Re: [Phys-l] cooling hints

[Bill Nettles <bnettles@uu.edu>]

My AC fan continues to run about 30 s after the thermostat relay clicks to turn off the unit. I also hear the compressor cut out when the relay clicks, so yes, there are units that do that, but it's not a function of the thermostat. It is a function of the control panel in the AC unit.  I've had to replace mine and I saw the jumpers which set the "run after" time for both AC and heat and the "delay" time for the heat circulation fan ( the flame actually comes on before the air is pushed through the plenum).  My old board had an AC run-after of zero but I thought "those coils are still cool for a bit after the compressor is off. I'll try this" so I changed the jumper. Of course, there is no old vs new data to see if there is a difference.

Bill Nettles

> -----Original Message-----
> From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
> bounces@carnot.physics.buffalo.edu] On Behalf Of John Clement
> Sent: Wednesday, August 24, 2011 10:30 AM
> To: 'Forum for Physics Educators'
> Subject: Re: [Phys-l] cooling hints
>
> Running the fan may also decrease cooling efficiency by running air
> through
> the less insulated ductwork in the attic.  If the ducts are below the
> ceiling then this would probably not be a problem.  But running the fan
> for
> a few minutes after the AC or heater have turned off may increase
> efficiency
> slightly by allowing a bit more heating or cooling.  Gas furnaces have
> fans
> that stay on depending on a temperature sensor, but AC generally cuts
> off
> immediately with the compressor.
>
> I wonder if there is a thermostat that could keep the fan running for a
> little bit after the normal cutoff.  It should not be a big cost item
> because most thermostats are now digital, but I suspect the savings
> might be
> small.  But even a couple of percent would be welcome during the
> Houston
> high cost summers.
>
> John M. Clement
> Houston, TX