I found this sticky thread in this forum: http://www.doityourself.com/forum/ai...ing-chart.html


Is this list for standard height walls (9-10 feet in height)?

I am curious. If I have a room that is about 300 sq feet and also has a vaulted ceiling (an additional 9 feet in height with a 45 slant), do I add another 150 sq feet and get a 12K BTU instead of a 8K BTU?

Forum Moderator: I apologize that I posted this in the wrong forum. I was reading between threads and accidentally posted here. Can you move this to the AC forum? Thanks

 

Standard ceiling height is eight feet. That chart is a VERY rough rule of thumb and cannot be relied upon for anything more than a rough idea. You could run a quick and dirty heat gain calculation on the single room and probably get a better answer. Here's one that takes into account the total volume of the room. Warmair.com - Window Air Conditioners Figure the height as 1/2 the difference between the top of the walls and the top of the peak plus the height of the walls.

Example: Walls are ten feet high and the peak is nineteen feet high. Divide the extra nine feet by two and 4-1/2 feet. Add this to the ten feet of the walls for a calculated height of 14-1/2 feet.

Do NOT oversize the unit. Much of the comfort in air conditioning comes from the reduction in the relative humidity (RH) in the room. An oversized A/C will quickly chill the room and then shut off under control of the thermostat. That leaves you with an even higher RH plus cold temperatures, a miserable condition. Ideally the A/C should run continuously on the hottest days.

 

Furd, thanks for providing the insights and knowledge.

I never knew that. I always thought that "oversized" is better. Very good knowledge to know.

The AC Window Unit that I have can continue to run without shutting off. It has a continuous mode. If I "OverSize", would that matter?

Thanks Furd.

 

The continuous mode merely keeps the fan running while the compressor cycles with the thermostat. Leaving the fan running while the compressor is off will cause the moisture that has condensed on the evaporator (cooling) coil to re-evaporate into the room air and that will raise the RH. There are very few instances when continuous fan operation of an air conditioner is beneficial.

 

Thanks for the follow up Furd.