Heoneywell RTH7500 indoor temp too high

Hello, We have had a Carrier Heat Pump with a Honeywell thermostate for over 5 years without a problem. Not sure what model number the T-stat is. When I came home Friday, it said te indoor temp was 92, yet it was very cold, mid 70's inside. I assumed the sensor went bad. We've wanted a programable unit, so we bought a Honeywell RTH7500 and installed it. This unit is also reading in the 90's for an indoor temp.
The T-Stat is mounted on an exterior wall, but a laser temp finder shows the wall to be 80 degrees and the T-State is 78 degrees. Any ideas on why the original unit, and the new unit are showing and indoor temp nearly 10 degrees higher than it should be?
Thanks, Doug
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On 30 May 2009 23:55:20 GMT, doug.scott70_at_gmail_dot snipped-for-privacy@foo.com (dougs70) wrote:

    Is there a lamp or heat source nearby? Is the thermostat level (I don't believe the newer stats have that problem).
    Why would you mount a thermostat on an outside wall???
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dougs70 wrote:

If you don't seal the hole in the wall that the wiring comes through, you're going to have a problem, especially on an exterior wall. In the winter you will have the opposite problem. Even if the thermostat is mounted on an interior wall, you should always close up the hole where the thermostat wiring comes through. New thermostats are sometimes supplied with a piece of foam to be installed behind them to block the air.
TDD
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