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?

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

RP wrote:



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

Make that 99 out of 100

every 100 programmable stats that I encounter in the field are running
in HOLD mode with no schedules set. HTH.

hvacrmedic

I should also add, while I'm at it, that some stats do offer
programmable options for backup heat, and may provide some savings over
a standard stat in which backup heat will always be energize during
recovery from setback. In multiple stage systems some programmables will
also offer net savings over the non-programmable by running the system
longer in the low stage. Sorry for these technical omissions.

hvacrmedic

"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"

It sure sounds like it would be a big issue on a heat pump system with
electric aux heat. I think his concern was if he set the setback delta
too big, the aux electric heat could kick in.

wrote:

"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"

It sure sounds like it would be a big issue on a heat pump system with
electric aux heat. I think his concern was if he set the setback delta
too big, the aux electric heat could kick in.

I immediately addressed this in the follow-up post that I made, but yes,
you are correct. This is however something that scheduling alone, what
he asked about, won't cure. That option, if available, will be in the
setup menu which is typically difficult for the average homeowner to
make sense of. They could easily change some settings that don't need to
be changed. If he is interested in optimizing recovery via heat strip
lockout, then he should check his literature or contact the manufacturer.

hvacrmedic

RP wrote:

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?

Switch off the strip heaters if any and turn off the heat at night and
set the thermostat timer ahead to make the house comfy when you get up.

The programmable-stat isn't going to magically save you any money vs a
non-programmable stat.

There's no magic: reducing the indoor-outdoor temp diff at night makes
less heat flow. That's Newton's (1642-1727) Law of Cooling.

IOW, you may find yourself frequently overriding the schedule on your
programmable stat,

Or opening all the house windows in wintertime.

...defeating its true purpose, which BTW isn't what you claimed it was above.

How can he be wrong about his true purpose? :-)

...that I bought the prog. thermostat for in the first place?

... 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.

Nonsense. He can be perfectly comfy sleeping under a quilt in a 40 F bedroom.

Nick

Nick Pine wrote:
RP wrote:

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?


Switch off the strip heaters if any and turn off the heat at night and
set the thermostat timer ahead to make the house comfy when you get up.


The programmable-stat isn't going to magically save you any money vs a
non-programmable stat.


There's no magic: reducing the indoor-outdoor temp diff at night makes
less heat flow. That's Newton's (1642-1727) Law of Cooling.


IOW, you may find yourself frequently overriding the schedule on your
programmable stat,


Or opening all the house windows in wintertime.


...defeating its true purpose, which BTW isn't what you claimed it was above.


How can he be wrong about his true purpose? :-)


...that I bought the prog. thermostat for in the first place?


... 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.


Nonsense. He can be perfectly comfy sleeping under a quilt in a 40 F bedroom.

Nick

From that angle one could simply turn the unit off, t-stat no longer a
factor

hvacrmedic

RP wrote:

... He can be perfectly comfy sleeping under a quilt in a 40 F bedroom.

From that angle one could simply turn the unit off, t-stat no longer a
factor

Sure. Swiss mountain folk believe it's healthy to climb under quilts with
bedroom windows wide open in freezing weather.

Nick

"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.

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.

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.

RP wrote:

Conventional Heat Pumps get an average COP of 2 to 1.

Depends on the climate, no? Might be 3:1.

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...

Thus? :-)

Nick

wrote:

RP wrote:


Conventional Heat Pumps get an average COP of 2 to 1.


Depends on the climate, no? Might be 3:1.


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...


Thus? :-)

Thus, yes, at some higher COP and/or warmer climate the trend will
reverse. That's why I said in summary that economy depends upon the
system

hvacrmedic



Nick

RP wrote:

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...

Thus? :-)

Thus, yes, at some higher COP and/or warmer climate the trend will reverse.
That's why I said in summary that economy depends upon the system

I don't quite see a "thus" there. Would you further explain your logic?

Would you agree that night setbacks always save energy if the heat strips are
switched off, even though that may require longer reheating after the setback?

Nick