Help with Coleman Pres III 7995C856


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Old 11-09-08, 09:27 PM
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Help with Coleman Pres III 7995C856

Hi, I need help with my Coleman Presidential III (natural gas) - the main fan does not come on for 8-10 min after the burner, and then it shuts off after about 1 minute while the burner stays on. I replaced the fan switch, so it's something else.
What are the possibilities?
All suggestions will help me.
Thanks in advance.
 
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Old 11-10-08, 06:23 AM
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Originally Posted by scaperdude
the main fan does not come on
Does the fan run when you switch the fan switch on the t-stat, or if you don't have it at the t-stat, then it's on front of the furnace.


I replaced the fan switch
Not sure what switch you are talking about... Fan switch on front of the furnace?
 
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Old 11-10-08, 10:10 PM
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Thank you so much for responding Jay.
The thermostat does not have a separate fan lever or switch on it - no off or anything but a lever to adjust the temperature setting.

The fan switch was the limit fan switch inside the electrical casing on the front of the furnace. I think it was 7957-***. The new switch did not have the number on it, but it's package had the same number as the old switch. So it's apparently some other problem.

I just need to know if anyone has any ideas what the problem areas are for these furnaces, and which ones might require a serviceman. I had one lady tell me she thought it was the burner, but it seems to be working fine -- almost all blue flame with orange tips. Throwing off plenty of heat.
 
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Old 11-11-08, 06:42 AM
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Isn't there any switch near the blower for fan ON?

Do you have A/C on this furnace?
 
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Old 11-11-08, 07:37 AM
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What kind of fan switch does it have?: Is it adjustable?

This sounds like some behavior as if the fan switch "on" setting were set to within a few degrees of the high limit setting. If that happened, the fan would come on late, but then could shut off on high limit soon after coming on, IF your furnace had other issues with heat retention. But I doubt this is happening. For one thing, it likely would limit well before even 8 minutes

Since you already changed the fan switch, your connections are likely good.

Have you used a voltmeter to test the fan switch (heat)tap for 120 when the blower should be coming on? Are you able to do this test for us. You would put one voltmeter probe to that tab and the other voltmeter probe to any metal ground. This test will show if it is in the electrical somewhere, or if the problem is motor or motor capacitor.
 
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Old 11-11-08, 09:24 AM
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Jay --
There is a manual switch on top of the box containing the relay and limit switches. It turns off everything except the pilot light (this furnace has an exhaust blower too).
There is no A/C.

Ecman --
"Have you used a voltmeter to test the fan switch (heat)tap for 120 when the blower should be coming on? Are you able to do this test for us. You would put one voltmeter probe to that tab and the other voltmeter probe to any metal ground. This test will show if it is in the electrical somewhere, or if the problem is motor or motor capacitor."

I will try the voltmeter. I also have the other limit switch - I could try changing it. What should the voltmeter read or do? Does the switch open or close at the limit? Are you saying the voltage should be 120? (I assume you are not talking about the temp).
The main fan motor in the unit is not the original one speed. It is a three speed of max 1/5 HP instead of the 1/4 HP one speed that's supposed to be in it. It also has no capacitor. Of course it comes on and seems to work, but also shuts off after about a minute of running. I bought a new fan motor and capacitor, because I knew this, but I haven't installed it yet because I figured I could return it if I couldn't clear up these problems, and decide to get a new furnace. I already switched the wiring to the motor from the lo speed wire to the high speed wire, but it still doesn't seem to blow as hard as other Coleman furnaces around here.
 
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Old 11-11-08, 09:53 AM
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Take a reading across the high limit switch.. If you are reading 120 v on that, then it's open.. If it read 0 v, then it's closed, and is fine.

Most of the time the high limit switch will shut off the gas valve, and that it is doing for you now since you don't have a blower.

Take a reading on the fan switch, and if you read 120v, then it's open.

Not sure if your fan ties right to this fan switch or not?
 
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Old 11-11-08, 06:21 PM
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Spider?

Becha we got us a spider nest.

Scaperdude; Make sure no other gas appliances are running for this test: (pilots are OK)

Turn up the thermostat, make sure the main burner lights, & go outside to your gas meter. There should be a dial marked either 1/2 or 1 cu. ft. Time it & report back how long it takes for the dial to make a complete revolution. We also need to know the input rating (BTU) of the furnace & if that dial is marked 1/2 or 1 cu. ft.
 
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Old 11-15-08, 09:01 PM
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Hi again, and thank you to everyone who has been helping me. I have some stuff to report. 1. The fan switch tested at 120V while the fan was off, so it is OK. 2. Tried replacing the other limit switch same results, except this switch doesn't have measurable voltage on the meter. 3. If I connect both of the lead wires to the fan switch to each other, the fan of course runs until I shut off the unit, but the burner will eventually shut down. 4. Also tried bypassing the limit switch on top of the burner cabinet(next to the fan), and then fan wouldn't run. 5. I had a furnace installer there helping with other stuff, and he said the burner flame seems low, but it seems the size the owner's manual shows. Not as big though as my flame in my old Coleman Presidential. 6. Checked the gas meter, and the 1/4 ft dial took about 45 sec to make a revolution. The furnace is rated at 95,500 btuh input, and 72,000 btuh output. Wouldn't a spider been burned out long ago?
 
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Old 11-16-08, 07:05 AM
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Gas input

With the ratings on your furnace, that 1/4 cu. ft. dial should make a revolution about every 10 seconds. With it taking 45 seconds you are only getting about 21,000 btu/hr input (based on 1050 btu/cu. ft.).

The spider actually crawls in thru the orifice & makes a nest behind the orifice blocking it. No flame is ever there, or at least better not be. I've cleaned out 3 or 4 already this year.

I just re-read your last post & you say the flame is not like your old furnace. Is this the first heating season for this furnace? If so, check the orifice size. It might have the LP orifice installed.
 
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Old 11-17-08, 10:29 AM
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Thanks Grady. I omitted that the furnace doesn't get very warm when it does blow, so this problem is probably related to the gas valve unless I have a pinch in the line somewhere. If it is a spider I sure will be relieved. I bought the place in October, and the furnace has basically been sitting idle for at least 2 yrs. It was a foreclosure. I did look at the spare nozzle hanging on the gas valve, and it was probably the lp nozzle but I don't know how to tell for sure. i will try checking it first against the parts list I have. Thanks again. I will report back.
 
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Old 11-17-08, 02:18 PM
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Another Possibility

Other than a spider or some other insect, there is the possibility the gas valve is a step opening valve & it is not opening to the second step. If you will give me the numbers off the valve, I'll see what I can find out but I'd still bet on the spider.
 
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Old 11-17-08, 05:29 PM
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As soon as I read the speed of the meter dial - I concur that something is way off. I have a 1/2 foot dial, and mine just flies around the circle in no time flat! And I only have about a 40-60,000 (can't remember, I better go look! )
 

Last edited by ecman51; 11-17-08 at 06:16 PM.
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Old 11-17-08, 05:50 PM
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Well, I think we found the problem.
Some idjit had the lp nozzle in the burner. It has been there a long time by the looks of being a little rusted in.
So we took the gas nozzle out of the bag, and put it in.
What a difference. I think the fan was turning off because the furnace just wasn't generating enough heat.

The furnace with the new fan motor heated up the house 5 degrees in 15 minutes. The fan did turn on once more after 5 or 10 min for just a few minutes I guess to blow out a little extra heat built up in the unit. As far as I can tell it is basically running correctly. The flame color appears quite blue when I open the pilot light hatch, but gets more orange as I watch it - i guess the extra air going in affects it.

I think I have to get the thing green stickered by Feb, so if I have any more problems "I'll be back!"

A big thank you to everyone who has helped me - I couldn't have done it by myself.
 
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Old 11-17-08, 05:55 PM
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Wrong Orifice

Now you have the right orifice in there, check the gas meter again. That 1/4 cubic foot dial should be making about 6 revolutions per minute.
 
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Old 11-17-08, 06:00 PM
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Oh, and there were some cobwebs in the lp nozzle too, which were blocking off even some of that small opening.

I am just so relieved that I don't have to buy a new furnace right now. Funds are getting low.
 
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Old 11-17-08, 06:09 PM
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You're Welcome

We are here & glad to help when & where we can. Glad you go it running & are now warm.
 
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Old 11-21-08, 11:22 AM
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Well, I'm back already.
Been relighting the pilot light the last several days.
This furnace was so messed up. My guess is when they ran out of money and were going in foreclosure, they tried to switch the furnace over to propane so they could run it off portable tanks. Who knows what they did to it.

But anyway the pilot light was staying lit with the lp nozzle, but now that I have put in the natural gas nozzle, it goes out after only one or two cycles. Given the above info (that they may have tried to alter this furnace to propane) what kind of things could be causing the pilot light to go out?

1. It might be possible that we unadjusted something when we changed the gas nozzle back to natural gas, but I doubt this one.
2. I've had a suggestion that it is probably the thermocouple, but before I buy any more parts, I wanted your opinions as to whether it might be a different part or just some adjustment that needs to be made.

Thanks again.
 
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Old 11-21-08, 07:46 PM
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Natural/LP

I am not sure if the pilot orifice is the same for LP & natural or not but the label on the furnace should tell you.

One more thing to check is the pressure regulator on the gas valve. It is about 3/4" long & should be marked either with a red band, a blue band (or both) or "LP" on one end & "Nat" on the other. Again, the label should tell you where it is, how it is marked, & how to convert from one gas to the other.

The thermocouple certainly could be the problem but so could a very small piece of debris in the pilot burner. The pilot flame should be large & strong enough to envelope the top 1/3 of the thermocouple. Pay particular attention to it's size with the main burner on as compared to when the main burner is off. It should remain nearly the same (just slightly smaller with the main burner on).
 
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Old 11-22-08, 09:15 AM
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"The pilot flame should be large & strong enough to envelope the top 1/3 of the thermocouple."
I looked at it closer this morning. I have noted that the pilot flame doesn't seem quite as big as before I changed the gas nozzle. It is not quite touching the lip above it - the thermocouple I guess.

Then when the exhaust fan comes on, the pilot light looks even smaller, as it is getting blown out into the burn chamber a little.
Last, when the burner and blower come on, the pilot light goes out. I thought that may have been the case when I was looking at it the other day, but this morning I confirmed it by turning off the unit while everything was going, and the pilot light was already out while the burner subsided(the pilot light will stay on with the unit off, if it is not blown out or whatever).

So what the heck now?
Will a bad thermouple cause this problem?

I am not sure what the regulator is. There is the black nob you turn & push to start the pilot light, and it has a red mark on one side where you point the arrow. To turn on the gas valve, you turn this nob so the arrow reading "on" faces the red mark. I guess it's possible the gas pressure is now not high enough to keep the pilot lit.

I will review my manual and see if it helps with installing for natural gas, but it has nothing on troubleshooting or repair. It only has installation instructions and parts lists.

Thanks again for all your help.
 
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Old 11-22-08, 02:16 PM
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Thermocouple

The thermocouple has no influence on pilot size but pilot size has has a major impact on the thermocouple. The thermocouple is mearly a sensor which generates a small electrical current to keep the pilot valve open.

The owner's & installation manual should tell you all you need to know.
 
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Old 11-22-08, 04:27 PM
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Does the blower come on after the burner flame? If so, closesly observe the burner flames as well, to make sure the blower does not move them, when it kicks in. If it does, you have a bad heat exchanger that is moving the burner and pilot flames. And then when the pilot flame goes out, the whole thing goes out.
 
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Old 11-23-08, 04:18 PM
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"closesly observe the burner flames as well, to make sure the blower does not move them, when it kicks in. If it does, you have a bad heat exchanger that is moving the burner and pilot flames."

I've seen the blower on without the burner, and the pilot was as still as stone, so I guess the exchanger has no holes.

Here's the latest:
The gas valve is a White-Rogers 36C67 which is Evcon part# 7956-336. It is a step valve. You can see and feel when the valve fully opens. Now when fully open the furnace is burning 1c.f. of gas in about 45 sec.

I guess I'm as big an idgit as the one who changed it over to propane, because I didn't read the conversion instructions on the unit. They say to unscrew a brass 'valve' on the regulator and switch it to the Nat Gas side. They also say to "open" the pilot light adjusting screw all the way. So I did these things.
Pilot light still went out. The adjustment screw seems to make no or negligible difference in pilot light size.

Took out the burner, and found a small ball of ash at the pilot light orifice which I removed. There is no exchangeable orifice here - I guess it is all supposed to be regulated by the pilot light screw.
-- still same result -- pilot light goes out.

The pilot light is touching the tip of the thermocouple which you can see is glowing red. It overlaps a bit.

Here's the skinny. When the gas valve turns on, I really don't see the pilot light going, but if I switch off the power button, as the burner subsides the pilot light is lit or relit. But once the valve fully opens then I think the pilot light definitely goes out. If I turn off the power switch at this stage, there is no pilot light. I can flip back on the switch before all the burner flames subside, and as the burner relights, so does the pilot light. So if I switch it back off at this point the pilot light will stay lit.

Grady, my manual has absolutely nothing on the thermocouple - not even a part number.

Does what I am describing sound like the thermocouple is bad?

more info on the unit:
Gas Valve - 7900 series gas valves are 100% shut-off
type and will fail safe if for some reason the gas is turned
off or the pilot goes out. They are also of the modulating
or "step-open" type valves which means they open to a
"low fire" position and after a few seconds "step-open"
to "high fire".

For natural gas operation, the furnace is designed for 7"
W.c. inlet gas pressure. Pressure to main burner is then reduced
to 3-1/2" W.c.
For L.P. gas operation, pressure measured at the gas valve
must be II" W.c_
Note: pressure is effectively increased, because we are at 1 mi altitude.
 

Last edited by scaperdude; 11-23-08 at 04:21 PM. Reason: add info
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Old 11-23-08, 05:31 PM
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High Altitude

If your natural gas is 1050 BTU/cu.ft., you have an input of about 83,800 BTU/hr., a major improvement over what you had. Nearly all gas furnaces have modifications for high altitude & since I live virtually at sea level, I honestly don't know what those modifications are. You might want to check with the gas company or a Coleman dealer. You might also want to get the gas company to check the gas pressure.

I'm all but positive the pilot orifice is removable. You will need a piece very small wire to pass thru the orifice. Often the contaminant is so small you can't see it yet when the orifice is cleaned, the pilot works well. DO NOT ream out the orifice opening.
 
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Old 11-23-08, 09:41 PM
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Sounds like you're thinking the thermocouple is not bad.

The pilot light looks to me like the simple end of the gas tube but with a fastener wrapped around it. There is a small "flame guide" tray-like structure which curves up from the fastener, but at about a 30 degree angle toward the burner opening. This seems fixed to the gas tube ie no adjustable angle. There really does not look like there is anything that can be changed - no removable orifice.

What you're suggesting sounds like I should detach the pilot tube from the gas valve, and then run a wire up through it & out the pilot light opening to push out any accumulated debris. Of course with everything off. I will report back as to the effects of this hopefully by tomorrow.
 
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Old 11-24-08, 08:30 AM
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Originally Posted by scaperdude
What you're suggesting sounds like I should detach the pilot tube from the gas valve, and then run a wire up through it & out the pilot light opening to push out any accumulated debris. Of course with everything off.
You'd never hit the teeny hole like this, since you are going about it from the wrong end. The orifice would be out at the end of the pilot tube, in the burner end, inside the part you remove at the end of the tube. It is located IN that end piece, usually as this little cup with a really tiny hole in it. So tiny you can barely see light through the hole, when it is clear! But you can indeed see a perfectly round tiny hole of light, if it is clear. It is so small that even if you blow through it, you feel so much resistance that you'd presume it is blocked, when it is not (as long as you see the round light hole after it has been poked clean.)
 
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Old 11-26-08, 01:02 AM
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You're right. It was a tiny hole. I blew the line out. Poked a needle in the pilot light orifice.
I also blew out the corrugation at the end of the burner.
I then discovered that the steel screw on top of the gas valve which I thought was the pilot light adjustment, just covered up the real (brass) pilot light adjustment screw down inside the valve, which I unscrewed.

Put it all back together, and the pilot light stopped going out.

However, after being on a while, my wife smelled gas. Tonight I went back to it. I put thread compound on the pilot tube connection and on the brass pilot light adjustment screw. I finally discovered that when I covered a "vent" on the left side of the valve with my finger, the gas smell went away. This vent is directly below the regulator, and on the left side of the valve.

I wonder if I blew some little part out of this vent, because it wasn't doing this before. I made a temporary fix to the "leak."

It seems like it's one thing after another with this furnace. New problems keep rearing their heads.

So what now?
Thanks in advance.
 
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Old 11-26-08, 07:02 AM
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I hope your fix was not in sealing that which is supposed to vent.

I can't imagine having something actaully getting blown out of there. Maybe a piece of debris stuck under a seating surface there.

If this does not go away on it's own and you know that is where the gas is coming from, maybe you could shut off the gas line, while the furnace is running, so that the gas pressure goes down. Then allow the furnace to shut off or turn it off, and then turn back on the inline gas valve to see if maybe it slaps it open or closed better? I'd be trying something like that anyway.

I recently had a furnace that had first of season fire up issues and I turned off the inline gas valve, and after I turned it back on, an inline regulator/vent made a very noticeable activation sound.

IF that does not work, and say you attempt this a couple times, then you may have to try to disconnect the line and try to blow in there and hope.
 
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Old 11-26-08, 03:34 PM
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Blew it out?

I hope you didn't use compressed air to blow out anything while attached to the gas valve. If you did, you may have damaged the valve.
 
 

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