Condensation on sealed combustion boiler intake
I’m building a new home and the plumber just finished installing the propane boiler. This is a sealed combustion unit and, to accommodate the air intake duct he came up into the first floor stud cavity. After he finished the installation I stopped by to check on things and saw that this intake duct was covered in condensation (I am in climate zone 6).
I brought this to the plumber’s attention and asked if he was planning to insulate this duct (there isn’t a lot of room to put adequate pipe insulation around the intake). He said that the reason there was condensation was due to the high indoor humidity from the temporary job site heaters we have been running. He isn’t planning to insulate the intake and said it would be fine once the indoor humidity comes down to normal.
The wall will be insulated with dense pack cellulose. Although the humidity is high currently I’m still concerned that this duct will cause condensation in the wall going forward. So, am I concerned over nothing or does this need to be corrected?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Anyone have any opinions on this? If it’s going to be altered I need to do so soon. Thanks!
I would insulate that pipe. My condensing furnace combustion supply air (PVC) runs through a basement hoist cavity and drips condensation in very cold weather. Have them throw some insulation sleeve on there, urethane or closed cell.
Dennis,
Interesting to hear of this, never noticed this on my direct vent PVC intake in Minneapolis. We used what I think is called a concentric vent in that just one hole is used for both the intake and the exhaust. The intake being right near the siding and the exhaust is a short length of PVC venting away from the wall. I suppose there could be just a bit of heat exchange with this type of vent, slightly warming the intake air with the exhaust vent in the proximity.
Doug
@Doug, there would be some warming, yes, but keep in mind we get quite a bit colder here, down to -35C. With our condensing (high efficiency) furnace the exhaust and intake PVC pipes are two seperate runs, about 12 feet total. The exhaust of course is no problem, it's just the intake, with the furnace running. That combustion intake air PVC gets cold enough to cause condensation pretty regularly. The home is 100 plus years old so the mechanical room is not an an outside wall, but in the middle of the basement. Part of that intake is exposed and part of it runs in an enclosed floor joist cavity to the furnace (which is now impossible to insulate). Insulating that intake in particular should be standard practice in cold climates, similar to insulating an ERV/HRV intake/exhaust.
The on demand hot water system does use a concentric (metal) vent, but is also mounted right on an outside wall. No issues there.