Quote:
Originally Posted by jeff5may
Beware of positive pressure!
In winter, any positive pressure inside the house will push warm, moist air into your envelope. This moisture will condense, soaking insulation or whatever occupies the warm/cold boundary. If the supplied moisture freezes, it can cause severe damage to your home in places that are hard to detect. If it doesn't have a way to dissipate, it will remain and grow mold in the cooling season.
This is the major reason most all gas heaters have an exhaust but no real intake. Unless your house is sealed up like a submarine, it doesn't take much pressure to move enough water vapor to cause problems. The manufacturers have learned the hard way not to expose themselves to this liability.
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Virtually all condensing furnaces today have a dedicated intake pipe. No liability issues since the intake/exhaust of the furnace is sealed off from the house.