I purchased a heat pump from a national company and it was installed in early January, 2008. When the thermostat is set, it will not get to that temperature, but usually stays 2 degree under and runs constantly. Only once or twice has it cut off by itself at the desired setting, and then only for 10 minutes or so. Defrost board and thermostat has been replace by installer. Also downstairs vent has been reduced to get more air flow to upsairs return. Is this the expected operation, and if not, what could be the problem, It is a 3- 1/2 ton unit for 3,000 square foot, brick home. Thanks.

Not a necessary expected operation. You have a warranty. You will void the warranty you mess with it. Call the dealer and have them fix. BTW some thermostats do vary 2 to 3 degrees.

Could be that the replaced unit is too small. What was the original heat pump condenser size? Did they also change out the fan coil? Do seers match?

What area of the country do you live and what are your winter temperatures?I need this for a sizing chart guess to see if your unit is to small.