Stretch wrote:

Nick,

If you check the data, or measure it yourself, Strips cost 2 to 3.5
times as much per BTU as the heat pump. So you set back by shutting
off the cheap heat (Heat Pump Compressor), and recover by using the
expensive strip heat. Setback increases the electric bill if the heat
strips come on during recovery. In addition to warming the air during
recovery, you also have to warm the entire thermal mass of the house
and all it's contents. THAT is ia big part of the added power
consumption.

Here in Myrtle Beach, SC, a lot of homeowners are relocated yankees
(like me), used to setback. They setback in the winter and their
electric bills go up. They call me to check their system, I explain
proper operation, they stop using setback in the winter and their
electric bill goes back down. Happens every winter, many times.

Many heat pump setback thermostats lock out strips during the beginning
of setback recovery. On those, recovery takes a LONG time. After a
number of hours, the strips are allowed back on, recovery occurs and
electric bill goes up.

Note that recovery without strips only works in mild weather, so
setback efficiency depends on your climate. It does not work well in
my climate.

By the way, my wife, like many women, has to make several potty trips
to pee every night. If I installed another setback thermostat and her
tush gets cold on a regular basis every night, I will get the cold
shoulder when it comes to sex. That alone is enough reason not to use
a setback thermostat.

Stretch

The short answer seems to be that setting back will either save energy
or it won't.

I keep mentioning, and everyone seems to continue excluding in their
calculations, the fact that even with resistance heat engaged the heat
pump will also be running. The net COP during recovery will be the
weighted average of the heat pump COP and 1:1, and thus greater than 1:1
when the heat pump COP is greater than 1:1. The recovery COP will depend
upon the ratio of heat pump and resistance heat capacities. This is why
I suggested trimming the backup resistance heat to a minimal number of
elements, staging the remainder in with either t-stat programming, time
delay, or return air temp. Regardless of overnight setback there will be
times when the system has been set back, such as vacation, and quick
recovery will be desired. During these times the remainder of the
resistance heat will be called upon as needed, but will never come in
during normal operation.

Now here's something that nobody has yet addressed:
If it is sufficiently cold in the house and outside, as in after the
unit has been turned off for a long period in sub freezing weather, or
perhaps was down due to some failure, then the heat pump alone may never
*ever* recover at all. In fact the temperature could actually drop
during the recovery attempt. Don't doubt it for an instant, as a
lifelong tech I have encountered this more than once.

This is a situation in which the COP of the heat pump is a misleading
number. Though it may be greater than 1:1, no energy is saved over the
heat strips because the house simply never warms up, and all of the
runtime is effectively purely wasted energy and thus wasted dollars. The
capacity is simply not enough to overcome the current level of heat
loss. Thus when it is exceptionally cold in the house the heat strips
will save energy during recovery. While this doesn't typically apply to
normal overnight setback, it does however illustrate that the difference
in COP of two heat sources doesn't equate directly to the difference in
energy used and/or money saved. IOW, though the energy savings may be
intact on paper, they are meaningless because you are getting nothing ot
show for the money spent.

During recovery without backup heat, the heat pump has to start recovery
sooner, so that the added runtime of this method of recovery can indeed
be greater than the cost of strip heat during recovery, infinitely
greater if the heat pump's capacity has dropped so that it just equals
the heat load in which case there is no recovery, period, just hours and
hours of runtime with nothing to show. But even under normal
circumstances we won't instantly switch to greater savings without
backup heat, there is a curve involved, it's a continuum, and there must
be some specific point along that curve in which the energy use is
exactly the same with either method of recovery. That point depends
entirely upon the system installed [it's configuration and sizing], the
house in which it is installed[R values, infiltration, and thermal
mass], the outdoor temp, and the indoor setback temp. There are even
more variables to consider than these, but these are the major ones.

You should be able to figure out from the argument above and those made
in the thread by all of you, how each of these variables affect the
balance, i.e. which direction of any changes in these will tip the
balance in favor of backup without resistance heat or backup with
resistance heat.

In summary, the energy savings gained or lost depend upon very many
factors and no blanket statement should even be attempted. Every system
would have to be tested in the field in order to determine its energy
saving potential during various ambient/indoor temp combinations, and/or
off or set-back periods. IOW it's too complicated an issue for anyone
here to claim to have made the correct statements about savings unless
its those who said "it depends" Easiest way is to check your light
bill before and after you changed your setback habits. Even if you show
a savings for the one month out of two tested, this will still be
meaningless unless you are sure that the month with the lower bill was
as cold as or colder on average than the other and you used exactly as
much additional electricity or less on other appliances, all of which
contribute to the total heat load.

hvacrmedic