Greetings all. I have a 3 ton Hydro-Delta system in a 1600 sq. ft. house in coastal CT (only gets below 15* 10% of the time here).
The house is a modular ranch, 2 years old, with R19 walls, R51 ceiling. I've been using the setback on the thermostat but I'm wondering at what point does it make the unit work more than it should. I have it set for 68* in the evening and set it back to 63* at night. I don't use the water heater option in the morning so the unit puts all effort into heating the house in the morning from 63* up to 68*.
Any thoughts?

Hi Sailor, most heatpump systems are programmed so that if the room temp falls more than 2° below the set point on the thermostat the backup heat is energized. You don't need to worry about over working your heatpump by setting back the thermostat, but having the back up heat come on is a waste of energy.

Welcome to the forums

I disagree. So long as the temperature setback is at least 8 hours, it saves energy even if the system has to work harder to to bring the temperature back up to normal. If it's less than 8 hours then yes it does waste energy, but anything over that is going to save money in the long run. I have my downstairs Geothermal system to go down to 58 degrees at night (the bedrooms are upstairs) and heat back up before I go downstairs for breakfast, it also turns down to 58 when I'm at work and turns back up to 68 degrees before I arrive home. Why should I spend more money to heat a house when no one is home? I've been considering playing with the numbers to allow the tempature to go even lower, but I will have to study the numbers to see how much it saves me.

Source:
LINK

You missed the point. 

If you are using the electric resistance back up heat to bring up the temperature after the set back, then you are definitely wasting energy.  Heat pumps produce heat at 300% - 400% efficiency.   Resistance heat is 100%.  The site you listed is for regular furnaces not heat pumps.  The method you described is true for furnaces,  not for heat pumps that use electric resistance back up heat.  There is a big difference.

Sailor55,

If you were to disable the back up heat, then doing a setback at night makes more sense. I did this on my heat pump by disconnecting the "W" wire in my thermostat. Now when my heat pump is recovering from the low temp at night, my back up heat strip is not energized.

Hope this helps.

Geodean,

What would be the harm in turning off the circuit breakers for the emergency heat strips on a geothermal unit or for that matter on a air to air unit?

Good Question.  On my heatpump, the fan is on the same circuit as the heat strips.   If the heat strips have a breaker all to themselves, then this would work fine.

It's almost always the case that the fan unit is powered by the same circuit as the electric heat. So if you turn it off, the fan won't blow, and most likely, the heat pump won't function, either.

With geothermal heat pumps, the least energy is used when a setback of only 2 or 3 degrees is used for "unoccupied" time. Because of the recovery time and electric heat needs, it will cost less in operation costs and be better for the geothermal system if there is little to no setback. It's best just to leave the temperature set at one point, rather than keep changing it. It's kind of like maintaining the same speed on the freeway. It'll use more gas to slow down, speed up, slow down, speed up, etc. than it will to just maintain the same speed.

You could solve the issue by using a thermostat with adaptive intelligent recovery. This allows the thermostat to learn how your home looses energy, allows for set back and warm up without using the secondary heat source. Many thermostats have this feature. It typically takes 7 days for it to learn the cycles of your home. Most Honeywell programmable thermostats [heat pump stats] have this feature. Carrier has it on theirs also.

Good point. Robertshaw also has that capability with the 9825i.