Heil furnace - cant find flame sensor

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Old 01-19-09, 02:00 PM
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Heil furnace - cant find flame sensor

hello,
My Dad has an older electronic ignition Heil furnace that has a problem with the burners cycling on and off. From reading posts here, it seems that a good place to start is by cleaning the flame sensor.
Problem is, I cant find it.
The only thing extending into the burner area is the ignitor and it doesnt appear to have a sensor built into it. Just an orange wire for the ignitor and the gas line.
Any idea if the sensor could somehow be "built" into this?
The link below is what I have now.
Appliancedirectparts - Detail
 

Last edited by msuchodolski; 01-19-09 at 02:26 PM.
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Old 01-19-09, 02:50 PM
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It's usually a mistake to say "Hey! my problem sounds like that, I must have the same problem!"

And it's unlikely you have that problem.

I'm presuming you have a Honeywell S86 ignition control module which uses the pilot light you dispaly in your image.

That's an intermittent pilot system, so the main burner isn't supposed to turn on at all unless the pilot light has lit properly.

And that module tends to be very reliable, although they can fail.

I'd start out by disconnecting the wire on the gas valve to the "MV" connection, which would prevent the main burner from lighting up. It the pilot light then lights and stays lit, you may have a gas supply that is inadequate or unreliable, causing the gas pressure to drop to zero when the main burner turn on, shutting off the system and the causing it to relight repeatedly.

Anyway, I'd try that for openers.
 
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Old 01-19-09, 04:22 PM
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Thanks SeattleP,
You are correct, its a Honeywell S8600M module.
the furnace is a Heil Nuge 100 AG01
Maybe I should have started with the problem.
When I turn up the thermostat, a fan comes on and then the burners light, after about 1 min. another fan (blower?) starts to come on but then the burners go out. The burners will then relight but only stay on for 5-10 secs. it keeps doing this until the desired temp in met.
Still out of curiosity - is there a flame sensor on these?
 
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Old 01-19-09, 04:51 PM
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Any portion of the steel pilot burner struck by flame functions as a flame sensor. Frankly, I have never seen this kind of ignition system have the flame rectification problems common on furnaces with a separate rod used as a flame sensor.

Switching on the main furnace fan should not cause the burners to go out --- not unless they are being blown out by a cracked heat exchanger or something.

Still, if you have a way to manually switch the fan on, I'd do that repeatedly to determine if the fan is blowing the flame on the pilot light and/or burners around significantly.


I'd be more suspicious of the pressure switch opening. I'd check that with a multimeter set to measure AV voltages and see if the 24 VAC gets interrupted by the pressure switch in time with the burners being shut off.
 
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Old 01-19-09, 05:17 PM
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Oddly enough, the Heil I had to work on today had a similar sporadic problem that had me going there for a while. Yes, it did just what yours was doing.

I ended up checking the pressure switch tubing and it's connections on each end and the nipple it plugged into and reaming out where the vacuum tube plugs into the inducer fan housing, and checking the exhaust vent outside for ice or ?, snaking it(after I determined the the volume of discharge was way less outside than at the inducer -turns out it IS way less because the baffle in the inducer reduces outflow by about 50%), removing vent at inducer, checking inducer fanblade, and discharged the exhaust directly out the exhaust hole (after I disconnected a main 25v wire on the control module so that I was not sending burnt exhaust gas out the inducer into the basement).

I concluded the pressure switch was in some way defective, after I ruled out those other issues.

With pressure switches, it is most common for people to think that when they fail there is diaphram failure. And by checking how it behaves when you suck on the tube(or using a test equipment for this that measures in water column, such as 4.0 -4.3 inches) is not the end-all in diagnosing one of these things! Not only is there concern about whether if it can hold a vacuum. The other ingredient is if the 24v electrical contacts inside, that allow the switch to remain closed, are not somehow arced/pitted and causing a situation where the outgoing 24 volts gets interrupted by a bad contact. Causing it to behave sporadically -one minute or two furnace working fine, and then sudenly flame goes out, then may come back on in seconds or in a minute or not at all until I reset the furnace.

That is what _I_ had going on with the pressure switch. I discovered this when I got the burner flame to shut off or come on by me rapping on the pressure switch at the 2-wire terminal.

Not that this is YOUR problem, but it's a possibilty, especially if when the flame goes out, that the inducer motor is still running.

If the inducer motor goes out with the flame, you may have some issue with the signal coming from the thermostat.
 

Last edited by ecman51; 01-19-09 at 05:28 PM. Reason: kept adding more
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Old 01-20-09, 08:56 AM
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thanks for your responses.
I was hoping for an easier solution that an untrained homeowner could try, sounds like this is beyond me so I called a professional this morning. I will post back their findings.
Thanks
 
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Old 01-20-09, 02:03 PM
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Turned out to be a bad pressure switch.
 
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Old 01-20-09, 05:27 PM
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Sorry to hear that you felt you could not DIY it. Hope it did not cost you an arm and a leg.

The one I am waiting to have come into a supply house, got sent to one of their wrong supply houses in some other state today by mistake. So I called the tenant and he said all has been good so far.

I had made the claim, yesterday, that it conclusively has to be the PS. Not necessarily, after I thought a bit more about this last night. I STILL could have some venting issue on the cusp, where let's say if I was within 1/10th of an inch of W.C. (water column)on the switch rating, that by me whacking it with my screwdriver, I caused it to jump inside the switch enough to open the contacts, shut off the flame, and then back on again. Hopefully though it IS the switch. Because if I do not first invest in a testing gauge for this, I will break the old one apart and see. And - if I was wrong and made a mistake, then at least I will console my pride in the fact that the landlord I work for has saved oodles of money by using me on such repairs over the years.
 
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Old 01-20-09, 06:43 PM
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Diagnosing Pressure Switch Problems

Hello Ecaman---

Diagnosing baressure switch problems can be tricky, in large part because the correct pressure switch opening and closing pressures usually aren't provided by manufacturers. If that information were routinely listed on the rating plate of the furnace, problems could be diagnosed more accurately. And often the pressure switches themselves don't list that information.

Judging from the troubleshooting guides and methods recommended in furnace, the manufacturers are happy with the way things are. From what I see, most recommend that the furnace be torn apart to check for problems causing the furnace or vent to become plugged.

That's not as bad as it might be, because in my experience bad pressure switches are only infrequently the problem. Usually a pressure switch that's opening IS caused by a plugged furnace or vent.

Of course, it seems like 90% of the time repairman condemn the pressure switch and replace it. That the furnace continues to malfunction after they leave so often doesn't seem to bother them.

Actually, taking reads regularly with a sloped manometer or suitable mechanical manometer can over come the problems listed above fairly often. If memory serves me, I've typically found that a -1.4"wc typically suggests that the pressure switch should be making. If the pressure switch isn't making with around that pressure, then it's it's reasonable to suspect that the pressure switch is bad.

On the other hand, if the pressure found is down to around -.5" or so, then the furnace or vent is generally plugged up and replacing the pressure switch will do no good.


The best way to improve accuracy in diagnosing pressure switch problems is to measure the pressure often, and on furnaces that are working properly. That will give you a feelfor typical good reads, and you bad reads with problems furnaces will stick out as being unusual. Also it will give repairman experience with using their test instrument, and they will be prepared with the needed tubing and fittings.

Heh, heh! I've often suggested that the way to identify a competent repairman is to ask to see their manometer---- if it's shiny and new in a box --- hire another guy! If it's obviously been used and has various hoses and fittings hanging off of it --- that's the guy to hire!
 
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Old 01-20-09, 07:43 PM
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Thanks SP,

You have pretty much stated that which was discussed between me and my favorite HVAC pro at a supply house in town. It sounds like you were in on the phone conversation!

She is going to call me back and tell me availabilty and prices of various testing equipment, including both positive AND negative pressure testing. If I buy one, I will be sure to scruff it up in the sand box before showing up on the job with it.

IF I have a venting problem, I'd almost bet it's in the exhaust pipe that I snaked 4 times. Because when I disconnected the 2 inch PVC from inducer and ran inducer it blasts out real nice. And the 1/8 inch nipple out of the inducer, to the PS, has been rodded out. And the red rubber 1/8 inch hose between PS and inducer nipple is real soft and supple and no cracks and barbs are good and no crack in nipple.

I was thinking about having the boss bring his most powerful shop vac to pull a suction on that exhaust pipe. But I changed my mind after snake made it to bottom with not so much as a hair, feather or ice on the end of the snake, nor anything that got pushed into the condensate trap.

Tomorrow the new part comes. Tomorrow I first will redisconnect the exhaust pipe real briefly, and then rap on the PS again to see if it goes out. (I neglected to do this test while the flame was on the other day) If it does, I think that will confirm to me it is the PS, since the discharge air from the PS is quite great.

I'll let you know the outcome. If need be I will break open the old PS after all is said and done.
 
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Old 01-20-09, 11:10 PM
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Originally Posted by ecman51` View Post
She is going to call me back and tell me availabilty and prices of various testing equipment, including both positive AND negative pressure testing. If I buy one, I will be sure to scruff it up in the sand box before showing up on the job with it.

Heh, heh! You are very creative, Ecman!

But really, try that test on some of your fellow repairman and see what it produces.

The gas utility I worked for supplied us with Taylor (I think it was) sloped manometer that used an oil as the liquid. It was sensitive down to a half inch water column or so. The down side was that it was finicky to set up in someone's basement or crawl space and the magnets to hold it on the furnace came unstuck easily, making measurements a pain.

I'll bet there are nifty digital manometers these days that would make life a lot simpler.

Once you get your new instrument, I really suggest you use it often on equipment that's working properly. That will give you the experience and confidence to use it effectively on equipment that may have a problem.


Another thing --- if you can't inspect a long run of PVC adequately, I'd get permission to cut it to inspect it and then use a rubber coupling to seal up the cut. It doesn't take much to cause a pressure switch to open in a vent line --- a single leaf will do it.

And that's actually something else to inspect for --- bushes near the PVC intake that might allow a leaf to be sucked in or allow vented combustion gasses to mix in with fresh combustion air, causing combustion problems.

Prune away or preferably eliminate bushes around those PVC pipes!
 
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Old 01-20-09, 11:23 PM
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Actually, I used a Dwyer Manometer just like this one:

Amazon.com: Dwyer 1227 U Inclined Manometer: Home Improvement
 
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Old 01-21-09, 07:16 AM
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I looked at the pic. Thanks. And I am going over there this morning with my home made one: A clear shot glass and a clear drinking straw and I will attach the inducer 1/8 tube onto the straw I will stick in shot glass of water and see how high up the water gets pulled up the straw. I can't wait. Hopefully tenant is up and about or off to class or work.

I am indeed concerned now, with that order I placed, since supposedly the stupid thing has been working now! Oddly, my last test was rapping on the switch. I did no further cleaning out of the vent or inducer, nor fiddling with hoses after that, nro blowing out condensate after that. All that stuff was done earlier and did no good. If I DID, I would think I improved something. But the way it stands now, I do not know what to think. My theory about bad contacts inside the PS I am not so sure about since we are dealing with potential arcs from only 24 volts; not 120 volt, that IS realy known to arc contacts.

First off I am going outside to feel the discharge out that vent pipe. I can remeber in my head about what that felt like. Then I will do my straw test.

BTW, 2 furnace pros told me the gauge of choice used for testing pressure switches is called something that has like 10 letters in it or there abouts and starts with "heli-------", ??, ....something or another, when we were discussing manometers.
 
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Old 01-21-09, 07:27 AM
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Originally Posted by SeattlePioneer View Post
Once you get your new instrument, I really suggest you use it often on equipment that's working properly. That will give you the experience and confidence to use it effectively on equipment that may have a problem.
Great word of advice. We think alike.


Another thing --- if you can't inspect a long run of PVC adequately, I'd get permission to cut it to inspect it and then use a rubber coupling to seal up the cut. It doesn't take much to cause a pressure switch to open in a vent line --- a single leaf will do it.
Another great word of wisdom for all readers here to remember. I am very good at chopping apart pipes. Something I have lots of experince in. Lots! But I may just get the powerful shop vac because I may even stick it right in the open inducer hole, also. I saw no signs of bats. That basement and inside the furnace has had bats. But none was in the combustion compartment. No bat droppings in the combustion compartment either. (That is their 'calling card'). I have only fished them out of the blower compartment. And I've never seen bat droppings on the ground at where the vent pipe exhausts the side of the house, about 1 1/2 feet off the ground. Generally bats enter things that are higher off the ground, perhaps out of caution for predatory animals. Only on the other side of the house where they were getting in the house behind the chimney. Vent pipe in outside inside corner of house where no signs that leaves swirl around out there either. And when I snaked, not one leaf crumb, nor anything else, dropped down into the condensate trap at the end of the vent line. The 2 inch vent has about one 6' vertical rise and a wide sweep L and then goes another few feet horizontal and L's to the outside. Nothing long nor with too many L's in it.

And that's actually something else to inspect for --- bushes near the PVC intake that might allow a leaf to be sucked in or allow vented combustion gasses to mix in with fresh combustion air, causing combustion problems.

Prune away or preferably eliminate bushes around those PVC pipes!
None around.
 

Last edited by ecman51; 01-21-09 at 07:39 AM. Reason: added more
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Old 01-21-09, 08:52 AM
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Originally Posted by ecman51` View Post
I am indeed concerned now, with that order I placed, since supposedly the stupid thing has been working now!


I always preferred the jobs that had a nice problem that would just sit there until you could diagnose it. My preference would be to leave all the tough intermittent problems for someone like you, Ecman!
 
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Old 01-21-09, 04:38 PM
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It's a done deal. I installed the new PS.

It is the type I like. The kind with the plunger that is visible. (The old one was not.) When I start up the inducer, the plunger draws in as far as it can go. I made sure by compressing it manually until it bottomed out. And it is in the same spot when induced, as when I manually depress it.

Then I observed that after it runs for say 10 minutes straight, the plunger stays buried, as one would like to see. But then, it started to move in and out some. Hmmm. So then I took the vent off the inducer to observe the water flying around inside it. Yet the trap and drainline is not obstructed (I had the trap apart and hose off this morning), and water is coming out the drainhose. Hmmm.

Then I ran the inducer with the vent pipe disconnected and the PS plunger STILL moved in and out 1/64th of an inch (a guesstimate) the same way as it was when the ventpipe was connected. I do not believe the vent pipe is the issue. Not only that, I read the specs for the ventpipe installation (it is printed on the trap that has a styrofoam float cylinder inside it that wil shut off the inducer if the float rises) and the length of pipe and number elbows are WELL under the permissible max amount).

I think the water in the inducer is the potential problem. And there si no seperate drainline for it. It is designed to exit out the inducer, only. Or, perhaps a problem exists where perhaps gook is built up where the inducer pipe hooks up to the secondary exchanger. That will be my next spare time project (maybe) in taking off the inducer and checking that out.

Then I made an interesting discovery: The Fernco-type connector between inducer exit and vent pipe had a rubber ridge of a good 1/16th inch. That maybe could have caused spraying condensate water, that gets whirled around by the inducer, to strike that lip and fly back into the inducer, or simply hold back more water in the inducer, like a dam. I cut off that rubber. Then observed that the plunger did not seem to be waivering in and out as much. At least then.

So now, about the old inducer. WAS there a problem with it:? Maybe, maybe not. I thought that since these things are so sensitive between being closed and open, that it might be possible that if that PS was the original in this old Heil furnace, that the plunger maybe wore a number of thousandths of an inch, which would make the switch, if it waivered any, to "open".

I will debate whether to open up the old inducer, or to hang onto it for an emergency.
 
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Old 01-21-09, 05:52 PM
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Heh, heh! Even when you fix it you don't know what was wrong and whether the problem will recur. Sounds like one of the reasons why I retired!


Better you than me, Ecaman!


Still, it would be interesting to be able to see what was happening to the pressure on the pressure switch while these various changes and tests were being made. That would give you more precise information than guessing about what a movement of the pressure switch might mean.
 
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Old 01-21-09, 06:14 PM
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Well, we KNOW that the pressure is pulsating after it runs a while. When it runs a while, more condensate occurs, and I saw it flying around in the inducer fan assembly.

And for all I might not know?: Who KNOWS how long maybe this house has had some problem and nobody ever knew it before? Now I have that type of PS where you can actually see the plunger as it moves. A couple days ago was the first day in the furnace's history that this problem occured, and would not relight on it's own.

But there were times when I caught the flame going out and yet the furnace relit on it's own without me needign to reset the furnace. This malady may have occured for years and gone unnoticed, say if there was this oddball problem the furnace has by design, in trying to get condensate water out of the inducer fan assembly. BTW, the fan blades spin balanced and still look shiny and new.

Don't tempt me though. I may crack that old one open. But, I'll sit on this for a while. As for today's work; after I cut that rubber that was sticking up in that sleeve, the pulsing of the PS plunger was not doing it as much. But I need to spend more time observing, to be sure.

I enjoy stuff like this even on my own time, without pay, for the sake of science, and befuddlement over a good mystery.
 
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Old 01-22-09, 12:04 PM
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Thanks for both your responses, turns out you were both on the same path.
I dont have much knowledge in this area but in hindsight i do have a meter and should have tried testing it - to be honest i wasnt sure where to check for the 24v.
The cost wasnt bad, $89 for the call. He didnt have another with him so he was going to order one, but jumpered it so it would work. Meanwhile i found one and ordered it myself, (i did call to let them know this) so i will install it myself and save some $$.
One question for you on connecting it back, since the tech took both blue wires off - i dont know which wire goes to which connector. the top has a C by it, the bottom On - I assume common and hot. which blue wire connects to the module control board?
 
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Old 01-22-09, 12:17 PM
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It is not going to matter. It's just a simple contact switch that causes the two wires to make connection inside. Whether it comes in the right and goes out the left, or comes in the left and goes out the right, does not matter. Probably that is why they even used 2 blue wires, rather than different color ones.
 
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