must remove the wires from the limit to ensure you are not getting a false reading
 
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Old 02-18-11, 03:17 PM
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I'm back from checking out the heater. I was incorrect when I said there was no 24v in the top area. Here's what I found:

there is 24v to gnd on both sides of the vacuum switch, thus no voltage across it.
And there is 24v to grnd is on the 24V connector on the S9600 Ignition control which is what I would expect unless the lead from the vacuum switch to the connector was bad.
There is 24v to gnd on the PV connector.
There is 24v to gnd on the PV connector on the gas valve. I also measured voltage from the PV to the MPV connetors on the gas valve and found 24v there.

There is voltage on the spark connector on the S8600. I had trouble seeing exactly what it was. Seemed to vary with time. And after a period of time (presumably 90 seconds), there was no voltage there, nor was there any on the PV connector.

I removed the wire from the spark connector and measured resistance to ground. It measures open. I guess that is what I should measure if it is in fact a spark gap device.

At this point, I believe the S8600 and the fan control board are working correctly. Seems to me (a novice, remember) that the problem has to be with one of three things. 1) the spark device is bad, 2) the gas valve is bad, or 3) there is no gas from the gas supply.

I added the last one simply because I don't smell any gas, but since the exhaust fan is blowing fine, I assume that any gas which does not light would get sucked up by the fan and sent out the exhaust. But just to be sure, I turned the gas valve 90 degrees and tried it again. No change.

If the spark device is bad, I am at a loss for how to remove it. It is on the other side of a piece of sheet metal, same place as the pilot gas tube. There are two screws in the sheet metal from this side, but not knowing what they hold on the other side, I am a bit reluctant to just remove them.

I would welcome any input on my conclusions, and also how to remove the spark device for cleaning and/or repair and or replacement.

Thanks very much.
 
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Old 02-18-11, 03:51 PM
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Refresh my memory..... do you have spark or no? Also, remove the 2 visible screw holding down the spark module, just that simple.
 
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Old 02-18-11, 08:32 PM
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I have never seen a spark. But as I said, the spark mechanism seems to be behind a piece of sheet metal. I have a mirror on an extension that I plan to use the next time I can get down there to look at it while it tries to light. Also, I may just remove the two screws and pull it out. Does the pilot light gas tube need to be disconnected? It disappears behind the sheet metal also.
 
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Old 02-18-11, 08:36 PM
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I'm kinda lost, I thought you were talking about the spark control module, now it sounds like you are talking about the pilot burner. A picture would do wonders. You should be able to hear when the control module is sparking.
http://forum.doityourself.com/thermo...st-photos.html
 
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Old 02-18-11, 08:44 PM
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I guess I am not being clear. I would think that I would only SEE a spark at the pilot burner, or whatever you call the place where the gas line for the pilot light ends up. That appears to be behind a piece of sheet metal. There are two wires that go to it. One comes from the Spark terminal on the Spark Control Module and one comes from a ground connection on the same module. That wire is white and is much smaller than the one from the Spark terminal.

Where should I hear the sound from? Are you saying the Spark Control Module itself makes noise, or is the sound from the spark which occurs at the pilot burner???
 
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Old 02-19-11, 12:54 AM
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>



This indicates normal operation of the furnace to this point.


The 24 VAC to PV should be turning on the gas to operate the pilot burner.

Also, you should have high voltage on the spark wire to ignite the pilot burner gas.


>


You aren't going to be able to measure the high voltage, which is 5,000-8,000 volts. You can disconnect the high voltage wire at the ignition module. Ground a screwdrive to the chassis and bring the end of the screwdriver near the high voltage connection on the ignition module. You should get a spark about a half inch long or more which would prove the spark.

Do that and report what you find.
 
 

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