Forced Hot Air Adds Water to Exterior Walls To the Editor: I'd like to add a couple of thoughts to Stephen Smulski's excellent article "Water in the Walls" (8/94). In Case 2, the humidifiers on the home's two forced-hot-air systems are listed first among a number of probable causes. My own experience confirms this, and suggests that even without a humidifier, a forced-hot-air (FHA) system can be the key factor driving moisture into exterior walls. It's common for FHA systems to have undersized returns and/or large return leaks in the basement, both of which result in the house being pressurized. This in turn drives airborne moisture in the house into the walls and ceilings. To add insult to injury, in the