Furnace Ignition Problem


  #1  
Old 01-05-03, 02:36 AM
jedelsol
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Question My Lennox gas is freezing my ...

Model G16Q3-50-5
History:
Has been working fine this winter until last night.
Observations:
Responds to Thermostat (White Rogers 1F72-310) signals. Heating cycle triggers when setpoint temp is higher than curent temperature. Blower runs in "On" position. Heating cycle appears normal up to and including the lighting of the pilot. Then the pilot stays lit and the induced draft fan runs continuously, but the burners never light. As a result the blower motor never kicks on in "Auto".
I have a wiggy and can find the terminals that are shown in the electrical diagram. I also have a multimeter if that will help in the diagnosis. It appears the main gas valve is not opening. There is a sticker on the gas valve warning not to short across the main valve and/or pilot valve contacts. Does this mean I can take a voltage measurement, but not a continuity measurement?
Assumptions and guesses:
I assume I have power through the Limit Control and Induced Draft Proving Switches or the ignition control unit would not have lit the pilot.
I guess the sensor must be returning a "pilot lit" signal or I would think the ignition control would time out and shut off the pilot valve.
What should I check to narrow the problem further?
Do you know where I can get parts for this unit?

Your help is much appreciated.
Thank you,
Joe
 
  #2  
Old 01-05-03, 04:01 AM
fjrachel
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Sounds like the problem may be in your ignition controls(Pilot lite flame sensor or ignition control module or safety switches in main gas valve circuit). What type/brand control module do you have on the unit?
 
  #3  
Old 01-05-03, 05:11 AM
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Hello jedelsol. Welcome to the Heating & Cooling forum & the D-I-Y Web Site.

Once the sparking ignitor ignites the pilot flame, the flame heats an element. The element is a metal rod like part attached to the pilot assembly. The rod like element is the flame sensor.

Remove the element carefully, clean it with fine grit sand paper or steel wool and replace it. Insure it is properly connected at it's connection point.

Retry the heater and all should be working fine. If not, replace the flame sensor.

For additional information & problem possiblities, read the questions within this forum pertaining to heaters with ignition problems. These prior questions and the replies offered within them contain almost all the required information needed.

Be sure the electrical power to the appliance is turned off, the thermostat is turned down and the gas control valve is in the off postion before attempting any repairs.

Retail parts dealers and heating part supplers can also help determine what the possible problem may be. Bring the make, model and serial numbers. Dealers are an excellent source for original replacement parts and are listed in the phone book.

If you need further assistance, use the REPLY button to add any additional information or ask additional questions. Attempt the repair solutions or correctional method provided first.

Regards & Good Luck, Forum Host & Multiple Topic Moderator.
TCB4U2B2B Company Enterprises. Energy Conservation Consultant & Gas Appliance Diagnostics Technician.
 
  #4  
Old 01-05-03, 10:27 AM
jedelsol
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Thank you for your prompt replies!

The ignition control is a Robertshaw SP735L (Lennox P/N 18G9101). The gas valve is a Robertshaw 7100 DER S7C 71F-11A-017.

In order to clean the sensor as you suggest, it looks like access will be difficult without removing the burner assembly. The sensor is tucked just far enough in that I can't see how it's mounted or how I can take it out. Is this correct? If so, let me check what looks like the procedure. There are four cap screws holding two gas pipe support bracketsto the furnace body. Do I shut the gas off upstream of the flex entering the furnace, unscrew the coupling at the flex, and after loosening the 4 cap screws, lift the brackets, gas valve, burners, and associated piping up and out?
 
  #5  
Old 01-05-03, 12:12 PM
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Attack!!!!!!

write down anything you take apart or even better use a digital camera, shut the gas of and power and take apart anything that gets in the way of getting to the sensor. a 5/16 nut driver or socket with lots of different extentions will help, you may have to break the union(un-do), and take it out as an assembly. I don't see too many Lennoxes where I am. But they're all basically the same.
 
  #6  
Old 01-05-03, 12:21 PM
jedelsol
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Thanks.

I'm on it!

I'll keep you posted.
 
  #7  
Old 01-05-03, 02:47 PM
jedelsol
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There's fire in the hole! The house is warming up and smiles are returning to faces.
Although the sensor did not appear to have any buildup, I gave it the sand paper treatment anyway. It appears to have been the problem.
Thank you very much for the guidance.
Joe
 
  #8  
Old 01-05-03, 04:57 PM
jedelsol
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OOPS! I claimed victory prematurely. After running a while and bring the house temperature up to a few degrees of set point, I started feeling a cool draft coming out of the vents - arrrgh!
I found the pilot still burning, induced air and blower still running but no burners. Using set point adjustment I turned it off and on several times, including waiting 15 minutes or so between tries. All cycles were the same - everything normal up to burner lighting - then nothing.
Is there any measurements I can take on the sensor wire to see if I'm getting a signal or not? How about across the main valve input signal connection?
Joe
 
  #9  
Old 01-05-03, 07:17 PM
fjrachel
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Seems as though the pilot sensor is working properly. It is keeping the pilot lite lit. You may measure the sensor signal in micro amps dc at the control module. Still say you got a problem in the module or main valve control wiring...Read your wiring schematic, and ck the switches and limits in that circuit.
 
  #10  
Old 01-05-03, 07:44 PM
bigjohn
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OK First, the warning about shorting across the terminals means don't short across them when the power is on. You can take voltage and continuity readings. That module is looking for verification of the pilot flame, and as long as it senses the pilot flame, it will keep the pilot lit. The next step in the sequence is to open the gas valve to the main burners. At this point the sparking continues for a few seconds to stabilize the main burner, then the sparking stops. IOW- if there were something wrong with the circuitry that verifies the pilot flame, the gas to the pilot would be shut off. So, I think the problem is in the next step of the sequence. We need to determine if the gas valve is getting 24 volts to the #2 valve. [ the gas valve has 2 valves inside- #1 opens to the pilot burner and is also a redundant opening to the main burner. #2 opens to the main burner BUT gas has to flow thru both #! and #2 inside the gas valve body] Shut off the power and clip your multimeter [not the Wiggy] leads to terminals M and C on the gas valve. Turn the power back on and try for a startup. You may have to wait over 90 seconds before checking for voltage. If you get voltage, then the gas valve is bad or the burner orifices are plugged which I doubt because you did get the burners to lite once. If you don't get voltage, most likely the module is faulty. Before condeming the module, check for 0 ohms between the tip of the flame rod to the sense terminal on the module. Also, check for NO CONTINUITY from the tip of the flame rod to the GND. terminal on the module. If these check out ok and you don't have 24 volts to the gas valve, then replace the module. The fact that the pilot stays lit makes me think it's going to be the gas valve but do the checks anyway. Let us know what U find.
 
  #11  
Old 01-05-03, 08:46 PM
jedelsol
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Measurements confirmed the following:
1) 26.67 volts at ignition control input when thermostat calls for heat, so all upline switches are normal.
2) I didn't have equipment to measure microvolts DC on the sensor, but I did get definite action (-10v DC relative to ground) on the meter when the pilot was lit and nothing when it was not.
3) I got 26.67v AC on pilot valve signal when the pilot came on.
4) I got no change on the main valve input connections.

Once the ignition control validates the pilot is lit via the sensor feedback (if -10v is enough) I guess it should be sending the main valve the signal to open.

So... is -10v read with the positive probe an idicator that the sensor is providing the correct signal to the ignition control?

If so, does that mean I've got a bad ignition control? If so, why did it work for a short time again this afternoon after I cleaned the sensor probe?

It's getting late in NJ - I hope you are a night owl.

Joe
 
  #12  
Old 01-06-03, 03:01 AM
bigjohn
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I'm sorry- I had to get to bed. Yes, once the module validates the pilot flame, it should energize the gas valve at terminals M and C. I really don't think the problem is in the flame proving circuit because, if it was, the module would shut off gas flow to the pilot. What is the condition if the wire going to terminal M on the gas valve? Have U checked for power at terminals MV ad MV/PV at the module? IOW- we don't want to condemn the module if the wire in between the module and the gas valve is faulty. Also, did U do the other checks? If all that checks out ok, then it has to be the module. We are either getting voltage to gas valve or we're not. Why did it work one time? That's like asking if I understand women.
 
  #13  
Old 01-06-03, 03:05 PM
jedelsol
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Sorry Big John, I didn't see your second-to-last post until a few minutes ago. When I read in your last post "Also, did U do the other checks?" I thought what other checks? Then I scolled up to see your very informative directions!

Anyway I did these. I confirmed 0 ohms between the tip of the flame rod to the sense terminal on the module. Also, there was NO CONTINUITY from the tip of the flame rod to the GND. terminal on the module.

I had been measuring the voltage at the main valve connections, so I eliminated the wires as the problem both by confirming continuity of each as well as by pulling the connectors off the ignition control and measuring voltage directly on the ignition control connectors. Here's the kicker - I found that I had 24v at the output connectors (marked MV - MV/PV on my unit) when the wire on the MV connector was not attached, BUT as soon as I connected the wire back on the voltage dropped to 3 or 4 mV. Then I pulled the wire off the M connector on the valve and tried again - still only 3 or 4 mV. I checked continuity between M and C looking for a short, but there was no continuity.

Now the wires at the M and C terminals of the valve are piggybacked with wires coming from the Honeywell Fan and Limit Control unit. According to my schematic, the fan control heater (if used) is located in the Honeywell unit and is wired across these two terminals as well. So I disconnected the leads across the heater in the Honeywell unit and without attaching the connector to the M terminal measured 24 V at the connector. I then reconnected to the M terminal while leaving the heater wires disconnected on the Honeywell side. I thought I was going to get the burners, but alas, I only had 10 V at the M terminal.

So Big John my friend, where do we go from here?
 
  #14  
Old 01-06-03, 03:31 PM
bigjohn
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Which wire from the gas valve is connected to the HW control? The one from C terminal on the gas valve or the one from M terminal? Try taking both wires out of the HW control, splicing them together with all the other wires in place and see what happens. [or is this what U did?] IOW- we want to eliminate the possibility of a voltage drop in the HW control. If it's the wire from C terminal, you wouldn't have the pilot flame. Make darn sure you are splicing together wires from the LOW VOLTAGE side of the HW control. Is the HW control a combination fan and limit control or is it a limit control only? If it has 4 wires going in and out, it's a combination control. The FAN side will be 120 volts so don't get the wires crossed. If the limit is ok, then it has to be the module. I'll be online for a while tonite. I looked up some modules today when I went to work. Both Robershaw and HW make a module that will replace yours. Johnstone Supply sells them.
 
  #15  
Old 01-06-03, 05:18 PM
jedelsol
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There is a wire from M and one from C. According to the schematic the fan control heater is between those two terminals. If I were to connect the two wires it would seem to be shorting across the M - C terminals.
The HW control is a fan and limit control (4 wires for these functions) plus it includes the fan control heater (2 more wires - one to M and 1 to C on the gas valve. I have disconnected the later two on the HW side.
Are you on the East Coast too? When I made that comment about being up late I was talking to fjrachel.
Joe
 
  #16  
Old 01-06-03, 06:23 PM
bigjohn
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OK Now I understand what you're saying. You have what is called a "heat assisted" fan control. You're right, pairing those wires would be a short. I misunderstod and thought that you were saying that the limit side of the switch was wired in series with the gas valve wiring. I thought that sounded strange, because normally the limit breaks either the primary or secondary side of the transformer. The heater in the HW control is wired in parallel with the gas valve. The next thing we need to do is eliminate the possibility that the gas valve operator is stuck/malfunctioning and drawing too much current which could be bogging down the module. You'll need a switch [plain old on/off switch like a light switch] and some wire. Connect a length of wire to each side of the switch. [12" to 18" or so] Locate R terminal on the low voltage terminal strip in the furnace. Connect one wire from the switch to R terminal. Disconnect the wire at M terminal on the gas valve and set it aside but not touching anything. Connect the other wire from the switch to Mterminal on the gas valve. Turn the switch off and start the furnace. Wait a couple minutes for the pilot flame to stabilize and the turn the switch on. What we're doing is putting 24 volts directly to the gas valve, and bypassing the module. If the gas valve opens, then I would say replace the module. If the gas valve does not open, verify that you have 24 volts at M and C terminals and also at P and C terminalst on the gas valve. If you do, then replace the gas valve, but be prepared that the gas valve may have damaged the module. Actually, these days I live in the Caribbean. I'm the HVAC guy at a US missle tracking station. We are on Atlantic Standard Time which, right now, is 1 hour later than Eastern Standard time. AST is the same year round, no changing of the clocks.
 
  #17  
Old 01-06-03, 06:29 PM
jedelsol
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By the way, it seems to me by disconnecting both sides of the fan control heater I have isolated the main gas valve. Since I only got 10 V doing this, is this an indicator that the main valve is somehow the problem?
 
  #18  
Old 01-06-03, 06:35 PM
jedelsol
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I guess our posts are crossing in the mail. Again I didn't see yours until after I posted my last. Please ignore it.
Living in the Caribbean sounds pretty nice - is this a tour of duty or a permanent job? My sister lives on St. John.
 
  #19  
Old 01-06-03, 07:33 PM
jedelsol
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Putting 24 V to the main valve opened it both times I tried. It looks like the Ignition Control is putting out 24V, but for some reason can't maintain it under load.



How much is one of these likely to set me back?
 
  #20  
Old 01-07-03, 02:47 AM
bigjohn
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A new module will probably be at least $100. That's why I try to exhaust all other possibilities first, because, you can't return them. If you want to see the instruction sheet for the HW module in advance, go to http://hbctechlit.honeywell.com Click on accept, then on the next page select MODEL and ALL LITERATURE, then type S8610U in the search box. You'll be able to download the installation instructions which include a cross reference. Let us know how you make out. I work for a private company contracting with the Air Force.
 
  #21  
Old 01-07-03, 03:01 AM
fjrachel
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Getting Lennox OEM parts are tough if you are not a Lennox dealer, and they do not sell to the public. A generic module made by Honeywell, Johnston Controls, White/Rodgers, or Robertshaw will work and be the cheapest way out.
 
 

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