Thread: B-Vent Length, Height

I see a lot of B-vents that show evidence of natural gas exhaust condensation happening inside the vent tube before it exits the top. This runs down the inside of the tube till it finds a joint, then it runs down the outside, corroding the galvanizing and then the tube itself, and dripping into the attic insulation.

This usually occurs when there is too much of a run in an unconditioned attic area, or in the case today where the height after it came through the roof was higher than what we normally see. I know there is a minimum height above the roof, but is there a standard, rule of thumb, code, etc. that spells out a maximum unconditioned length, or height above the roof?

Photos show extra high B-vent above the roof, condensation stains coming out of the elbow below the roof in the attic, and the stain in the attic insulation from condensation dripping into it. This particular B-vent came up the front of the house into the attic, then angled back across the attic to get to the rear area of the roof, then up again through the roof.

Thanks a lot for your thoughts and information on this.

Mike Chambers
St. Louis

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Originally Posted by Michael Chambers

I see a lot of B-vents that show evidence of natural gas exhaust condensation happening inside the vent tube before it exits the top. This runs down the inside of the tube till it finds a joint, then it runs down the outside, corroding the galvanizing and then the tube itself, and dripping into the attic insulation.

This usually occurs when there is too much of a run in an unconditioned attic area, or in the case today where the height after it came through the roof was higher than what we normally see. I know there is a minimum height above the roof, but is there a standard, rule of thumb, code, etc. that spells out a maximum unconditioned length, or height above the roof?

Photos show extra high B-vent above the roof, condensation stains coming out of the elbow below the roof in the attic, and the stain in the attic insulation from condensation dripping into it. This particular B-vent came up the front of the house into the attic, then angled back across the attic to get to the rear area of the roof, then up again through the roof.

Thanks a lot for your thoughts and information on this.

Mike Chambers
St. Louis

DSC01106.JPGDSC01172.JPGDSC01174.JPG

Michael,

Yeah. I see that a lot too. The max height would probably be a manufacturer's installation requirement. However, if adjacent to a second story wall, it has to keep going, even in unconditioned space, so there might not be a max. The reality is all you can do is get a HVAC contractor in there to replace the gas vent pipe and service the furnace.

Not the question you asked and this may not apply here but I see corrosion from the interior when old low efficiency furnaces get replaced by high efficiency units but they leave the old larger pipe in place and reuse it. There is not enough heat in the newer furnace exhaust gas to keep moisture in vapor form, less heat in a larger and colder pipe leads to condensation and corrosion.

Here's the rule of thumb for attic off-sets.

You can off-set 1 1/2 times the diameter of the B vent in inches to feet.
In other words, a 6 inch b vent can off-set 9 feet.

Originally Posted by Darren Miller

Here's the rule of thumb for attic off-sets.

You can off-set 1 1/2 times the diameter of the B vent in inches to feet.
In other words, a 6 inch b vent can off-set 9 feet.

The maximum horizontal offset is 100% of the total vertical height between the draft hood and the top of the vent, as I recall without looking it up).

I also recall something about a maximum offset of 75% of that height - but don't recall right now the difference between the two.

Would need to look it up in the code.

I see a lot of condensation stains, too. It's so common that I usually only note it unless it is excessive and/or draining into the induction fan. Then I'll call out for correction. I recently saw condensate leaking from the PVC flue just above the furnace. Even though my CO sniffer didn't detect a leak, it seemed reasonable that if water was leaking out of the joint, then CO might leak out under different weather conditions.

Originally Posted by Jim Luttrall

Not the question you asked and this may not apply here but I see corrosion from the interior when old low efficiency furnaces get replaced by high efficiency units but they leave the old larger pipe in place and reuse it. There is not enough heat in the newer furnace exhaust gas to keep moisture in vapor form, less heat in a larger and colder pipe leads to condensation and corrosion.

No question that the higher efficiency furnaces are more susceptible to problems that can be caused by exhaust condensation, dirty filters, etc. Everything has to be set up right for the furnace being installed, or problems can occur.

Maximum horizontal run is 75% of total vent height. No limitation on how far you can extend a vent above a roof once you've met code except that 5 feet or more requires braces.

When you see condensation and rust issues it should start with a level II inspection. Assuming that inspection reveals the venting is properly sized and installed, Next would be to perform combustion analysis to see how its burning and measure the draft.

Let's assume the unit is an 80% NG furnace 100,000 BTU/hr. input, 5" B-vent routed through the center of the building with 3 ft. of vent exposed above the roofline and the combustion analysis reveals the following: stack temp. 275°F, supply plenum measures 155 though rated for 185°F, O2 10% and 20 ppm CO stable at light off, one minute, 5 min., 10 min, and shutdown with draft -0.03 wci and the manifold gas pressure is at the mfrs. spec. of 3.5" wc and its condensing. You've got a reasonably modern insulated house with double pane windows and weatherization. You have a digital setback thermostat. What could be wrong?

Believe it or not, increasing the B-vent to 6" will help improve flow and raise stack temps., which will minimize flue gas condensation.

Increasing the manifold gas pressure until you hit the supply plenum max. will increase efficiency, drop the O2 and raise the CO (must be kept under 100 ppm at all times and stable). Calculate the output to ensure you're not overfiring.

Install a bullhead tee with double acting barometric damper and an automatically resetting spill switch to break 'R'. The increased dilution air will reduce flue gas condensation by lowering the dewpoint and Rh%.

Most importantly, install a delay timer so the inducer becomes a post-purge fan to clear those flue gases and water vapor so there's nothing to condense. It typically takes about 4-5 min. to work.

Obviously, installing the post purge is the cheapest, quickest and easiest of these courses but it has a proven track record. You do all these recommendations and now the supply plenum is at its max. 185F, the stack temp. is 375F, the O2 is 6%, the CO is 85 ppm, manifold pressure 3.8 wci, the draft is still -0.03 wci, the condensation has disappeared and the unit burns 15% less fuel than previously.

Some people will freak out at such measures but they are legal and do work. Take the NCI CO and Combustion Analysis course to see why and how this works and much more.

Obviously, if you extend B-vent up like a missile launcher it will begin to cool the flue gases and cause condensation and possibly loss of flow.

Off-sets in attic are different.