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Old 12-27-2009, 07:49 PM   #31
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Perplexing problem, Ruud furnace


That is true. I had a aftermarket inducer (Rotom) with 50 less rpms than an OEM on a Ducane and it would not work. OEM is sometimes the best.
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Old 12-27-2009, 09:30 PM   #32
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Perplexing problem, Ruud furnace


I am using a water tube to check pressure. I suspect it is fairly accurate. When I checked manifold pressure, which is spec'd by this model furnace to be 10" WC, it measured dead on at 10".

So, I did buy the part THROUGH Amazon, but not really FROM Amazon. It was purchased from Electric Motor Warehouse. I compared both original fan and new fan. They are exactly the same. Both 1750 RPM. Both .75 amp. Both made by Fasco. I think the reason the new one isn't working is that I jumped the gun on presuming there was something wrong with the original fan to begin with. I have not yet spoken to tech support. I don't think this supplier actually has a tech support.
I do have reason to suspect the draft is insufficient. Let me provide more info:
6 weeks ago, problem started with flame not traveling down all seven burners. Obviously, flame sensor detected no heat, and then gas valve would close after a few seconds. Draft inducer ran a few more moments, then shut down, and cycle would repeat. This problem was intermittent. Given 2 or 3 attempts, flame would eventually propagate properly across the burner. I checked all orifices. All were clean. Pulled burner, found no major obstruction, cleaned it anyway, and problem persisted. At this point, once the burner lit, the furnace would run until thermostat was satisfied. A few weeks later, however, furnace would shut down before thermostat was satisfied. There was no flame rollout. Manifold pressure would instantly drop to zero as measured on manometer and burner would shut off. I suspect the limit switch was tripping. Poor draft, not pulling enough exhaust out, heat exchanger overheated, furnace shut down. Now, as you know problem has worsened to the point where furnace doesn't run because draft inducer doesn't pull enough vacuum to close pressure switch.
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Old 12-27-2009, 09:33 PM   #33
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Perplexing problem, Ruud furnace


Forgot to mention, I tore this furnace down pretty far a few weeks ago. Stopped short of actually removing the heat exchanger, but I did not see a way to pull the plastic front piece off the heat exchanger. I guess I should take a closer look. Anyone know for sure if there is a way to get in to clean secondary heat exchanger on this model?
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Old 12-28-2009, 02:13 AM   #34
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Perplexing problem, Ruud furnace


Quote:
Originally Posted by jbarker View Post
Forgot to mention, I tore this furnace down pretty far a few weeks ago. Stopped short of actually removing the heat exchanger, but I did not see a way to pull the plastic front piece off the heat exchanger. I guess I should take a closer look. Anyone know for sure if there is a way to get in to clean secondary heat exchanger on this model?


Is your draft inducer housing filling up with water? If you pull the plugs is it clean? If you had water from the combustion or draft inducer building up, it could cause this problem. Are there other flexible hose take-offs on the inducer housing? The one you have it hooked to could be prone to water pooling. Isn't that where they normally connect a tee to the condensate drain? I'd think you'd want the pressure take off higher up on the blower housing.

Also is that the same place the old inducer took off for the pressure switch? Looks like that is the high pressure side of the inducer, not the low pressure side. Low pressure is on the inside of the squirrel cage.

Last edited by zootjeff; 12-28-2009 at 03:29 AM.
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Old 12-28-2009, 07:37 AM   #35
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Perplexing problem, Ruud furnace


The manufacturer/design engineers are looking for a pressure differential across the exchanger. They decide where the ports are on the fan. Sounds like the exchanger is quickly plugging with soot.

Last edited by yuri; 12-28-2009 at 08:01 AM.
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Old 12-28-2009, 07:49 PM   #36
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Perplexing problem, Ruud furnace


Yuri-
I agree with you. After you mentioned the possibility of plugged secondary heat exchanger, it seemed like the only logical cause of the problem. Is this something that is typically repaired, replaced, or just get a new furnace? Thanks so much for all your help.
-Jay
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Old 12-28-2009, 08:27 PM   #37
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Perplexing problem, Ruud furnace


I don't do a lot of Propane but have heard that is not uncommon to foul up the exchanger. I did several thousand posts at defunct HVACMech and other people have had your problem. Propane is hard to burn clean and the soot/carbon mixes with the condensate/condensed water. Looks like you would have to replace the heat exchanger or find a Pro who can dismantle it and clean it. Probably cost $600-$1000 to do so. If it is mid 90's then I would replace it. We never clean heat exchangers as it would require some kind of chemical flushing etc and would cost a fortune. It may also crack soon or have other problems and be a $$ pit so I would go with a new furnace. And make sure it is properly checked and setup by an experienced Propane tech once a year. Lots of techs work on Natural gas but only a few are good at Propane or have the necessary experience. In my area the Propane supplier has techs which are good.
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Old 12-28-2009, 08:53 PM   #38
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Perplexing problem, Ruud furnace


Propane is a dirty fuel. But most of the problems come from lack of proper set when installed. And from lack of maintainence.
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Old 12-28-2009, 09:56 PM   #39
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Perplexing problem, Ruud furnace


In the picture I can not tell but it look as if there is a drain from the exaust vent positive pressure. ties directly to the drain off the heat exchanger a negative pressure. Could the positive be blowing into the negitive side causing the lack of vacuum?

Edit
Nevermind they look like They are seperate. Make sure the trap has water in it.

Last edited by JohnH1; 12-28-2009 at 10:01 PM.
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Old 12-29-2009, 08:32 AM   #40
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Perplexing problem, Ruud furnace


Thanks for all the info Yuri. Your insight has been invaluable.

Beenthere: When you refer to "proper set" - is there anything to be set properly besides inlet and manifold pressures? I mean, of course you need to have proper size orifices in the manifold, and spring in gas valve needs to be replaced with one for propane. But I wasn't sure if there were any other factors that I am overlooking.
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Old 12-29-2009, 08:37 AM   #41
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Perplexing problem, Ruud furnace


Temp rise. Too cool, and you get condensation in the primary heat exchanger.
Too hot, and you get fatigue in both heat exchangers.

Next, combustion should be checked. To know if its burning correctly. You can have factory speced manifold pressure. And still be burning dirty, or too clean. Hard to tell by just manifold pressure and temp rise. The length of the vent and intake have a big effect on that.
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Old 12-30-2009, 07:22 AM   #42
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Perplexing problem, Ruud furnace


i once replaced a inducer motor. and furnace wouldn't run. finally figured out that it didn't seal on furnace plate good enough. i would apply pressure with my hand on inducer and furnace would fire up. so i sealed the inducer with a gasket along the edges of the inducer motor. furnace finally fired up. furnace was an aireflo model.
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Old 01-07-2010, 09:19 PM   #43
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Perplexing problem, Ruud furnace


Has the issue ever been found on this?
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Old 01-20-2010, 12:03 PM   #44
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Perplexing problem, Ruud furnace


Never was able to find a satisfactory answer to why this thing wouldn't run right. Tore it all down, removed heat exchanger, separated the primary heat exchanger from the secondary. It had some debris in it, but not a ton. It wasn't what I would call clogged up. I decided to get a new furnace.
Bought a Rheem 95%, 105,000 BTU. Appears quite similar to old furnace. I installed it this past weekend. It is now up and running, but I will continue to use wood stove primarily until I am satisfied that it is set up properly. Thanks again to everyone for your help. I have found this experience very interesting and I have learned a lot. This has been a good reminder to me that "the more I know, the more I realize how much I don't know".
-Jay
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