This is a tough one: ignitor never powered


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Old 11-18-07, 02:15 PM
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Angry This is a tough one: ignitor never powered

I've been working on this problem for a couple of weeks now. I'm hoping someone can help me (it's been very cold here lately :-))
- System
Make: Comformaker
Model: N8MPN075B12A1 (manufactured in May 2003)
Time of use: less than 4.5 years
Gas valve: Honeywell SV9541
Control board: ST9160B-1084

-Measurements:
Igntor resistance: 4 ohms
Pressure switch resistance (when closed): ~ 0.2 ohms
Voltage on flame sensor: ~105VAC (drops to 98VAC when I hear a click - probably the pilot valve opening);
Voltage accross the limit and roll-out switches: 0VAC
Green light: constant heart-beat
1/2 wave rectified: ~15VAC
Power from control board: ~24VAC
All measurements matched the spec;

- Problem
1. I turn on the furnace (through the main power switch) and the combustion blower starts to run (I've set up the thermostat to call for heat). The pressure switch is initally open (~12VCAC accross its terminals);
2. Pressure switch closes (0VAC accross its terminals);
3. I hear a click (probably the pilot valve opening);
4. The voltage on the return side of the igntor stays at ~24VAC (indicating that it's not been powered. It should be around 1VAC when powered). The voltage on the supply side is ~24VAC;
5. This cycle repeats about every 30 seconds;
6. Sometimes, the ignotor does come on and everything works fine. After a while (~24 hours- heater on and off), it stops working again;
7. The gas valve was recently replaced for a brand new one;
8. I had a serviceman come in and he said the control board was bad (wasn't supplying proper voltage). However, based on my measurements, the control board seems to be working just fine;

Any ideas?
 
  #2  
Old 11-18-07, 08:33 PM
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Hi RookkieGuy:

This isn't so tough. Basically, sometimes you're getting 24 volts to the ignitor sometimes you're not. For starters, a good ignitor should read 10 ohms or less, so that base is covered. It sounds like all the functions are working except for the ignitor glowing [no fault codes either]. You need to check for 24 volts across the ignitor terminals on the gas valve. IWO- the ignitor is a load, so you need to know if it's being supplied with voltage. My guess is you'll find that sometimes it is not. If you're getting 24 volts to the gas valve from the board AND at the same time you're NOT getting 24 volts to the ignitor [t-stat is calling for heat of course], the gas valve is bad. If you're getting 24 volts to both the gas valve and the ignitor, but the ignitor won't glow, replace it. Now, I read where you replaced the gas valve, but with Smart Valves, all bets are off. Let us know what you find.
 
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Old 11-19-07, 06:26 AM
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Angry

daddyjohn, thanks for your reply!

As I was reviewing my post, I realized that I had measured both sides of the ignitor in reference to ground but not across the ignitor. So, I went back to the furnace and took that measurement.

To my surprise, the voltage was around 24VAC and the ignitor glowed! The furnace worked for several hours and then finally quit. At this point, I checked the voltage across the ignitor again an it was 0VAC.

In summary, you're correct! Sometimes there's voltage across the ignitor (in which case everything works) and most of the time there's no voltage (even though the 24VAC power is available to the gas valve).

I thought it could be the flame sensor. The service manual states that the flame sensor is checked before the ignitor is turned on. I checked that too and it seemed to be okay (105VAC and, per the manual, it should be above 85VAC).

I agree w/ your assessment, though. Everything seems to indicate a bad gas valve. What's puzzling, though, is the fact that this is a brand new valve, less than 2 weeks old!

I don't know if it matters or not, but the new gas valve was installed at an angle to the front panel. The original one had its face (i.e., where the on-off switch is mounted on) parallel to the furnace front panel. The new one is at a slight angle. Could this be an issue?

Thanks again!
 
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Old 11-19-07, 08:13 AM
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The complexities of the ignitor system. Found this online:

"Titleevice and method for triggering a gas furnace ignitor Document Type and Number:United States Patent 6474979 Link to this page:http://www.freepatentsonline.com/6474979.html Abstract:An ignitor controller and method of providing the same triggers an ignitor switch of a gas appliance, such as a furnace, subsequent to the zero-crossing of the line voltage, but before the voltage exceeds +/-five volts. This effectively reduces EMI levels caused by the constant triggering of AC line voltage to the ignitor switch after +/-five volts, which constant triggering is needed to properly operate ignitors (e.g., silicon nitride ignitors) in electronic ignition systems. The controller includes a voltage detector circuit (i.e., a two transistor circuit), a series switch (i.e., a darlington array) and an energy storage device (i.e., a capacitor), which trigger the ignitor switch (e.g., a triac) sufficiently close to the line voltage zero-crossing to reduce line conducted interference."

[Silicone nitride ignitors are HSI's that are better than their counterpart, silicone carbide.]
 
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Old 11-19-07, 08:59 AM
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Hi RG:

HW says that 0 to 90 degrees from upright, including vertically is acceptable. The problem is in the circuit board on top of the valve itself.
 
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Old 11-19-07, 06:20 PM
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Is it possible to replace this circuit board by itself or does the whole valve have to be replaced?

Thanks again.
 

Last edited by RookieGuy; 11-19-07 at 07:14 PM.
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Old 11-20-07, 12:51 PM
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Unfortuneately, the entire valve has to be replaced.
 
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Old 11-20-07, 01:11 PM
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Last year I had to replace a smart valve on a powervent gas water heater and I was nervous that maybe I would be putting in the wrong (most expensive) part. I tested all the safety switches, and ventor and they came up good, so I chanced it, even though the blinking light signified a problem in the pressure switch circuit. Well, I wasn't wrong. It was indeed it. So, you can get even a false troubleshoot blink pattern. I thought about this and concluded that the board only knew that the pressure system was not working, because, IT wasn't working because the inducer motor would not come on to create any pressure. And the reason the inducer motor was not coming on was because the board-part of the smart valve was screwed up. I was going to call up the national name-brand company to tell them of their improper error code but never got around to it. Or, I tried and got tired of waiting after listening to menu options and put on hold (I go thru this all the time).
 
 

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