Blower fan turning off furnace flame

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Old 01-02-10, 02:08 PM
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Blower fan turning off furnace flame

I woke up this morning to a chilly house. I went down to look at the furnace, and only the induction motor was running.

After further investigation, the furnace starts up as usual (induction, ignitor, gas, blower). As soon as the blower starts to run, the flames almost immediately go out.

After removing the access panel, I found the furnace operates completely normal, with no issues (yet). As of now, I just taped the safety switch down so the furnace runs without the access panel, and it seems fine.

It is a very windy day outside. My first thought was wind effecting the pressure switch. Is this possible? The induction motor and control board were both replaced within the last 3 months.

Thanks for your help.
 
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Old 01-02-10, 03:15 PM
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Flame Goes Out

If it is working properly with the blower panel removed, the furnace is probably going off on high limit. Try removing the air filter & re-installing the panel.
 
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Old 01-02-10, 07:12 PM
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That didn't work - what I've done now is replaced the access panels, but removed one of those black circles that plug open holes up on the combustion part of the furnace.

Whenever the furnace is running and I close the plug, the flames immediately go away. Its now becoming very windy outside and I am thinking that the induction motor isn't able to keep up with the wind. By allowing the motor to use indoor air, its able to maintain draft. The induction motor still runs, so all exhaust air should be venting to outside.

One other thought is to install PVC T's on the intake pipe outside to try and protect from a gust stopping the negative pressure in the combustion chamber.

This is a Rheem 90 Plus
 
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Old 01-02-10, 07:29 PM
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Your seat of the pants engineering solutions are wrong and potentially dangerous.

The fan compartment pressure switch is a safety switch to prevent the furnace from operating when the door is off and sucking air needed for gas combustion away from the furnace, and perhaps causing large amounts of carbon monoxide to be produced, which would also be circulated around the house by the furnace fan.


More than likely something is plugging up the air circulation system of the furnace, causing the furnace to shut off on the high temperature limit switch.

Commonly that is a plugged filter, a return air grill covered over, too many warm air vents closed off, or an air conditioning evaporator coil plugged with dust and crud.

Do not operate the furnace with the door off or otherwise defeat the purpose of the door switch other than briefly for test purposes.
 
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Old 01-02-10, 07:50 PM
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The doors are back on now, just the small black round plug removed. I tried putting that back in, and removing the filter and visually checking for clogs, debris in vent lines. The furnace starts, but then stops as soon as blower comes on like before. I don't see how the blower fan and the pressure of the combistion chamber relate.

I did find this thread touching on the exact same issue I have:

Rheem Classic 90 Plus issue - HVAC-Talk: Heating, Air & Refrigeration Discussion


Measuring my vent piping, I've got over 22' of pipe from intake of the furnace to the terminus outside. Its all 2". This seems long to me, but probably OK.
 
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Old 01-02-10, 09:13 PM
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Check the pressure switch to see if that is what is opening and shutting off the burners.

Also check the limit switch to see if that is turning off the furnace.

Do you know how to do that?
 
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Old 01-03-10, 06:46 AM
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How old is the furnace? Has anyone considered the heat exchanger being bad??
 
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Old 01-03-10, 09:22 AM
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Not sure how to check the pressure switch to see if that's whats happening, since the opening of the access panels makes the furnace work fine.

I did blow/suck into one of the pressure switches and heard the click, the one that attaches to the induction motor. There is another that attaches to the back wall.

I seriously doubt its the limit switch, since the furnace is working fine with even the slightest opening of that top panel. If I close off all external room air to that top panel, and then watch the furnace start up, as soon as the blower fan comes on, you can see the panels suck in and turn the flames off.

So the blower fan is somehow affecting the pressure in the combustion chamber, and just allowing the panels not to be air tight lets the pressure somehow equalize better when the blower is on.

I suppose the symptoms do point to a possible cracked heat exchanger, seeing as the blower may be taking combustion air away. But it doesn't add up, since the furnace works with either the bottom panel off or the top panel. Having the bottom panel off wouldn't fix the problem if it was a cracked heat exchanger right?

The furnace is 1994, so about 15 (now almost 16) years old.

I'm going to get a technician out soon to have a look

Thanks
 
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Old 01-03-10, 12:34 PM
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Panels Suck In

The panels sucking in is a dead give away of not enough return air for one reason or another. Is your duct work metal or fiberglass board?

Do you have one or two PVC pipes from the furnace to the outdoors? If only one & the furnace can be set up for two, I strongly encourage doing so. This not only reduces the negative pressure in the house it virtually negates the effect of wind since equal pressure is being applied to both the intake & exhaust.
 

Last edited by Grady; 01-03-10 at 12:43 PM. Reason: More info
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Old 01-03-10, 12:45 PM
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Duct work is all metal. I haven't touched any return air vents.
 
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Old 01-03-10, 12:56 PM
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There are 2 PVC pipes. I can feel air flow on the intake pipe. But as I said earlier, it no longer works with just using the intake pipe as the only source of air, I have to open it up and use room air as well.
 
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Old 01-03-10, 01:07 PM
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Metal Duct

If you have some sheet metal sheers you can cut an 8x14 hole in the cold air return drop. This will provide for extra return air & can be later fitted with a register you can open or close as you so desire.
 
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Old 01-03-10, 01:21 PM
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HVAC-Talk

Just read some of your posts on HVAC-Talk. Seems you have been fighting this issue for a couple of years now. It also seems you have not yet gone to the 3" pipe as suggested there. I work on few Rheems but I've had other brands which said in the install manual 2" was ok but once I went to 3", problems went away. When you talk to tech service they MIGHT admit the 2" is marginal.
 
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Old 01-04-10, 06:35 AM
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Those were not my post on the other forum, just something I found similar.

However, this morning I went downstairs again and plugged up the furnace as its supposed to be, and lo and behold, it worked. The gas valve seemed to almost struggle a bit at first, but it did work.

It is much less windy out today, and I still theorize that the induction is just marginal with normal weather conditions, and it was just having a problem sucking air with high winds the past few days. Probably need 3" pipe, or maybe my pressure switch is flaky??

I left the port off for today however, as I didn't want the furnace to stop working while I'm gone at work. I will put it back on tonight and watch it over a few hours to make sure its back to functioning properly.
 
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Old 01-04-10, 02:59 PM
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Pressure Switches

I attended a service seminar a couple of years ago & the instructor (factory rep.) made it a point of talking about pressure switches. He said 99+% of the ones which are replaced under warranty are not defective. In the 20+ years I've been doing service work, I can count on one hand the number of bad pressure switches I've found.
 
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Old 01-05-10, 04:59 PM
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Turns out it was the pressure switch!

Had a technician look at it today. We found the intake to be good. It looks like the pressure switch, with that little plunger thing, wasn't quite making good contact with the switch, or was getting stuck. All he did was bend the electrical switch just slightly off the pressure diaphram section to allow more room for the pressure switch plunger to open and close fully.

I am not sure how that happened, but at least its fixed now.

I will see how that works for now, and I am sure I'll need a replacement in the future.

Thanks for everyone's help!
 
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Old 01-05-10, 05:35 PM
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Frankly the pressure switch is about the last thing someone should be doctoring up.

As suggested by Grady, when a pressure switch is failing to close the reason 90% of the time or more is that there is a defect in the furnace or venting system and the pressure switch is staying open for good reason.

Now you have some repairman doctoring it up. That probably means he changed the pressure at which it will close ---not what the manufacturer intended.

Did the repairman use a manometer to measure the pressure being applied to the pressure switch? If so, what was the pressure he obtained?
 
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Old 01-06-10, 04:01 AM
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No, he did not measure the pressure. However, one thing he did notice was that the flames were not at all starved for air after the pressure switch was moved. Also he did an experiment with the induction motor on, he sealed the outdoor intake pipe, and noticed that the furnace panels sucked in real hard, indicating we had draft.

I think its a combination of a flaky pressure switch and marginal intake pipes. He was slightly concerned why the installers chose to run the pipes the way they did, 2" with 8 or 9 90's. It clearly passed inspection when it was installed, so I don't know.

He also tried snaking the pipe, but was struggling with all the 90's. We ended up not cutting the pipe up since he believed we had solid intake draft.
 
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Old 01-06-10, 06:43 PM
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2" pipe

I would bet a weeks pay your furnace is not installed in accordance with the manufacturer's instructions.
A quick calculation says you have the equivalent of 102 feet of straight pipe. Eight ellbows, with each being equal in resitance to 10 feet of straight pipe (80 equivalent feet) + 22 feet of straight pipe = 102 equivalent feet. MAYBE if you went to 3" it would comply but even then I'd imagine you'd be close. The maximum number of ells I've ever seen allowed is 6.
Also make sure there is a steady uphill slope away from the furnace of at least 1/4" rise per foot of run with no droop in the pipe.
I still don't believe there is anything wrong with the pressure switch.
 
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Old 01-08-10, 11:47 AM
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The rise of the pipe is fine. You are saying that a marginal installation, combined with a few years, has probably just finally made the furnace stop. I agree with you that the installation probably isn't ideal.

I still somehow doubt that it would just happen overnight - I would think the draft would get worse over time, and the furnace may not work everytime, but it would fire sometimes and still heat the house. Yet, the furnace has been working perfectly until that one night.

I am not going to redo the installation when the adjustment to the pressure switch did the job. Certainly seems a lot easier then redoing the piping.

I guess the concern is why are inspectors approving these types of installations?, and why are furnace repairmen 'doctoring' pressure switches?

Again, thank you for all of your help! I certainly learned a lot during this issue, and I'll keep everyone posted on the status of the pressure switch in the future.
 
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Old 01-08-10, 11:57 AM
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Originally Posted by adamol View Post
<<I am not going to redo the installation when the adjustment to the pressure switch did the job.>>

<<Certainly seems a lot easier then redoing the piping.>>


The "adjustment" to the pressure switch very likely destroyed the pressure switch as a safety device. That's why the furnace is working despite the poor condition of the vent.


Suppose you had a carbon monoxide detector that kept going off and was annoying, so you took the battery out. It's fixed! No more annoying problem!


Same idea.



Seattle Pioneer
 
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Old 01-08-10, 12:04 PM
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The pressure switch must be working still, or the furnace wouldn't even work at all.

The furnace checks for an open switch before turning on induction, then checks for the closed switch before turning on gas. This is to prevent someone from just jumpering the switch.

So the fact that the furnace isn't complaining, means the pressure switch is doing its job.


And my CO detector hasn't reported anything during this whole ordeal - and yes it has batteries.
 
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Old 01-08-10, 12:19 PM
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Originally Posted by adamol View Post
The pressure switch must be working still, or the furnace wouldn't even work at all.

The furnace checks for an open switch before turning on induction, then checks for the closed switch before turning on gas. This is to prevent someone from just jumpering the switch.

So the fact that the furnace isn't complaining, means the pressure switch is doing its job.


And my CO detector hasn't reported anything during this whole ordeal - and yes it has batteries.

You prove yourself to be a fool with these comments.
 
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Old 01-08-10, 01:05 PM
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Thanks for the great explanation on why I was mistaken. You are really helpful.



I came to this forum looking for ideas to help diagnose what was wrong with the furnace. I am not about to cut new holes in my house and reroute pipe, especially after I had a furnace repairman look at it, and did what he thought was best for the situation. If the furnace fails again, then maybe we'll have to go down that route, but its definately not the first choice. Especially when both the repairman and myself have a hard time believing it to be an intake problem.

Anyways - I apologize for not knowing as much as you do about furnace operation, I guess that makes me a fool. I guess you will always know my furnace better than me.
 
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Old 01-08-10, 03:40 PM
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Furnace malfunction from wind

All I can tell you is that since I got my Carrier in 2006, it has gone out every time a front blows in with strong winds - code 13. I had techs out to check it every year and they were all stumped. This year I wasn't going to take "I don't know" for an answer. This guy's theory (which I tend to think might be valid based on what I've read on theories of wind-affected furnaces) was that the configuration of the vent (it went horizontal before going up through the roof) was allowing back draft etc, etc.

So today I let them install a new vent with new hole in the roof, etc. It's not quite as windy today as yesterday, so it hasn't been fully put to the test, but so far so good.

Putting the new hole in the roof wasn't as traumatic an experience as I thought it would be, so you might consider it. It's better than freezing every time there's a strong wind. ;-)
 
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