A c question

[steiconi]

My brother is an engineer, and designed ac for large buildings for 40 years.

He closes off his spare room when it's not in use, so I think it's probably a good idea.

[aslowdodge]

Good thinking points.
I counted 9 vents and had 4 shut.
Does a fan really work that much harder? I figured the air still moves since it now goes to the areas of least resistance. So if half the vents were closed, it’s not like the resistance the fan meets is doubled is it?

[harry chickpea]

Originally Posted by aslowdodge

Good thinking points.
I counted 9 vents and had 4 shut.
Does a fan really work that much harder? I figured the air still moves since it now goes to the areas of least resistance. So if half the vents were closed, it’s not like the resistance the fan meets is doubled is it?

Considering all the factors and the possible variations, each situation will be unique. Rather than try to count angels on the head of a pin using formulas and inaccurate estimates of effects, the more reliable answer is going to be an empirical test. A problem with that is that clouds, air temps and humidity will vary from day to day and hour to hour. Still, on similar days you can get a sense of overall effectiveness. Put a run-time meter or other method of recording how long the unit runs, keep the thermostat at the same setting and measure that on a day when the vents are open, and again on a day when the vents are closed.

As an aside, if I have one or two rooms that I don't actively use, the vents are mostly closed in them. I don't expect much summer savings, but in winter, especially when I'm using propane for heat, the savings can add up.

[btuhack]

Originally Posted by Curly Q. Bobalink

Harry, in a simple residential system, wouldn't the fan still move significantly more than half the airflow, even though half the vents (in the unused rooms) were closed off?

The issue is magnified when you consider most duct systems are undersized to begin with. The blower amps fall off as the blower max's out but the inefficiency relates mostly to the condensing unit and/or heat exchanger.


Closing off rooms is a bad idea in terms of efficiency and wear and tear on equipment. The same applies to simplified zoned ductwork...especially with a bypass duct.