Russell Coleman wrote:

I have a heat pump with aux heat for which I bought a programmable
thermostat. The RTH7400D to be exact

What is the recommendation on setting the temp difference between wake and
sleep times so that I am not going to make the system work to hard and not
get the energy savings that I bought the prog. thermostat for in the first
place?

The programmable-stat isn't going to magically save you any money vs a
non-programmable stat. It has one purpose: To control temperature, just
like any other t-stat. A thermidistat OTOH also controls RH, within
limits providing constant comfort at constant temp, something the
regular thermostat won't do. IOW, a constant temp setting won't provide
uniform comfort on a regular stat, either programmable or
non-programmable. IOW, you may find yourself frequently overriding the
schedule on your programmable stat, thus defeating its true purpose,
which BTW isn't what you claimed it was above.

The energy savings enter in only in the fact that you are setting back
the temperature for periods of the day, something that a programmable
stat isn't a requirement for. Basically any time the unit is off you are
saving. It isn't the stat that will save you money, it does nothing to
the system by its mere connection to the system to improve its inherent
efficiency, it is only the lower than comfort temp settings scheduled in
winter, and higher in summer that save you the money. Thus for the
greatest savings just turn the system off when you aren't home, and set
back at night to whatever freeze level in winter or sweating level that
you care to withstand in the name of savings.

Recovery time between sleep and wake settings isn't an issue as far as
energy cost is concerned, but it should be to be taken into account when
adjusting the schedule times.

BTW, in reference to my above comment: People don't buy programmable
stats to save money, they buy them to save them from the 2-second hassle
of manually setting back every night, and/or for improved comfort via RH
control when available. Comfort control OTOH typically reduces overall
economy, FWIW. Thus, unless you aren't home, then the only way that you
are going to save money with *any* stat is to live uncomfortably in your
own house. I'm almost positive that this is the reason that 1 out of
every 100 programmable stats that I encounter in the field are running
in HOLD mode with no schedules set. HTH.

hvacrmedic