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Dirt in furnace by exhaust ven


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I have seen this build up of dirt several times but never this bad.

What causes this? This is in a 8 year old furnace that had not been serviced at all.

I always just call it out as needing to be evaluated by a HVAC guy.

Thanks David

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Looks like the remains of condensation from flue gases that reacted with the zinc coating of the pipe, dripped back down to the furnace, leaked out then evaporated leaving the zinc compound - the white stuff. That happens when the temperature of the flue gases drops too much before exiting out the flue pipe.

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Exactly right, Marc.

A furnace guy is liable to come out and say it's perfectly normal. Look at the flue pipe. Is it double-walled or single walled where it leaves the appliance? Does it have sharp 90-degree bend in it anywhere? Is the furnace installed in a garage or other 'cold' areas? Above the roof is the end of the flue so rusted it's the color of a dark brown shoe? If it is, there's a good chance if you go up there you'll find vertical cracks in the outer shell of the B-vent or even big tissue thin areas you can poke your finger through.

I often know when there is a flue issue the second I get out of my car, if the flue is visible from the driveway and I can see that it's badly rusted. When I see that, I know that something is causing the exhaust gases to cool too rapidly; so they lose buoyancy and are barely rising as they leave the end of the vent. The warm air carrying those acidic gases hits the cold air and if it is already too cool it immediately settles around the end of the vent and over time eat away at the outer shell. When that corrosion gets bad enough, the continued expansion/contraction of the vent causes those vertical cracks to appear. Virtually every time I see a vent like that above the roofline I enter the garage to find that the furnace has been replaced but not the vent or I find an older furnace - sometimes not too old - where there is a vent issue.

The vent should have a double-walled connector all the way from the collar to terminus to keep the exhaust gases as hot as possible and maintain exhaust gas buoyancy. The code used to say that a connector in a 'cold' area had to be double-walled; it doesn't say that anymore. Somewhere around the 2006 code, just as it seemed HVAC guys were finally getting the message and had learned to install double-walled connectors on any appliance installed in cold areas, the code stopped calling for a double-walled vent in 'cold' areas and everything went backwards again. Combine the single-walled connectors with the 2, 3, or 4 90-degree bends they configure in those vents in contravention of the code, in order to route them out of the way of furnace filter holders, and you have a situation where exhaust gases are losing so much buoyancy that they are struggling to rise up the vent and in the process lot of that white mineral salt is produced by the reaction between the acid in the exhaust and the zinc used in the galvanizing.

That stuff is corrosive. Unless the furnace is due for a service or there is something else wrong with it, the call should be to have the exhaust vent throughly inspected, reconfigured with fewer bends with less angles, and any sections badly corroded replaced as needed and then clean the furnace.

If the vent configuration is fine - even with a single-walled connector - and there's just a little bit of that accumulated after 8 years the vent is probably going to be fine. Just show the owner how to open up the furnace and properly close it and recommend he/she vacuum/blow all of that mineral salt out of the burner tray area annually. Make sure they know to be really fussy about getting it off any electronic components, soldered terminal ends, control valve etc.

ONE TEAM - ONE FIGHT!!!

Mike

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One other venting issue I see that causes condensation is on retrofit of higher efficiency furnaces and using the old vent pipe that is woefully oversized. When the 3 or 4 inch vent gets transitioned to a larger size, I see dripping of condensation at the joints. There is just not enough heat left in the exhaust stream to keep the larger size vents warm enough to prevent condensation from forming inside the pipe. I have never seen it this bad at the furnace but most of this in my area occurs on horizontal units in the attic so there is an elbow or other joint before it gets all the way back to the furnace.

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Condensation in a vent or chimney is generally beyond the understanding of most HVAC contractors around here. Quite often I'm inclined to recommend a certified chimney sweep make an initial evaluation. But what I find with that approach is that someone calls the HVAC guy anyway and in those cases the condition is rarely (never) corrected.

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Just so ya'll know, most all Chicagoland contractors don't have their heads up their asses. There's a smug-ass inspector out this way, though, that belittles most every single tradesperson around on a fairly regular basis, mistakenly thinking he knows far more than they do. Heck, he even told Doug Hansen that Doug didn't know what he was talking about re: bonding/grounding.

If you ever come across one of his posts, ignore it.

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Just so ya'll know, most all Chicagoland contractors don't have their heads up their asses. There's a smug-ass inspector out this way, though, that belittles most every single tradesperson around on a fairly regular basis, mistakenly thinking he knows far more than they do. Heck, he even told Doug Hansen that Doug didn't know what he was talking about re: bonding/grounding.

If you ever come across one of his posts, ignore it.

Jeez,

Waddueyegotta do to find out who the horse's ass with teeth is? Call Douglas?

ONE TEAM - ONE FIGHT!!!

Mike

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The vent should have a double-walled connector all the way from the collar to terminus to keep the exhaust gases as hot as possible and maintain exhaust gas buoyancy. The code used to say that a connector in a 'cold' area had to be double-walled; it doesn't say that anymore. Somewhere around the 2006 code, just as it seemed HVAC guys were finally getting the message and had learned to install double-walled connectors on any appliance installed in cold areas, the code stopped calling for a double-walled vent in 'cold' areas and everything went backwards again.

I'm not sure what you mean here. IRC still doesn't allow single wall connector in an attic or crawl space. 2012 -- G2427.10.2.2 (503.10.2.2) Vent connectors located in unconditioned areas.

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It's definitely caused by condensation build up in the vent. However, it's not necessarily a venting issue. It's possible there has been a history of the furnace short cycling due to design issues, lack of proper maintenance, etc.

I'll add another vote for short cycling.

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