CBHVAC wrote:

"Russell Coleman" wrote in message
llsouth.net...

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 same across the board....
Or, no more than 2F degrees difference between the two.

The most common question asked is : What temp saves me the most money?
The answer is, as low as you can stand it.

With programmable stats, any real savings you think you had, you kill when
the aux heat comes on...if needed for recovery.

Conventional Heat Pumps get an average COP of 2 to 1. Thus if the
overnight (set back) runtime is reduced by an amount of time that is
twice the recovery time with heat strips enabled), then energy is saved
over not setting back. Keep in mind that the COP during recover will be
greater than 1 to 1 because the heat pump will be running along with the
heat strips.

At low ambients, where the COP of the heat pump is barely, if at all,
Greater than 1 to 1, then it is impossible to imaging that heat strip
assisted recovery will overcome the savings of setting back. OTOH, at
higher ambients, it hardly matters much--the savings, or loss, whatever
the case may be, only amounts to pennies per day.

Unless I see some studies that supersede this, then I say setting back
overnight will save money. I also suggest that the system be set up to
run minimal backup heat, not at the stat, but at the unit. Rheem used to
install a klixon in the air handler that locked out a strip or two until
the air temp dropped below 65ºF. Not a bad thing. Staging the strips is
also a good idea. This can be done in conjunction with a 3 stage heat
stat, or by installing a long time delay on a portion of the strip heat.
There are plenty of options, but none of them are going to save
*substantial* amounts on the energy bill. If energy efficiency is that
great of an obstacle then either shut it off, or get a higher efficiency
system. I really don't think that piddling with t-stat settings is worth
the time unless you've done the calcs that prove that any savings from
doing so are going to be noticeable. It sometimes amounts to spending a
dollar to save a dime. Figuring in the cost of the programmable stat vs
a non-programmable, payoff might take 10 years, it might take one, and
it may never pay for itself, depending upon system design.

hvacrmedic



Any decent digital stat maintains a 1F differential, so that alone saves you
a ton over a mercury stat.
I said decent...the one you have is a marginal Honeywell....bottom end of
the scale, particularly if you got it at Lowes or Home Depot.