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Old 12-23-2009, 08:10 AM   #1
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Perplexing problem, Ruud furnace


Hi fellas-
Great forum you have here.
Anyone figures this one out, and you have my utmost respect:
I have a Ruud 90 plus furnace running on propane, installed in the mid 90's. Seems like I have a problem with insufficient draft.
Draft inducer kicks on when thermostat calls for heat, but it does not generate enough vacuum to close one of the pressure switches. Obviously, the furnace sequence stops here. The hot surface ignitor never heats up. Draft inducer shuts off after about 2 minutes. It repeats this cycle a few times and then gives up. The draft inducer seemed to get a bit too hot when it was running, so I replaced it with a new one. Problem persists. I check exhaust vent EXTREMELY thoroughly, to the point of actually cutting apart the PVC pipe to visually inspect every inch for possible obstruction. Vent was clear. It is now properly reassembled.
This furnace has 2 pressure switches. One appears to be plumbed into the heat exchanger. This switch closes appropriately. The other pressure switch is plumbed into the housing on the draft inducer fan. This is the switch that does not close. It DOES close when a slight vacuum is applied to it manually. I confirmed this with ohm meter. The port on draft inducer housing that the pressure switch plugs into is clear. There are no holes in the tubing that runs to pressure switch. The port on the draft inducer fan that pressure switch hose connects to is not clogged.
So...if you're still with me here...
I put a manometer on the port on draft inducer housing, and it is only generating about 3/4 " WC of vacuum. The pressure switch that is not closing is rated to close at 1.11" of WC. So clearly, the switch is not at fault. This system is simply not generating enough draft. But why? I'm stuck. Vent is clear. Draft inducer is brand new. It IS moving some air, as I can feel it briskly moving out of the exhaust vent outside. By the way, I do have a woodstove in the house. Someone suggested that perhaps it was creating a huge negative pressure in the house that the draft inducer could not overcome. So I let the stove burn all the way down. Opened a door in the same room as the furnace, and problem persists.
Thank you in advance for any help you can offer.
-Jay

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Old 12-23-2009, 08:13 AM   #2
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Perplexing problem, Ruud furnace


Forgot to mention- I have confirmed that I am getting 120 volts to the draft inducer.

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Old 12-23-2009, 07:44 PM   #3
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Perplexing problem, Ruud furnace


Did you check the INTAKE pipe, not the exhaust pipe? Propane can burn dirty and you may have a partly plugged secondary heat exchanger or a cracked heat exchanger.
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Old 12-24-2009, 11:38 AM   #4
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Perplexing problem, Ruud furnace


Thanks for the reply Yuri.
I am quite sure that it is the exhaust pipe that I checked. That is a good question to raise though.
I don't think the heat exchanger is cracked, as we have not had any CO problems. Also, the flame pattern on the burner doesn't change when the blower fan turns on. I did wonder about the possibility of clogged heat exchanger. I didn't know how common that is.
Please correct me if my logic is faulty, but the way I look at it, to get sufficient draft, you need 3 things:
1. no exhaust obstruction
2. no intake obstruction
3. good draft inducer fan

This may be an oversimplification, but if this is correct, I have eliminated 1 and 3, leaving intake obstruction (clogged heat exchanger) as the logical problem.
Is this common on propane fired furnaces?
Can it be unclogged, or am I looking at getting a new furnace?
Thanks for you insight. Merry Christmas.
-Jay
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Old 12-24-2009, 12:01 PM   #5
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Perplexing problem, Ruud furnace


You need to pull all tube to draft motor and heat exchangers. That a paper clip and clean out the holes. You have crap in one of those ports and need cleaning. If you had crack or plug heat exchanger it will trip roll out
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Old 12-24-2009, 03:02 PM   #6
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Perplexing problem, Ruud furnace


Make sure the drain trap has water in it. It will not pull a good vacuum with water in it. Also confirm the gasket behind the Induser is in place. Will the switch pull in with the exaust pvc disconected? I am assuming this is a older istall and the vent is within venting distance guide.
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Old 12-24-2009, 03:36 PM   #7
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Perplexing problem, Ruud furnace


Gasket behind inducer is in place. Vent pipe is the same length as it has been when it was installed professionally about 13 years ago, so this should not be an issue. Thepressure switch does pull in when the exhaust is disconnected. However, I don't think that this necessarily proves there is an exhaust obstruction. (Actually, I know for a fact there is no exhaust obstruction after close inspection). My understanding is that there are 2 loads on the inducer: the intake restriction and the exhaust restriction. By changing the exhaust restriction to zero, the draft inducer is able to create enough vacuum to close the switch, even with the (presumably) partially clogged heat exchanger in the circuit.

Tator- I have already checked all ports, and none are clogged. My original post was quite long, so many people probably missed this- But the draft inducer is simply not creating enough vacuum to close the switch. I checked by placing a manometer directly on the port on the draft inducer housing. It is only creating 3/4" WC of vacuum. Switch is rated to close at 1.11" WC of vacuum.

I am all ears to more suggestions, but I suspect Yuri is right about clogged heat exchanger. Now I just need to find out if this is a fixable problem. Let me know what you think. Thanks everyone.
-Jay
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Old 12-24-2009, 04:00 PM   #8
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Perplexing problem, Ruud furnace


Simmiler to the plugged heat exchanger the secondary exchanger could be flooded maby due to a plugged drain.
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Old 12-24-2009, 04:52 PM   #9
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Perplexing problem, Ruud furnace


A clogged intake pipe is a different issue than a flooded or clogged secondary heat exchanger. There may be a screen where the intake pipe attaches to the furnace or burner box. Rheem/Ruud usually won't light properly with the burner box cover off.
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Old 12-24-2009, 05:54 PM   #10
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Perplexing problem, Ruud furnace


Gentlemen,
I neglected to mention that I did check for a clogged drain/water backup and there is no problem in this department.
As for the intake, it has a screen over the opening of it outside. So I know that there is no varmint in there, although conceivably there could be a beehive or something in it. Although I did not cut the intake pipe apart to check every inch of it, I know there is no obstruction in the first or last several feet of it. This furnace has run fine in the past when I had the access cover off it. I do not suspect that the intake pipe is the culprit here.
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Old 12-24-2009, 06:00 PM   #11
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Perplexing problem, Ruud furnace


Is it possable the induction blower wheel is loose, or coverd with dust?
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Old 12-24-2009, 06:07 PM   #12
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Perplexing problem, Ruud furnace


I had wasps build a nest on the intake of a Lennox G26. Summer ended, wasps went South. Nest dehydrated and shrunk. Furnace fired up in early October and nest got sucked in. Never say never. If you cut the intake pipe off a foot or 2 B4 the furnace and it runs OK, the pipe may be blocked. Use a plumbers MJ/Fernco rubber coupling to reattach it.
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Old 12-25-2009, 05:44 AM   #13
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Perplexing problem, Ruud furnace


Well, that was easy enough to try, so I gave it a shot. No change in the problem after I cut the intake pipe.
3 legged dog- The blower wheel is not covered in dust, as it is brand new. I know that it is secured to its shaft, as it does produce an exhaust draft. Just not strong enough.
I appreciate your advice everyone. Have a great Christmas today!
-Jay
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Old 12-25-2009, 06:43 AM   #14
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Perplexing problem, Ruud furnace


Is this one of the drum units?
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Old 12-26-2009, 12:28 PM   #15
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Perplexing problem, Ruud furnace


I'm not quite sure what you are referring to. If you are referring to the inducer itself, here it is:
http://www.amazon.com/gp/product/B00..._ya_oh_product

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