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Old 11-20-2009, 12:27 PM   #1
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Rheem 80% furnace


I have a 1997 Rheem Criterion II 80% 75k BTU furnace that is vented with 3" vent pipe. I have just learned that this is not correct. The 3" outlet on the furnace should be adapted to 4" according to the specs. The furnace seems to run well. The vent pipe is a straight shot up through the roof, no elbows or adapters. I had the roof replaced last fall. At that time the 3" pipe terminated into a 4" at the roof. The roofers swithced the 4" that was there with the 3" that was on the gas water heater.

Is this okay or will it cause me problems later or damage the furnace? It has been running like this for years with no problems. Since the roof part was changed to 3", it will drip water every now and then when the furnace is running, not very much, but it never did this until the roofers switched 4" to the 3".

Thanks for you help.


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Old 11-21-2009, 07:57 PM   #2
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Well, the roofers had no business messing with the furnace venting! I think the 3" pipe should be fine - how long is it? 80% furnaces are not condensing, so the water shouldn't be coming from the furnace. I suspect the water is just a leak but does it occur when it's raining or just when the furnace is running? They aren't running the furnace AND water heater out the same 3" pipe are they?


Last edited by sktn77a; 11-21-2009 at 08:00 PM.
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Old 11-23-2009, 08:59 AM   #3
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I agree they had no business , I think it was just another of the mistakes that occured because they were in a hurry. The furnace vent is right at 7 feet straight up. It only drips when the furnace is running and it is only a drop every so often. The water heater is on the other end of the house, they must have torn all the flashings up and got them mixed up when they put them back on. I think the 3" pipe is okay as long as it terminates in a 4" combination cap at the roof, I think that cap restricts it too much and thats why it drips. I mentioned this to the roofer when he was back to fix a big leak around a turbine vent and his solution was to bend the cap a little over to one side. If anything this made it worse. I didn't figure out the 4"-3" thing until much later.
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