Rust on 1 of 2 exhaust vents

**Terry Beck** · 10-30-2009, 09:12 PM

Why is one (not both) so rusty (see pic)?

Two exhaust vents on top of 1997 2-story condo. One is for gas water heater on main level, other is for Cat I furnace in crawlspace. I did not confirm which vent is for which device (I know, should have). The rust appears just like this on one of the two vents on EVERY one of the 9 units in this condo building, and also true for other similar buildings I could see nearby.

Would the rust be due to more water vapor coming from the exhaust of one vs the other?
Ok, assuming that, I will note that the Cat I furnace is in an unvented crawl (not conditioned, no dedicated source of combustion/dilution air). I did note a small amount of elevated humidity in crawl (not unusual for area), but no mold or mildew conditions. Crawlspace is probably around 800 sq ft.

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**Jim Luttrall** · 10-30-2009, 09:20 PM

Terry, you can't run a gas appliance in an unvented crawl unless there is a dedicated combustion air source. That may be your problem right there.

**Terry Beck** · 10-30-2009, 09:44 PM

Maybe I didn't word my question quite right.
I am aware of the requirements for combustion air for gas appliances, as well as ventilation (of one type or another) for crawlspaces. Yeah, I call out stuff like that as well, almost on a daily or weekly basis. Builders here love to ignore rules, and there is no one to stop them. Plus, I run into a fair number of moisture issues even when the crawlspace is built with vents.

My question was searching more for other's experience as to what you would think when you saw this much rust on an exhaust vent. (What kind of clues should alert me to possible problems, and what would be the possible causes)

For example - - if I saw the vents on the roof before I was in the crawlspace, is this a good clue that should alert me to the possibility of inadequate combustion air (or a damp supply of combustion air)?

I was also worrying about possible downdrafting, but if that was a factor, why rust on only one of two vents.

**Jim Luttrall** · 10-30-2009, 09:57 PM

If you have a lack of combustion / dilution air then you will get concentrated exhaust nasties that will tend to eat up the vent stacks. Given the picture of the roof top vents, I would be looking to find something wrong with one of those appliances unless they were of vastly different ages. Bob H and DavidR can get more into the specifics that I but anytime I see soot or excessive corrosion on a b-vent it set off the red flags in my thought process. I don't have much experience with gas appliances in crawls, but the humid air does not seem to be much of a concern to the vent since the by product of proper combustion is water vapor. I just don't see humid air doing that, but I may be wrong. From what I understand though, CO is very corrosive. Starve the unit for combustion and dilution air in an unvented crawl space and I could believe that result. Let's wait and see what the gas heat experts have to add (or correct.)

**Terry Beck** · 10-30-2009, 10:16 PM

Jim
Good point about the effects of CO versus water vapor.

**Rick Cantrell** · 10-31-2009, 06:19 AM

The flue gas temperature on an HE furnace is lower than on non a non HE furnace. If the flue has a long run the gasses can condense in the flue, especially above the roof. Like Jim said flue gasses can be quite corrosive.

**DavidR** · 10-31-2009, 06:34 AM

It's the chemical reaction in the combustion process that produces all the moisture.
It doesn't have anything to do with where the unit is located or the supply of combustion air it's getting though that's what has been spread around as truth for years.

If you take a basic methane combustion reaction with the perfect amount of excess air that it needs for combustion you end up with 1-CO2 and 2-H2O
Problem is most show this as being where the chemical reaction stops but it continues.

The CO2 and H2O will continue to react and form H2CO3 which is the formula for carbonic acid.

This is where all the rust you see comes from in regards to flue gas.

Hope that helps.

**John Arnold** · 10-31-2009, 06:55 AM

It appears the vents are the same size, but I assume the water heater BTU is considerably less than the cat 1 furnace. Perhaps the water heater vent is oversized, resulting in a cooler vent and more condensation?

**Jim Luttrall** · 10-31-2009, 07:03 AM

So David, given the general set of circumstances (I know there is not much to go on) what would you be looking at for further clues and what would your suspicions be regarding defects in this installation? Would lack of combustion/dilution air be suspect for a corroded vent or possible improper sizing/installation of the vent? 12 years old with repeating issues on all of the units tells me something is not to spec. I'm just trying to learn here since this is outside the norm on installation and climate from my area.

**Bob Harper** · 10-31-2009, 07:57 AM

Good discussion. The first answer is, a properly trained tech needs to perform combustion analysis to figure out what's going on including taking draft interference measurements.

If you have just one WH venting into an oversized two story B-vent, you are trying to heat that vent with about 10,000 BTUs/hr. Due to the climate involved, this vent will cool near the top rather easily. Once the WH is tuned and firing to its optimum, you could:
-remove the existing B-vent and replace it with properly sized venting
-construct a chase from the attic-up to keep the vent warm until discharge
-replace the WH with a power vented model

The CAT I furnace is way down in the basement. We don't have the rating numbers but most likely, it will have higher stack losses in total BTUs than the WH, which will warm the vent and reduce the chance of condensation except for the fact you hit a point of diminishing return since this stack is taller and thus has more cooling surface. So, once the MUA issue is resolved and the unit tuned using combustion analysis, you have the same options as the WH. One thing you can do is measure actual flue gas discharge temps. Another important clue is Rh%. If the air entrained by the vent is already soggy, then the stack Rh will reflect that and when you hit dewpoint, could have a flood. Any signs of condensation below at the units or vent connectors? Are they using B-vent connectors in that unconditioned space? Are there offsets in either vent?

FYI, you make carbonic acid in your body which is buffered by bicarbonate in the liver until it disassociates back into CO2 in the lungs to be exhaled. You breahe to blow off acid--not take in O2. There are other acids found in flue gases.

The HOA should hire someone like DavidR to inspect and test the place then make recommendations. With that kind of consistent venting malfunctions, you can bet there will be others.

Bob

**DavidR** · 10-31-2009, 09:02 AM

Originally Posted by Jim Luttrall

So David, given the general set of circumstances (I know there is not much to go on) what would you be looking at for further clues and what would your suspicions be regarding defects in this installation? Would lack of combustion/dilution air be suspect for a corroded vent or possible improper sizing/installation of the vent? 12 years old with repeating issues on all of the units tells me something is not to spec. I'm just trying to learn here since this is outside the norm on installation and climate from my area.

Bob covered it better than I could have Jim.

There are so many variables that can contribute to this it's not funny.

The concern that I would have is that the appliance attached to the clean flue might not be venting while the other appliance's venting might be marginal on the rusting flue.

My start on something like that would be full blown combustion testing and draft interference testing.
Other test would be performed depending on the data I found from these test.