Condensation on water heater air intake

Climate zone 6
I recently put an AO Smith lp gas, direct vent, sealed combustion water heater into service that utilizes a 7” air intake that houses the internal exhaust vent. This is a non-powered direct vent heater.
Condensation is forming on about a 3”length of the outer air intake pipe right where the pipe enters the outside wall. The points where the pipe enters the wall and exits are effectively sealed.
There’s always some condensation, and during subzero temps it frosts right up and of course drips down the wall when the temps climb to the double digits.
The condensation is within the portion of the vent that has a zero clearance requirement, and I wonder if insulating that portion would be any help.
Thanks, Daniel
GBA Detail Library
A collection of one thousand construction details organized by climate and house part


Replies
I don't see a clear solution to this issue but I didn't know what type of water heater this was so I googled it and found this image of what I think you have going on so that maybe someone else can provide some additional recommendations. This may be an inherent flaw with this type of ventilation system?
So, what is possibly happening is that some of the warm exhaust is being sucked back into the outside air intake and forming condensation. Is there an obstruction that the exhaust air is blowing up against like another house or a bush or something where it is deflected back into the outside air intake?
No, there’s no obstructions anywhere near the exhaust. Humidity may be slightly higher in the utility room where the water heater is located than in the rest of the house where it’s about 37-40% right now, and I’m wondering if that’s a factor. Daniel
Google:
Condensation on the outside air intake of a direct vent sealed combustion appliance, like a furnace or fireplace, usually indicates a significant temperature difference between the outside air and the appliance's combustion chamber, causing moisture in the air to condense on the colder intake pipe when the system is running.
Key reasons for this condensation:
High humidity outdoors:
When the outside air is particularly humid, more moisture is present to condense on the cold intake pipe.
Low outdoor temperatures:
A large temperature difference between the outside air and the appliance's combustion chamber leads to more condensation.
Improper venting:
If the vent pipe is not properly insulated or is too long, it can cause excessive cooling of the intake air, leading to more condensation.
Poor air sealing in the home:
If the home is not properly sealed, it can create a negative pressure, drawing more cold air into the combustion chamber and causing condensation.
What to do about it:
Check for proper insulation:
Ensure the vent pipe is adequately insulated to minimize heat loss and reduce condensation.
Inspect the vent pipe for damage:
Look for cracks or leaks in the vent pipe which could allow moisture to enter and condense.
Address home air sealing issues:
Seal any drafts or gaps around windows, doors, and other openings to improve home pressurization and reduce cold air infiltration.
Consider a condensate drain pan:
If condensation is excessive, installing a condensate drain pan under the intake pipe can help collect and direct water away from the appliance.
When to call a professional:
If you notice large amounts of condensation dripping from the intake pipe.
If you suspect issues with the vent pipe installation or design.
If the condensation is accompanied by other problems like unusual combustion noises or poor heating performance.
The issue is if you insulate and fix the condensation the vent, it will now be colder inside and might start condensing in the tank somewhere near the burners.
Just thinking out loud. My thinking the problem is a convective loop even when not running. Cold outside air comes in through the fresh air supply, drops into the tank, gets heated by the tank and exhausted out. A quick check is to see if you feel a warm draft out of the exhaust when not running. I'm guessing that is why the UHF is not great on these units.
I can't really see a solution to this. Maybe add a small fan that blows house air at the duct to help warm it up so there is less chance of condensation. Warming the duct might also help in reducing the convective loop I talked about before.
The condensation, I’m certain, is forming on the outside of the air intake right at the wall because it’s the coldest point, and the moisture in the warm indoor air is simply condensing there. While my concern is that insulating that short portion may just cause condensation to a few inches further down the outside of the vent towards the unit, I’m not sure I understand how that would cause condensation to form inside the tank. Also, the venting of this unit requires a .25”/foot up away from the heater, so wherever any condensation may form inside the intake, it’ll make its way downward.
I’m thinking that the condensation is a normal aspect of a situation like this, particularly in a cold climate and with a metal vent.
Wait, so the condensation is forming on the outside of the outside air intake duct, on the portion located in your utility room? Just insulate the outside of the duct. Ideally the insulation will have a vapor barrier of some sort included so you'll want to look for a type that is closed cell and can be painted or sealed to prevent moisture intrusion. I think armaflex and other brands are the type you're looking for.