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Ruud 90 Plus Furnace - condensate leak where PVC and inducer motor plenum meet

Ruud 90 Plus Furnace - condensate leak where PVC and inducer motor plenum meet


Old 02-20-16, 08:39 AM
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Ruud 90 Plus Furnace - condensate leak where PVC and inducer motor plenum meet

Hi All,

The equipment is a Ruud 90 Plus (condensing) 6 burner up flow natural gas furnace (around 1997) ...over the years I have replaced the HSI and have cleaned the furnace each season. It is located in a sometimes damp basement area ...I recently started running a dehumidifier near it that drains into a shared sump pump.

I have been trying to work off a couple of problems that I ignored until recently ... the first was a clogged flame propagation path and blocked burner/nozzle holes (only the first burner worked, but it lit normally) ...sometimes they would all light and sometimes they wouldn't - pulled the burner assembly and cleaned the light rust and opened the propagation holes on the side wings – the rust likely due to where the unit is located in a damp basement. Worked fine after that. See photo of the burner assembly on the bench - the holes showing where almost completely blocked by rusty deposits … the flame channel along the front edge of the “wings” was also rusty. Used a wire brush and a small ice pick to carefully open the holes.

I then replaced the condensate lines from the heat exchanger and the bottom of the inducer blower housing. They run from the sources to the two input trap on the inside of the lower case and thence through the case to the condensate pump sitting just outside the case on the floor; the pump runs to a barbed fitting into a bathroom drain. Photo of the left inside area showing the bottom drain T on the inducer fan plenum.

I had also been having some unburned gas smell in the exhaust (the intake and exhaust run together about 20 ft horizontally and then up through a wall and exist about four feet up outside the garage). So, since I was getting more educated on furnaces, I got a digital manometer and checked the input and output pressures ... input was 6.3"WC (measured with furnace operating) and output was 3.8"WC ... I tuned the output back to 3.5" (about 2 degrees screw turn to counterclockwise) ... can't smell the unburned gas any longer, but this doesn't seem like much of change to account for that ... however, 0.3/3.5 does mean it was 8.5% "rich" (except, I don't much frame of reference to even say this as I don't know what the curve looks like or even if it is sensitive to this to use the word "rich" - it may just mean the furnace was putting out too much heat for its rating and the 8.5% over-gas condition was being burned just as efficiently as the baseline 3.5" of pressure - it could also be that when I cleaned the burners, I started getting better combustion so the unburned gas smell went away). Thoughts?

To the problem that I need help on (see photo) ... the area around the bottom of the fitting where the PVC exhaust plumbs into the inducer plenum stays wet - and, the moisture apparently is wicking into the insulation attached inside under the top of the case. There is a fabric gasket (it stays wet) under where the 2” PVC meets the inducer "air pipe" through the case in one ... the underside case insulation is wicking the condensate away from the joint and it drips onto the gas pipe, and, if run long enough will fill up the shallow lower case pan and run down onto the floor.

I have clear drain lines so I can see the lines passing water to the trap and watch them drain ...I pulled the front and was able to see there is no water draining from the A coil area (photo) (I assume this would be making water and draining in the summer with A/C use) - I also noted no dust or gunk build up on the A coil. Other than the A/C coil drain, there is nothing that high up that could drain and cause the water … so, it has to come from the interface between the PVC exhaust pipe and the inducer blower

I have located the part - I think ... PN 68-24021-01 ...I don't see the gasket that is under mine, so I am assuming it was factory installed as there is a plug for the air pipe shown in the installation manual with the air pipe already connected to the inducer plenum.

So, unless the 2" pvc has become disconnected, or the plastic connector cracked or wasn't glued up in the first place (it does seem connected and supported by strapping), I don't see any other way for condensate to get to the connection ...unless it is wicking from the insulation inside the top of the casing ...but there does not appear to be a source for that ... so back to the joint between the white PVC and the black "air pipe" that runs from the inducer plenum "T" (old installation docs show that it is PVC also, but some call it plastic".

I have ordered the part and will install it with the hope this is the issue ... the part number brings up a slightly more beefed up part than the original looks like and I did find a single thread online that sort of discusses this - does anyone know of this sort of leak?


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Old 03-06-16, 12:18 PM
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I tried getting advice on hvac-talk, but they were prickly about any DIY ...their standard answer was "do nothing yourself or you will put your eye out."

So, I kept at my issue ...

Recall the last of my furnace issues was water dripping from the insulation inside the top of the case ....as some others were helpful in diagnosing, the inducer air pile was crack right at the case level ...condensate was blown under th exhaust pressure out through the the crack and into the area just below the steel case and above the insulation. The insulation absorbed the condensate, and after soaking during long run periods would drip onto the bottom of the case, then leak down through the edges into the lower case, then run out not the floor.

The redesigned part has a thicker wall where it fits into the case ....I am assuming this was the reason for the modification in 1998.

Repair was simple. My first pvc section was solvent glued to the air pipe ...should have been RTV'd. I cut the 2" pvc just below the 3x2 reducer, then about 18" up the 3". The air pipe then disconnects by holding the inducer T and twisting to overcome the small amount of RTV holding it in.

After removing the air pipe up, I had to enlarge the case opening to account for the larger diameter modified part ...used a drill with a small grinder wheel. Bit at a time till it fit snugly. Cleaned the inducer T, ran a bead of RTV on the inside, then more on the air pipe, then a bead around the base of the air pipe where it mates to the case, then assembled with a slight twist. Replaced the sheet metal screws with ones a bit longer since the casting on the new part was thicker, as well.

I had already fit and glued up the pvc sections except the final one, so did the final glue up, and adjusted the pvc to make sure there was some weight on the pvc to air pipe joint.

Few photos to show.

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Old 03-06-16, 12:22 PM
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Photos out of order, but you get the point.

By the way, checked furnace this morning ...absolutely dry inside upper burner area and lower fan motor and drain line area...I had mixed up some baking soda and water and spread it around to neutralize the acidic condensate ...was like powder after having been wet when I put it all back together.

RTV connections at top of inducer T and at the entry to case where pvc connects to the air pipe were dry and solidly connected. PVC connections where I connected new assembly all dry and airtight.
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