We seem to be having an odd problem with our heat pump/HVAC system.
As temperatures here have recently (finally) climbed into the mid to
upper 60s, our house heated itself up to about 78 degrees (per the
thermostat reading). The thermostat was set on "heat" and set to 72.
As the 78 degrees in the house was pretty warm last night, I went to
set the thermostat to "cool". I noticed the thermostat was flashing
to "replace batteries". I did so. I then set the thermostat to
"cool" and to 73 degrees. This morning the temperature still read 78.

This evening I'm fighting the same battle. The termperature inside
has actually gone UP to 79. Thermostat still set to cool. I checked
to verify that the heat pump outside has kicked on and it has.

The heat pump was replaced in 2001 and the inside system in 1999.
Could this be a switch sticking and not cliking over from "heat" to
"cool"? Does the system take time to move from heat to cool? I don't
remember this from past years, but I also don't rememeber big early
March temp swings from 20 to 70 in just a few days (though I'm sure
they happened). Could this be a weird result of the thermostat
batteris frittering out before we replaced them? Could this be a
freon issue? I'd like to get some level of education on this before
inviting the HVAC guys out to the house to explain how this is going
to cost me some hundreds of dollars.

After I originally posted this in alt.hvac last night, I noticed
something else that happened overnight. I turned off the system
alotgether since the outside temps were supposed to be right around 50
degrees overnight. The house is still 78 degrees inside even this
morning. I have a regular store bought thermometer sitting on top of
the thermostat just to doublecheck. I know there's probably some
physics lesson out there that could help explain, but I'm curious as
to why the house is hanging on at 78 with no mechanical intervention
even though it's never gotten that warm outside. Maybe the heat pump
wasn't warming the house, but it just wasn't cooling it either.....

Thanks very much for any help.

jeff