Thread: Furnace flue issues?

Ruud 80% furnace 125K BTU. 1992 vintage. I have not seen this personally until today. Lots of white flaky material at all the joints of the flue pipe. It also was the most prevelent where it leaked back into the furnace. I assume this is a venting problem?? Also about 2 feet out of the furnace the flue pipe 90s and I bumped my head on it, and it disintegrated. The bottom of that section of flue pipe had about 1" of the white flaky material. The furnace exhaust was 4" and transitioned to 6". The furnace was the only appliance utilizing that 6" flue. I was thinking maybe the flue pipe too big?? Also, furnace specs called for 40-70 Temp rise, this was geting 41 measured at the flex connectors at the furnace. Here, we never see more that 55-60 temp rise due to the derating because of the altitude (8500 ft.) But genrally always see at least 55. So any opinions on why the white material, flue pipe too big, furnace not operating as intended??? There was no installation instructions, and yes I deferred to a qualified HVAC guy but I was just curious.

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Paul, I think you are on the right track. I would guess the furnace was never tuned to the altitude, type of gas or pressure resulting in a lower than rated output. Couple that with the flue that was too large and you have massive amounts of water vapor condensing in the flue which eats it from the inside out.
But I would certainly defer to someone with more knowledge about furnaces and high altitude systems.

Paul,

I'll bet that these stains have been present in some of your previous inspections but you didn't pick up on it. In my experience it's most often due to an oversized vent that does not heat up enough to prevent condensation from forming from the water vapor in the combustion gas. It's very common for me to see these stains on the attic section of the vents. Pay attention to the attic floor insulation directly under the vent elbows - you'll often see where the condensation has been dripping on to.

I explain what's going on to the client because my money is on the contractor not recognizing what the problem is. To keep on the safe side, I tell clients that this is what I EXPECT the cause to be.

Thanks guys, thats what I was leaning to. This was in a vented crawl space, so that would probably magnify the problem when the flue is not hot enough and its 10 below outside. I have seen this in minor amounts, but never at this level.

I got this from past thread. Forgot from whom but I thought it explains the issue well.
The flue gases are condensing inside the vent pipe and running down. The water finds its way out at the rotation seems of the elbows and at the pipe joints. The white deposits are byproducts of natural gas combustion that are condensing out along with some of the moisture (flue gas discharge is very wet). The combination of the moisture and sulfur creates a mild acid that can eat the pipe up over time so the condition needs to be corrected.
The white stains are a sign that something is wrong with the furnace or with the flue pipe design. Could be that the flue piping is over sized or is so long that it does not heat up and stay hot enough to prevent condensation from forming within the flue pipe or it could be that the furnace is being under-fired and is not producing enough heat and volume of discharge to allow the gases to evacuate from the flue before the gases can begin to condense.

At 8,500 ft elevation with a 125MBTU input, that flue is oversized. At that elevation, there is very little natural draft. As a consequence, you need a really hot flue to compensate for this. As you efficiency goes up, your stack temp drops and your draft pressure weakens.

If that furnace is to remain and it is anticipated it will eventually be replaced with a Cat. I furnace, then it needs a new vent system. You did not state whether it was vented into a chimney or B-vent. If in a chimney, it needs a properly sized listed stainless steel liner. If B-vent, then the vent connectors, too should be B-vent. If they anticipate replacement with a mechanically vented unit (recommended) then do it now and abandon this atmospheric vent. Put those repairs into the new Cat. IV unit.

You have about a dozen various acids eating up that vent connector. Much of the white powdery material is zinc oxide. Any time you see these deposits, it warrants further investigation for the venting but also the appliance performance.

Once you get above about 2,500 ft. in elevation, most mfrs. of atmospherically vented appliances require you to de-rate the burner input by installing smaller orifices then adjusting the manifold pressure. The std. formula is to derate 2% per 1000 ft once above 2,500. The mfr. will provide this info. and the proper orifices & specs for that particular unit. However, it is most critical that a qualified technician thoroughly inspect that unit including the heat exchanger, then test fire it, adjust it and perform a combustion analysis and evaluate the overall performance. This includes measuring duct pressures/ flow, checking for duct leakage, delta T, and repeat the combustion analysis until it is maximized yet still within mfrs. specs.

Paul, any pics of the venting?

I see those stains on the flue pipes in at least half of every house I inspect that is 20 years old or less.

Stains = repair item.

Thanks Bob, sorry no more good pics of flue. As the flue goes through the floor it changes to a Bvent.