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Distance between a/c and water heater exhaust.


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IRC G2427.8 Item 3: The vent terminal of a direct vent appliance with an input of 10,000 BTU per hour or less shall be located at least 6 inches from any air opening into a building. An appliance with an input between 10,000 and 50,000 BTU per hour shall be installed with a 9" vent termination clearance, and an appliance with an input over 50,000 BTU per hour shall have at least a 12" vent termination clearance.

IRC G2427.8 Item 4: Through the wall vents for Category II and IV appliances and non-categorized condensing appliances shall not terminate over an area where condensate or vapor could create a nuisance or hazard, or could be detrimental to the operation of other equipment.

Or you could go with the manufactures installation instructions for the AC unit and those usually mention a 3' clearance.

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There are proximity rules for gas vents to electric meters (18" min.) but I have never seen distance restrictions for AC compressor units and gas vents.

Charles, your reference is for direct vent appliances. This is a non-direct vent which has more stringent specs.

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Non-direct? OK. What's that other PVC pipe for?

Marc

I see two pipes terminations. I assume one pipe is for the water heater and the other is for the furnace. If both pipes are for one direct vent appliance they have been installed too far apart.

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I'm struggling to find a reason for it to cause a problem. I wouldn't write it up without one.

Think about worst case scenario. The unit fails to ignite and dumps flammable fuel out the exhaust as it purges. Not something to dump onto an electrical appliance.

Also aren't gas exhaust fumes slightly corrosive?

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The one on the left is from a powered vent on a water heater. The one on the right is from a furnace.

I was thinking there might be an issue with the corrosive nature of the exhaust causing damage to the ac fins; however, it is 10 years old and no signs of an issue.

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I'm struggling to find a reason for it to cause a problem. I wouldn't write it up without one.

Think about worst case scenario. The unit fails to ignite and dumps flammable fuel out the exhaust as it purges. Not something to dump onto an electrical appliance.

Also aren't gas exhaust fumes slightly corrosive?

Wouldn't the unit shut the gas off if it didn't ignite? I don't know for sure so that is a serious question.

Even so I think it's a stretch to not imagine the gas being diluted by the open air to the point the chance of ignition is slim to none. Enough gas at a concentrated level would have to reach an ignition source. Big big stretch in my opinion.

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I searched and could not find any rule against it, so I did not write it up. I also could not come up with scenario in which it would be an issue.

I was watching holmes on homes a few weeks back and he moved and ac becuase the furnace exhausted directly behind it....

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How about this?

...More than one gallon of water, laced with sulfuric acid and hydrofluoric acid, is produced for every 100,000 BTUs of gas burned...

Marc

He gets the venting categories wrong in the first paragraph. How accurate can the rest of the article be?

Sure, these vapors are slightly acidic. How much vapor is going to condense on a coil that's at the ambient temperature most of the time and that's warmer than the ambient temperature the rest of the time? (When the AC is running.)

If it's a problem, why isn't there any sign of a problem after 10 years?

I've seen this setup a few times and never seen a problem associated with it. I see far more problems with dog urine than with furnace exhaust. It's a non-issue.

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He gets the venting categories wrong in the first paragraph. How accurate can the rest of the article be?

Yeah, that's true but I didn't see that as enough reason to ditch the entire article. I can look for another if you'd like.

Sure, these vapors are slightly acidic. How much vapor is going to condense on a coil that's at the ambient temperature most of the time and that's warmer than the ambient temperature the rest of the time? (When the AC is running.)

During the winter season, a lot, since ambient temperatures are far lower than the exiting flue gases. At that distance, the condenser fan doesn't need to be running for those gases to reach the fins.

If it's a problem, why isn't there any sign of a problem after 10 years?

Good question. I can't explain it but sometimes a manifestation isn't a requisite to report something.

I've seen the white residue on the above-roof-deck portion of water heater flue vents and the same residue on shingles nearby so I know there's consequences to flue gases that condense too soon. If the next house has the same condition, I might very well write it up even if the white residue isn't there.

I've seen this setup a few times and never seen a problem associated with it. I see far more problems with dog urine than with furnace exhaust. It's a non-issue.

I've only seen one Cat II and no cat IV in all my 11 years here in coastal Louisiana but that only means I don't have sufficient experience to disregard a condition that may very well cause problems, as evinced by what my eyes told me when I saw that white residue.

Regardless, we've all got a right to our own opinions.

Marc

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Ok, good points. I can imagine a scenario where it could be a problem. However, in the specific situation depicted in the original post, I still wouldn't say anything about it.

If another inspector were to call it a problem, I wouldn't mock him very much.

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I don't know about propane, but natural gas is only combustible between about 5 and 15% fuel/air. Given that natural gas is CH4 (molecular weight about 16) and air is 28.something, the gas will be moving smartly towards the heavens. Propane, OTOH is more dense than air, but I don't know how concentrated it could get in this situation and what the numbers are.

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I don't know about propane, but natural gas is only combustible between about 5 and 15% fuel/air. Given that natural gas is CH4 (molecular weight about 16) and air is 28.something, the gas will be moving smartly towards the heavens. Propane, OTOH is more dense than air, but I don't know how concentrated it could get in this situation and what the numbers are.

28 is probably the molar weight of a representative sample of air. Molecular weight refers to a molecule not a combination of gases like air.

Propane has a molar mass of approximately 44.

Marc

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