Furnace Inconsistant Light-Off

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Coleman residential (LP) doesn't alway light-off. Novice at furnaces (but not ignorant) DYI'er (aircraft mechanic)
I changed the igniter first. Local parts man believed (99%) this would be the fix based on the appearance (arcing) and probable age (probably original). Relatively cheap first try too :^) Still doesn't always light off. Cycles the igniter 3 times, then goes 'to sleep' just as it's supposed to (I'm told). Power off - then back on - try again.
Is the ignition/control unit next ?
I do have 24v to the ignition unit, but I don't get 24v output to the gas valve. At least not every time. One time, when it finally opened, it was toward the end of the igniter cycle (it was cooling down). Is the gas valve on a time delay, or should the valve be powered immediately ? How does the control unit know when it's time to open the gas valve, since there's no temp sensor at the igniter.
This wouldn't be so frustrating or hard to diagnose if it wasn't an intermittent problem. (we hate those on aircraft too !!!) Usually, circuit-card controllers either work or they don't. They don't usually "kinda" work.
Thankfully it's not cold around here right now (Cincinnati area) Hate to have this sucker completely give up the ghost when it gets colder.
Watta y'all think...
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I think you need to call a pro. LP isn't anything to mess with. and FWIW the primary cause is probably *not* the ignitor or its controls. You can change parts all day long and spend a lot of unnecessary time and money and still not correct the problem. Your only looking at symptoms, not the cause.
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The following message is written with respect to your profession !
As for the "LP isn't anything to mess with"... Don't take this wrong, but that falls under the normal 'understood' precautions. People might call me crazy, but I'm not stupid. Is there something specific you're trying to warn me of ?
Let's re-phrase my "novice" status - I'm not going to pretend to be a HVAC expert, but I'm a lot more advanced than your regular DYI'er. (I'm an aircraft maintenance instructor - on commercial aircraft) So, could you please respond on a higher level - please :^) I know you all hate it when a non-expert looks at the schematic and thinks he's got it all figured out. Well, I'll confess that's partly the case, but how complicated can this sucker be (I CAN grasp the concepts and understand system functions). And systems troubleshooting (including electrical)is part of my job, and I've got the test equipment. Yes, at some point I do have enough smarts to know when I'm in over my head.
So, back to the problem ... If it's not a control issue, then what's preventing the controller from applying the power to open the gas valve ? The vent blower switch and limit switch (which provide power to the controller) are good - there's 24v to the controller. I don't see any other inputs to the controller that it would use as logic for powering the gas valve. High voltage to the controller is good, and igniter works, so, that leaves the low voltage side of the controller - to the gas valve. So, what am I missing? I checked the voltages to/at the connector on the controller - it's easy to probe from the back of the unit.
I'd just like to understand... :^) but I also don't want to be lumped in the category from the "every year, there's got to be a few of these idiots" thread :^{
Remember, I meant this all in a respectful way !!! :^) Thanks Marc
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Like Mike said.... "once all the safties are proven"...... and *if* all of the safties are proven, then yes, look at the control module....
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hell who knows it could also be a slow opening gas valve or intermittant gas valve. might be a bad or intermittant grounding issue. maybe itsa honeywell smartvalve issue.
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Smartvalve?? not hardly...
"Coleman residential (LP)"
Can you say trailer??
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On Sun, 31 Dec 2006 15:07:52 -0500, Marc Anderson

If you have 24v input to the ignition module on a call for heat and no 24v output to the gas valve then probably the ignition module is bad. Its not often that they are intermittent like you say but it does happen.
On a call for heat and once all the safeties are proven the ignition module gets 24v at which point the ignitor glows for a predetermined amount of time. On some models if the ignitor doesnt draw the proper amperage the gas valve isnt allowed to open at which point you have to replace the ignitor. On other models the ignitor will go through a warm up period of say 45 sec and then the gas valve gets 24v and opens. If flame isnt sensed within a specified time the gas valve closes and may re try a few times and lock out or it may lock out immediately.
Thats basically how they work and should give you enough info to be dangerous or fix it. Good luck.
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Thanks for the info. Ah-hah ! Igniter current is sensed to verify igniter operation. Have seen similar idea used on aircraft.
Looks like I had the basic concepts figured out, just wondered what little secrets I didn't know.
I'll promise not to be 'dangerous' :^) If I don't play with it myself, at least I'll be a little more knowledgeable when the pro comes to call.
Thanks again Marc
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Marc Anderson wrote:

I said thats one way. Yours is more likely to be the other way. The valve gets powered open and if flame is established and sensed by a separate flame sensor within a certain amount of seconds it stays opened. If no flame is sensed the valve closes and the ignition module locks out.

Its not secret. Its basic operation. Each of the manufactures have certain twists they employ to achieve the same result. That result being a safe controlled explosion and fire.

I doubt you will be able to purchase a ignition module but if you do somehow manage to get one and install it, and when you go to light it for the first time Id advise keeping your face away from the burner area and keep one hand on the power switch in case you need to shut it off.
Youd be better off having a pro do it. He would be able to spot problems and correct them before they become bigger problems. The only reason I told you this info is to keep the scum bag rip off artists from selling you something you dont need so they can finance a fishing trip in Florida or something.
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I don't think I'll tackle this one. Sounds simple to change out the ignition module, but since there may be other gremlins ready to bite me, I'll probably not take the chance. Try to save a few bucks ... blow up the house ... not worth it. Then I'd join "the few of those idiots" :^{ Thanks all !! Marc ......................................... Power's Mechanical wrote:

The R&R of the ignition module is simple, but I'd agree that it could be kinda scary the first time you light it off. There's always that last moment of apprehension when you hit the switch.

Thanks for that info and advice. I do have a company I've had good dealings with, so I'm comfortable getting them to do the job. Marc
M
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Marc Anderson wrote:

Let us know what your HVAC tech finds.
--
Remove the TOS star ship captain to reply.

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Ain't it the truth. Did I ever tell you the story of the gas co. guy who USED to have a beard?
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And I think you have a white rogers igniton module in that furnace. Most of them follow the same sequence anyway. As long as the thermostat is calling for heat and the safeties, pressure switch close, then the module gets 24v usually on the TH terminal. They all have internal timing, the sticker should say pre purge, relight times ect... Your module is getting 24v and starting it's sequence, 120v to igniter, wait 20sec and send 24v to gas valve boom ignition, flame sensor rectifies signal back to module. Sounds like the timing to the gas valve has failed. Would be another issue if your in fact smelling gas during your inconsistant light offs hehehe
-CanadianHeat
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Now that is fricken scary: You mean that the jet engine stays lit because a $1.49 thermocouple tells the jet fuel valve to stay open?

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Oscar_Lives wrote:

A t/couple doesnt sense current.
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Well you took more of a risk changing the igniter, I would change the module now. It's obviously the problem. Do you smell gas when it doesn't fire? No because the module isn't sending 24v to the gas valve. It could only be the module.
-CanadianHeat
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DANgER ( snipped-for-privacy@heat.com) wrote

Like I posted earlier, the older Coleman's have a sequencer. That could very easily be the problem. Think "Clare Mega-junk"! If not, I agree about the module.
--
Respectfully, Bob

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How much current could a current sensor sense if a current sensor could sense current?
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two cents per millivolt. Currants are for flavor only.

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Oscar_Lives wrote: ...

There ain't NOTHIN on an aircraft that costs only $1.49 !
There's a computer worth a couple hundred thousand bucks (not kidding) that controls the engine, and it's monitoring a whole pile of sensors.
I only meant that the CONCEPT of monitoring heater current is used as verification of correct operation in some systems, and it's not necessarily on the engine (electrical de-icing systems for example).
Marc
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