As a technician, how many times have you gone out to the same customer's home because of unrealistic expectations? Some homeowners expect air-conditioners to work like an ice box, want you to size them for their big party on Fourth of July weekend, and expect you to show up at the drop of a dime when it doesn't meet their impossible notion. Of course, this could be avoided by establishing a standard during the sales process or a little customer education. I liked to talk about "The Occupied Zone" and work in a little Radiant Asymmetry, using more layman's terms of course.
The occupied zone is a concept used in the design process to properly heat and cool spaces without a customer feeling drafts. As you can see in the picture taken from ACCA Manual RS: Comfort, Air Quality, and Efficiency By Design, the zone is 2' inwards from all walls and ceilings. ACCA Manual J: Residential Load Calculation, states the occupied zone is only 6.5' in height. This area near walls and ceilings is use to mix conditioned air with room air. In fact, this mixing is what cuts down on occupants feeling convective currents or drafts in the heating season.
Not only will occupants feel drafts, but they will feel what is termed 'radiant asymmetry'. This is the phenomenon (not really, just to a homeowner) of a warm or cold temperature. If the wall is colder than you are, the wall will be pulling radiant heat from you. This makes you feel cold, even if it is 70F in the room, hence the statement my wife likes to use: "a cold 70F". The best example I ever heard of this is the temperature at an ice rink. It feels cold when on the ice, but the air temperature hovers around 65F! Anyhow, have you ever wondered how much asymmetry is too much?
ASHRAE Standard 55: Thermal Environmental Conditions for Human Occupancy was nice enough to spell this out for us. As you can see, a warm ceiling is the number one device that occupants will find uncomfortable with only a 10-20F difference. This would of course happen during the cooling season, and with any luck on Friday afternoon just before that Summer, holiday weekend/family reunion, right? I can see this happening more often in Cape style homes since there are more ceiling/roof combinations and occupants are more than likely outside of the occupied zone. The only answer to this issue is proper home air sealing and insulation. It does not matter how oversized your air-conditioner is, it will not be able to overcome the laws of physics and radiation.
This is just another reason to peer into the attic during your sales calls to verify the weatherization of the home and the thermal boundary. Don't set your system up for failure, and certainly establish some basic expectations with the homeowner. You don't want to leave this up to your technician on a holiday weekend, getting paid double-time for warranty work!
Occupied Zone - ACCA Manual RS
ASHRAE Standard 55
