Carrier 2-stage heat pump installed in 1995. Unit was working fine when I lowered thermostat setting (78 to 65 degrees)prior to going out of town last week. Now digital thermostat (NO BATTERY) is dark and heat pump is inop.

Low refrigerant levels (most common cause.)

it sounds like your outdoor unit is running, but the AC is blowing out hot air, start with cleaning the condenser coils. Spray the coils with a water hose, clearing away the loose dirt and foliage. If the AC continues to blow warm air, call an HVAC expert for help with refrigerant levels.

whats the model and serial # of the thermostat

If it 2 degree cooler then that's fine .adjust your thermostat 2 degrees higher so you will get the temperature

you likely have a "two stage" system, but not a two stage heat pump. the first stage is probably the heat pump itself, and the second stage is the auxilliary resistance heat that suppliments the heat pump at lower temps. if i knew the terminals that are available on the new stat, i could advise you on the proper wiring.

Remove the cover of the circuit board and controls on the outside unit. Some units have a thermostat that allows you to set the temperature at which it switches to the second stage of heat. Set that for about 34 degrees.

http://www.corp.carrier.com/www/v/index.jsp?vgnextoid=9d087afdef677010VgnVCM100000cb890b80RCRD
Try searching this Carrier web site. It has lots of manuals.
Hope this helps.

Need to have more numbers in model number to be sure There's about 30 T841 t'stats and some are1, 2, and 3 stage heat pump and some 1, 2, and 3stage AC.
If you have
R to transformer
G to fan relay
Y to contactor in outdoor unit
W to Gas valve
You have the thermostat wired right.
Depending on the T'stat B could be the common side of the transformer or 2nd stage heat.
W2 terminal are for 2nd stage heat
You need to put the thermostat on cool and lower the setting to bring on the cooling. Then go outside and see if you have power to the outdoor unit and 24 volts to the contactor coil.

if you have a mercury thermostat and it is not leveled properly it will turn the temperatyure on and off by a couple of degrees
some thermostats have the capability of running the unit longer than the set temperature
i would recommend calling a local heating and air conditioning contractor and get the unit cleaned and bring up the issue then

If it is Carrier tstat. Some have resets other dont. Kill power to insid unit. WAit five minutes turn back on. If it has batteries take them out. But the storm may have damaged the tstat. I had a thunderstorm damage 11 tstats in one building. You can buy a tsta at Home depot that will work and they are easy to wire. Just make sure you get the coorect tstat. If the unit is straight cool electric thats the easy one. A heat pump you need a Tsta with two stage heat one stage cool. You can pick one up for about 25 to 30 dollars. Please call back for more help be glad to assist.Rusty

Turn up your outside thermostat in the outside unit.
You should have about 3 degrees difference in one stage to the next. So I would change the thermosat with one that has three stages of heat.