Another Furnace Ignition Problem

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  #1  
Old 01-05-03, 10:16 PM
msw
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Unhappy Another Furnace Ignition Problem

Although the time I've spent reading previous posts has provided a little familiarity with the lingo, it hasn't solved my problem, which is a gas furnace that won't light. Here's what I think I know:

The unit is a 17-year old Payne 375A gas-fired induced-draft furnace.

The thermostat seems to work, because when I make it call for heat, the inducer fan runs. That's all that appears to happen.

The manual says that the call for heat should simultaneously do all of the following:

1. Supply power to the 24-volt safety circuit containing the limit switch, fusible link, manual-reset draft-safeguard switch, and manual reset auxiliary switch;

2. Energize pick coil of of gas valve;

3. Energize spark generator; and

4. Energize inducer-motor relay coil.

No. 4 is the only thing happening. The spark generator is supposed to light the pilot, which is detected by the flame sensing switch. In about 50-60 seconds, this is supposed to close a circuit to open the main gas valve. A time-delay circuit should then start the blower in about 50 seconds.

Where do I start? Any help would be most appreciated. Let me know what other information you need. Thanks!

- Mark
 
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  #2  
Old 01-06-03, 03:22 AM
fjrachel
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Reset all your manual safety switches and check your induced draft proving switch to make sure it is closing.
 
  #3  
Old 01-06-03, 05:04 AM
bigjohn
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OK I'm a little confused. Is the inducer motor running at all? If not, then see if you're getting voltage to the motor. If it is running, the next step in the startup sequence is the air sensing switch closes proving the airflow. Look aound for this control [usually diapragm shaped] and see if there is voltage leaving the switch. Also, check the hoses for leaks. Let us know what U find.
 
  #4  
Old 01-06-03, 11:04 PM
msw
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Thanks for your tips. I don't know how to check all the manual switches, but I think I found the flow sensing switch. I disconnected the hose at the switch and tried to blow into it to see if it leaks. I guess it doesn't, because I could not blow any air into it.

From left to right, the switch has red, brown, and yellow wires attached to terminals. At first, I got no voltage across any two of the terminals when I tested after closing the door safety switch. Then I disconnected the hose and tried it again, getting 200 volts across both the red-brown and brown-yellow pairs. Finally, I reconnected the hose and put my little multimeter probes on the terminals BEFORE I closed the door switch. I consistently found that, for both the red-brown and brown-yellow pairs, it would measure 200 volts for a second or two and then go to zero.

Are we making progress? What do I do next?
 
  #5  
Old 01-07-03, 07:20 AM
bigjohn
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Look closely at the terminals on the air proving switch with a bright lighand a magnifying glass. I'm hoping that you see a C, NO and NC. These would mean Common, Normally Open and Normally Closed. The Normally designations mean when the switch is in an at rest state. When the furnace is off, you should have continuity between C and NC and no continuity between C and NO. When the inducer motor is running, the switch will change positions so that you should now have continuity between C and NO and no continuity between C and NC. Any chance you can scan or take a photo of the wiring diagram and post it?
 
  #6  
Old 01-07-03, 10:29 PM
msw
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If things go as intended, a photo should be attached that shows the flow sensor. Upon close examination, the three terminals are clearly labeled:

Left terminal (red wire): Common
Center terminal (brown wire): Norm Open
Right terminal (yellow wire): Norm Closed

It is NOT behaving as you said it should. With the door switch off (open), I get no voltage anywhere. When I place the multimeter probes on two terminals and then close the door switch (which starts the inducer), I get the same results as previously described, which are 200 volts for about one second, then zero. This happens for both the Common - Norm Open and the Norm Open - Norm Closed pairs, but still nothing at all between Common and Norm Closed.

Should I try to measure anything else?
 
  #7  
Old 01-07-03, 11:06 PM
msw
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My attempt at including a photo did not work, so I placed three of them on the internet. The addresses are:

www.zeaonline.com/furnace/overview.jpg

www.zeaonline.com/furnace/inducer.jpg

www.zeaonline.com/furnace/flow sensor.jpg
 
  #8  
Old 01-08-03, 11:32 AM
msw
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Okay, so my attempt at providing a hyperlink didn't work either. Please bear with me as I try again to embed the photos:

[img]http://www.zealonline.com/furnace/overview.jpg[img]

[img]http://www.zealonline.com/furnace/inducer.jpg[img]

[img]http://www.zealonline.com/furnace/flow sensor.jpg[img]
 
  #9  
Old 01-08-03, 12:55 PM
bigjohn
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I'll try the links again. I couldn't get them either. Reading voltage on a switch is not the same as reading continuity. When you read voltage across two switch terminals it means the switch is open. BTW- are you sure it's 200 volts? Most gas furnaces are 120 volts. Here's what we can try. You need to get a 3-Way light switch, some wire and a roll of elecctrical tape. What we're going to do is remove the wires from the air flow sensor switch, wire up to our 3-way switch and duplicate the function of the air flow sensor switch manually. A 3-way switch is Single Pole/DoubleThrow, just like the airflow switch. Look at the 3-way switch terminals, one of them should look different than the other two. [usually lighter or darker color] That terminal is common to the other two. Take your ohmmeter and determine which set of terminals is Common to Norm Open & which Common is to Norm Closed. Mark NO & NC on the switch brackets. Now, remove the wire from the Common terminal on the air flow switch and hook it to Common terminal on the 3=way switch. Hook the wire from Norm Open on the air flow switch to Norm Open on the 3-way switch. Hook the wire from Norm Closed on the air flow switch to Norm Closed on the 3-way switch. Wrap some electrical tape around the switch terminals so you don't get a shock when handling the switch. Now, ensure that the switch handle is flipped to Norm Closed. Start the furnace, let the inducer motor come up to speed and run for a few seconds then flip the switch. If the furnace continues the heating cycle and lites the burners, the problem will be bad switch, leaky hoses, maybe the blower wheel in the inducer fan assembly has disintegrated or maybe the flue pipe is blocked somehow. If the burners don't lite, try to notice what is or isn't happening. Spark? No spark? Pilot flame or no? Pliot flame comes on but goes out? Let us know what happens.
 
  #10  
Old 01-08-03, 10:48 PM
msw
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I wired the system wires to the three way switch, but got an unexpected result. Regardless of the switch position, the inducer tries to start for a second and then gives up. I then have to wait 45 seconds before trying it again, or the inducer will not even TRY to start (all I get is the usual spark on the 24 volt circuit board when I close the door switch). When I connect the wires back to the flow sensor switch, the inducer starts and runs again. So then I tried a few tests.

Here's an observation that may be helpful: when I connected the common and EITHER the Norm Open or Norm Closed wires (but not both) to the flow sensor switch, the inducer would start and run.

In checking readings again, I think the voltage I'm getting is about 30 volts. I don't think it's a coincidence that the only way I get a voltage reading is if I place the probes on the terminals before I close the door switch, and then it's only for a second, which is the same amount of time the inducer would try to start when I used the three way switch. That one second of voltage reading is still occurring across the Common - Norm Open terminals, but not the Common - Norm Closed terminals.

Where do we go from here?
 
  #11  
Old 01-08-03, 10:54 PM
msw
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Let me clarify one part of my last post. The inducer will start and run either with the common and one or both of the other wires connected to the flow sensor switch. In other words, it doesn't matter whether I connect one or both of the Norm Open and Norm Closed wires.
 
  #12  
Old 01-09-03, 07:42 AM
bigjohn
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It sounds like the air flow switch is switching if C to NO shows voltage and then doesn't. Where does the wire from NO go to? Any chance you could post the wiring diagram? What is the brand/model # of the spark module?
 
  #13  
Old 01-09-03, 11:06 PM
msw
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The wires from the flow sensor switch all go to a printed circuit board. I don't know which parts are separate brands -- the parts replacement guide has the words Payne, Bryant, Day & Night, and Unipart on it, and the spark generator is part no. P771-1015.

For reference, I scanned some of the diagrams, legends, parts list, and even the sequence of operation from the manuals that came with the furnace. I uploaded them to a separate website and list their addresses, along with the previous photos, as follows (cut and paste the part between the [img] entries):

Photos:

[img]http://www.zealonline.com/furnace/overview.jpg[img]

[img]http://www.zealonline.com/furnace/inducer.jpg[img]

[img]http://www.zealonline.com/furnace/flowsensor.jpg[img]

[img]http://www.zealonline.com/furnace/gasvalve.jpg[img]

Diagrams:

[img]http://www.zealonline.com/furnace/partslist A.tif[img]

[img]http://www.zealonline.com/furnace/partslist B.tif[img]

[img]http://www.zealonline.com/furnace/wiringdiagram.tif[img]

[img]http://www.zealonline.com/furnace/wiringlegend.tif[img]

[img]http://www.zealonline.com/furnace/inducerdiagram.tif[img]

[img]http://www.zealonline.com/furnace/gasdiagram.tif[img]

[img]http://www.zealonline.com/furnace/sequence.tif[img]
 
  #14  
Old 01-10-03, 12:04 AM
fjrachel
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FYI, all your parts are Carrier....They manufacture payne,bryant,day and night and a few more. You got a whole lot of info at your disposal.
As per your schematic: With Normal call for heat- ck for 24v at points 5 & 3 on gas valve. Also at T1 & T2 of spark generator. If you got 24v at valve and spark generator...are you getting spark and is the pilot lighting?
 
  #15  
Old 01-10-03, 07:04 AM
bigjohn
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OK!! I have the diagram, this will be easy to troubleshoot. When terminal W goes hot [IOW- the t-stat calls for heat] 3 things happen at once, [this is assuming the limits are closed] relay 2D is energized which starts the inducer motor, the PICK coil in the gas valve and the spark generator are BOTH energized THRU the Normally Closed contacts in the pilot flame sensing switch- 6H. At this point the inducer motor is running and the spark genertor is producing a spark at the pilot burner/ignitor assembly. As soon as the inducer motor comes up to speed, the flow sensing switch- 7V -changes position which energizes the HOLD coil in the gas valve and gas flows to the pilot burner only. The pilot flame lites which warms up the flame sensing switch. When the flame sensing switch gets up to temperature, its' contacts switch and energize the MGV coil in the gas valve and also make a circuit to the heating blower relay- 2E. The pilot flame lites the burners and 2E times out and starts the circulating fan. You have probably read about flame rectification in some of the other posts, Your furnace DOES NOT use that principle. It depends on the flame sensing switch to prove the pilot flame. So, start the furnace and see what happens. When the thermostat calls for heat, you should see the spark right away. If not, check for 24 volts from the NO terminal on the air flow switch to the common side of the transformer. [the side that DOES NOT go to R terminal] If you're hot there, then the air flow switch has transfered its' contacts. Next, check for 24 volts from T1 to T2 on the spark module. If that's NOT hot, then the flame switch is faulty or the interconnecting wiring has a problem or 2D contacts in parallel with the air flow switch are not closing. If you DO have 24 volts there and no spark, then the module is bad, assuming the ground at the sparker is ok. The most common item prone to fail on these furnaces is the flame sensing switch-6H and that's where my bet is placed. The pilot burner/flame sensing switch is all one assembly. Johnstone Supply sells them for around $50 to $60. Let us know what you find.

Edit: I negelected to mention that, if you replace the pilot burner/switch assembly, be careful not to lose the orifice inside the pilot burner.
 

Last edited by bigjohn; 01-10-03 at 07:49 AM.
  #16  
Old 01-10-03, 11:32 PM
msw
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fjrachel - No voltage at points 5 & 3 on gas valve before, during, or after inducer motor startup. Same for T1 and T2. No spark (that I can tell) at any time.

bigjohn - You said to check voltage across the NO and the common side of the transformer. What I checked is the voltage across the NO and common terminals on the flow sensor switch. Is that what you meant? I measured 30 volts (I'm using an old, cheap Radio Shack multimeter) for a second, then zero.

Does this mean I need to replace Item 26 Pilot Assembly, which is Part No. P671-1302? If so, is this something I can pick up at a local supplier? Would I specify it as a Payne P671-1302 or Carrier P671-1302? How much should I expect to pay retail? Also, please explain your warning about the orifice - is that the small thing between the gas line and the pilot assembly? Are there any specific precautions I should take? Sheesh, are you getting tired of my questions yet?
 
  #17  
Old 01-11-03, 04:09 AM
bigjohn
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Read my last post again. You want to check FROM the NO terminal on the air flow switch TO the common side OF THE TRANSFORMER. Look at the wiring diagram, do you see sec1 and sec 2 at the transformer? Touch one meter lead to the NO on the air flow switch and the other to sec2 [you want the side of the transformer that DOES NOT go to R terminal on the board]. Now if you have 24 volts there, that means that the air flow switch is ok. When the air flow switch closes, the circuit to the spark module is made THRU the Normally Closed contacts in the flame sensing switch, so the next thing to check is for 24 volts at the terminals on the spark module. If you don't have voltage there, then the problem has to be the flame sensing switch or the interconnecting wiring. Since the flame switchRobertshaw Uniline makes a universal repalcement pilot burner & switch assembly that costs around $50 to $60 wholesale, I'm not sure what retail would be. Either the Carrier or Payne part # will do, it's the same part. I found it at www.heatcoolparts.com for $77. When replacing the assembly, you'll remove the tubing nut but you don't need it because the existing tubing has one. Up inside the cavity that the tubing nut unthreads from is the orifice. It can fall out if you're not watching for it. [wanna guess how I know?] No need to do anything special, just don't let it fall out as you're threading on the existing tubing nut. The only dumb question is the one you don't ask. Let us know how you make out.
 
  #18  
Old 01-11-03, 06:10 AM
fjrachel
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Next, during normal call for heat: Check for 24v at gas valve between 4 and 3. If you get voltage, the pressure switch is more than likely good. Next ck for continuity between points 1 and 3 at pilot assembly plug. If you get continuity, then the low voltage contacts on the 2D relay are bad. If not, the pilot assembly is bad.
 
  #19  
Old 01-11-03, 07:58 PM
msw
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I found the transformer and checked the voltage between the air flow sensor NO terminal and the sec2 terminal. It does have voltage. I then checked the spark generator terminals - no voltage. Next was a voltage check between gas valve terminals 3 and 4. At the call for heat, it displayed no voltage for a second, then voltage.

At this point I removed a sheet metal panel to get closer to the flame sensor so I could see how to replace it and check continuity. I found wiring leading to the flame sensor that is fairly charred and brittle. It appears that this is the same area that was repaired by a repairman in 1999, because the wires are spliced and covered with electrical tape. The following photos show an overview of the splice area and the front and back of the wiring plug:

[img]http://www.zealonline.com/furnace/spliceoverview.jpg[img]

[img]http://www.zealonline.com/furnace/plugfront.jpg[img]

[img]http://www.zealonline.com/furnace/plugback.jpg[img]

Although the back of the plug doesn't look too bad, the front does, and the wiring leading up to it does too. The two parts of the plug appear to be fused together. So it looks like the wiring may be the problem instead of the flame sensor.

How much do I replace? Exactly what kind of wire should I use? I assume that it's okay to splice and solder new wire onto the old. Can I splice directly to the thin wires coming from the flame sensor? Do I need to get a special plug?

From a broader perspective, might there be something wrong that keeps scorching the wiring in this area, or is it just because it is so close to the burner? The wiring is protected from the heat by a metal channel as it crosses the top of the chamber, but has no protection as it descends to the flame sensor. Is there some appropriate way to protect it in this area? You can see the metal channel in the following photo:

[img]http://www.zealonline.com/furnace/overview.jpg[img]
 
  #20  
Old 01-12-03, 04:08 AM
fjrachel
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Looks as though you have had the flame rolling out of the furnace. Not just the wiring being burnt, but also your pic shows that the spark generator casing melted. Tells me you got a bigger problem. Check heat exchanger for holes. Where is the unit located?. Does is get enough combustion air? Check for restricted flue.
 
  #21  
Old 01-12-03, 07:50 AM
bigjohn
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First, let me get this off my chest. Any technician working for me who went to a customers home and spliced wires like that would be unemployed. A temporary splice at night or on a weekend to get the heat going is one thing. Leaving it like that? No way. Ok- I feel better. It's you're call, but I would advise against splicing wires. You need to replace the flame switch/pilot burner assembly, the spark generator and the wiring harness. I agree that it looks like the flame is rolling out of the furnace. Notice how the wire adjacent the the flame switch is ok but it's burnt farther back? Also, see how the duct insulation is charred but the cabinet paint is ok? I would check the duct connection to make sure it's not sucking air. Also, take the burner assembly out and see if the heat exchager flue cells are obstructed. While it's out, clean the burner assembly. Watch the burner flame to see what happens when the circulating fan starts. I'm not sure, but the spark generator may have just been cooked by heat from the firewall. The wiring around it looks ok.
 
  #22  
Old 01-13-03, 06:24 PM
msw
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Okay, it looks like I've got my work cut out for me. But before I start dismantling the thing, here are yet more questions:

1. How much of this should I do immediately? For instance, one idea would be to just splice some wires so I can get the burner to work so I can see if it rolls out (which, I presume, means to shoot right back out towards where I was when I took my photos).

2. If I follow that approach and it does roll out, should I then determine if the heat exchanger needs replacement before I order any more parts (that would be the time to also order the wiring harness and spark generator)? Is there a specific way to identify a blockage or hole?

3. If the heat exchanger has a blockage, should I replace it (17 years old) or try to clean it? The documentation that came with the furnace identifies a procedure that sounds pretty ugly and involves a power drill and a plumber's snake, following some major dismantling. On the other hand, replacement would mean even more dismantling and more money.

4. If I need all this stuff, including a new heat exchanger, is it still worth doing the repair instead of replacing the furnace? I might be able to replace parts, but lack the confidence to install a new unit (I'm sure I could do it, but who knows what mistakes and oversights I would commit along the way). I'm guessing that it would still be more cost-effective to repair it, especially since I plan to sell the house soon.

5. How do I clean the burner assembly?
 
  #23  
Old 01-13-03, 07:19 PM
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Join Date: Nov 2002
Location: Massachusetts
Posts: 1,815
Initial inspection

Kill the gas ****, break the union, remove the logs, and vacuum the cells with a crevice tool. Get a powerful flashlight and the wifes compac mirror and look as deep as to can for cracks and holes, usually on the two side cells and where the pilot is are the chief areas where I see holes/cracks. Coldest and hotest areas of the heat exchanger. Also check by the exhaust manifold. If you find a bad exchanger, stop and get some estimates from a pro for replacement. If you get the heat exchanger, a good realitor will see how old the unit is and devalue the home knowing its life is limited. My sister sold a house in 1975 and the realator cut $500.00 from the house because they said the boiler would adventually need replacement in the next 10 years.
That was alot of money back then...
 
  #24  
Old 01-18-03, 02:29 PM
msw
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I removed the burner and looked inside the heat exchanger. I don't know what's normal, but I expected to see a lot of soot buildup. However, there's no sign of soot at all, except a tiny area right at the front! The inside of the exchanger looks like new. The burner is pretty clean too, although it shows some signs of flame. I took photos, but was unable to get a good shot inside the exchanger. The one shot I'll provide is pretty poor, and shows the only things visible inside, which are two sheet metal screws. The following links include photos of the burner, pilot assembly, spark ignitor, and a shot inside the cabinet with the burner removed:

[img]http://www.zealonline.com/furnace/exchanger.jpg[img]

[img]http://www.zealonline.com/furnace/exchangerclose.jpg[img]

[img]http://www.zealonline.com/furnace/burnerfront.jpg[img]

[img]http://www.zealonline.com/furnace/burnerside.jpg[img]

[img]http://www.zealonline.com/furnace/burnerfrontclose.jpg[img]

[img]http://www.zealonline.com/furnace/burnerback.jpg[img]

[img]http://www.zealonline.com/furnace/sparkignitor.jpg[img]

[img]http://www.zealonline.com/furnace/pilot.jpg[img]

[img]http://www.zealonline.com/furnace/insidecabinet.jpg[img]

Considering how clean everything is, should I just install a new pilot assembly, wiring harness, and spark ignitor, or am I still missing something? Does the burner even need to be cleaned? If so, how do I clean it?
 
  #25  
Old 01-18-03, 05:45 PM
bigjohn
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You need to find out why the flames are rolling out. I would put it back together, let it run and see what happens when the blower starts. [the fan that circulates the air thru the ducts] If all of a sudden the flame is blown out by the force of the indoor fan airflow, then the heat exchager has probably failed. Have you investigated the integrity of the return duct connection where it's charred? Is there a leak there that may be sucking the flame out? That flame comes out far enough, it will set the insulation on fire. If the heat exchanger is bad, no point in buying the other parts because you won't be able to get a new heat exchanger for a furnace that old. You'll have to replace the furnace which could be a selling point if you're going to sell the house.
 
  #26  
Old 01-18-03, 06:43 PM
msw
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Okay, I want to do this right, but I'm a little confused. I don't see anywhere where the ductwork is burned. The airflow is from left to right in my photos, so the cold air return duct is out of the photos to the left. The duct that appears in some of the photos is the one that carries the hot air. It is wrapped in insulation covered with a silver-colored material. But it doesn't seem to be damaged (that I can tell).

If I put everything back together, it still won't work because (I think) the wires are charred. It appears to me that I should try to splice the wires in an attempt to get it to work so I can see what happens when the blower kicks in. Does that seem appropriate at this point?
 
  #27  
Old 01-19-03, 12:41 PM
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Join Date: Nov 2002
Location: Massachusetts
Posts: 1,815
a burn

The burn is the area thats blackened outside of the heat exchanger cells. It an air flow problem I think. If the exhaust to the unit is blocked even partially, it may not start or be in and out! Check the exhaust tube by temporarily disconnecting the exhaust from the top of the unit, and see if the unit fires up. The ehaust will dump inside the house only for a minute or so, it wont hurt you. If this concerns you, open a door to vent out the place. This test, will tell you if there is a blockage in the exhaust tubing!, if not then look at the flue collector box area and check with a flashlight to see if its blocked. One other thing. when the combustion blower starts, check its rotation!
 
  #28  
Old 01-19-03, 07:49 PM
msw
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Here's the latest. After reinstalling the burner and pilot assembly, I disconnected the flue (the tube that exhausts the combustion gases out through the roof) at the top of the unit. Then, when I tried to start the furnace, the inducer blower blew lots of air out through the top of the unit. Of course, the pilot still did not light, but that wasn't a surprise since the wires were still charred. But it would seem to me that it means there is no blockage anywhere within the furnace itself (maybe somewhere in the flue as it goes up to the roof, but not in the furnace).

Next, I left the flue disconnected and spliced the wires leading to the pilot assembly by cutting out the charred portions (including the plug) and twisting the bared ends together and taping them. No solder or wire nuts, because I just wanted to see if the pilot would light. No such luck. That means that the main blower won't start either. As always, the inducer fan comes on, but nothing else visible happens.

At this point, I still can't get the pilot to light and have not determined why the wires got burned. Does anyone know how I can positively diagnose which parts are bad so I don't have to start buying parts blindly? And is there any way to determine the cause of flame rollout without first getting the pilot to light and the main blower to start?
 
  #29  
Old 01-20-03, 02:37 PM
bigjohn
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The first set of pictures you posted has one called splice overview. The insulation [or actually the duct tape] in the far right and near the top of the picture looks brown like it's been touched by flame. Compare that to the paint on the cabinet which shows no damage at all. Maybe it's just the lighting that makes it look that way? Take your wire splices to the flame switch apart and, using your ohmeter, test for continuity on the 3 wires going to the flame switch. If the flame switch is good, you should have continuity on 1 pair and no continuity on the other pair. I can't tell you which colors because the wiring diagram only shows numbers not colors. The best I can tell from the pictures is you should have continuity from yellow to green and no continuity from yellow to white. If you don't have continuity on any pair then the flame switch is bad. i've been assuming that air flow is right to left facing the front of the furnace, but you're saying the supply air exits the furnace at the burners not at the fan section? If so, then rule out an air leak sucking the flame out at that spot.
 
  #30  
Old 01-20-03, 07:58 PM
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Join Date: Nov 2002
Location: Massachusetts
Posts: 1,815
Carrier/BDP

Hook up one lead of your meter to the side of the transformer that is secondary, touch the other to the R low voltage terminal, it should read 24V if not switch the lead on the transformer to the other secondary terminal and you should read 24V to the R terminal. Once you get the 24V read leave this lead on the transformer for the rest of the testing. The pilot switch yellow is the common hot, power is yellow to green (NC) when it heats up the bimetalic strip it switches yellow to white (NO), so now the white wire carries the 24volts. Measure this to see if you have the 24Volts first at yellow and green. Are you getting the spark?
Check to see if you have 24V at the P terminal on the gas valve. Do you have 24V on any terminal on the gas valve?
Oh, the burnt up stuff REPLACE IT!
 
  #31  
Old 01-20-03, 10:18 PM
msw
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I think I was able to verify what both of you thought: the yellow is hot and the power is SUPPOSED to go through the flame sensor first to green, then white. The reason for my astounding verification detective work: I traced the wires from the flame sensor and the yellow goes to the air flow sensor, the green to the spark ignitor & gas valve terminal 5, and the white to gas valve terminal 1.

I first checked continuity through the flame sensor by disconnecting the wires and testing two at a time. No continuity on any pair.

Then I found the secondary terminal on the transformer that provided 24 volts to the R terminal. It was the one marked sec2. With the inducer fan running, I got 24 volts between sec2 and gas valve terminal 4, but not between sec2 and any other gas valve terminal. I don't see any gas valve terminal marked P - they are labeled 1 through 5. Does P stand for Pick? If so, that would correspond to terminal 1 but, like I said, there was no voltage between sec2 and gas valve 1 either.

I disconnected my temporary splices of the wires leading to the flame sensor. With the inducer fan running, I got voltage across the yellow & green and the yellow & white pairs. Since I've proven myself to be such a great detective, I'm therefore deducing that the wires up to this point are okay.

I then reconnected the yellow wire to the yellow flame sensor wire (but made no other connections) and checked for voltage between the following pairs:

Yellow and green coming out of flame sensor
Yellow and white coming out of flame sensor
sec2 and green coming out of flame sensor
sec2 and white coming out of flame sensor

There was no voltage across any of these pairs. Risking yet another deduction, it looks like the flame sensor is confirmed dead.

If these deductions are correct, I feel better about replacing the flame sensor, wiring harness, and spark ignitor, because I know it's the flame sensor that's bad. However, that still leaves the question of what caused the flame rollout. The air flow does indeed move from left to right in the photos, which means from cold air return duct to main blower across heat exchanger to hot air ducts. Despite what the photos may suggest, there does not appear to be any damage to the hot air duct or its insulation - the damage all seems to have been inside the furnace.

This boils down to two questions:

1. Is there agreement that the flame sensor is bad?
2. What do I do now? Do I replace parts or first do something else to diagnose cause of flame rollout?
 
  #32  
Old 01-21-03, 07:31 AM
bigjohn
Visiting Guest
Posts: n/a
Good job on tracing the wires. We'll make a technician out of you yet. Witn no continuity on any of the flame switch wires, the flame switch is bad. You'll find that the new flame switch will come with a one way plug that is supposed to couple to a companion plug at the end of the yellow/green/white wires. If you choose to not buy the wiring harness and just splice the new flame switch in, you may have a problem if a sharp home inspector spots it. [at one point you had mentioned you thinking of selling] It would probably violate a technicality of the furnaces' UL listing because the flame switch would be potentially defeatable. [how?- with a timer] My suggestion is to replace both the pilotburner/flame switch assembly and the wiring harness but the harness is your call. Let us know how you make out. BTW- some of the voltage readings you had were because of a backfeed thru the gas valve coils.
 
  #33  
Old 02-01-03, 06:32 PM
msw
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I haven't replaced any parts yet, but I spoke with a local technician who said that, if he had the time to come over (which he doesn't), he would be looking at the gas valve pretty carefully. They can apparently be open or partially open when they shouldn't be, which could be a cause of the charred wires. Any thoughts?
 
  #34  
Old 02-01-03, 06:52 PM
Member
Join Date: Nov 2002
Location: Massachusetts
Posts: 1,815
parts

sounds like a bad pilot burner switch, replace it.
 
  #35  
Old 02-01-03, 07:04 PM
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Join Date: Nov 2002
Location: Massachusetts
Posts: 1,815
OH!

P stands for pilot solenoid valve and the M is for main gas valve solenoid. The gas valve has two electromechanical solenoid valves, when the unit first starts the pilot by sparking and energizing the pilot valve by sending power thru the NC switch (yellow to green), as the bimetal heats up the switch, it sends 24V power to main gas (yellow to white)...this energizes the main gas solenoid and lights the unit. The puffback might be from a main gas valve sticking or hanging up. If this were the case though the furnace would never go out, unless it was a partial leak....hard to say...Flue cap might be missing, stack may be too low...
 
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