Are you sure it is constant--at least I was unaware that W-F used
single-speed units--they weren't when we did ours anyway, but that's
been quite a while ago.
Even if so, it does not lead to the conclusion. The savings of a
setback depend on the integral of the the demand over the time
period--if the average demand is lower, then the input required is lower
for a similar set of external conditions.
The only kicker in the mix is whether there really would be such a
significant loss in efficiency owing to the heat source "drawdown" that
the overall system efficiency drops sufficiently to cause more energy to
be used than is saved. I have an extremely difficult time believing
that to be at all likely.
See my other response for some suggested places to look for some more
definitive research and sources.
[...asking about setback and subject...]
Don't know if you gave up or got an answer but I mentioned the OSU site
earlier. Being bored, I went and found it -- here's the current link.
Didn't find a specific answer in the faq's, but they have a couple of
contact ways you can get to them.
i sent this email to Alliant Energy Geothermal
==============there is a discussion underway with this as topic: -
We heat and cool our home geothermally (water to air system.) We
would, of course, like to decrease our costs further if we can, and so
have explored the benefits of setting our thermostat lower at those
times when the house (or parts of it) are not occupied. The folks who
designed the heating system say that with these systems, it is best to
leave the set temp unchanged. Of course, I have asked "why", but when I
do, it seems that smoke starts to come out of the phone. In essence,
they say that it is "best" but seem unable to say why. Might any of you
know what would be best in this regard , and particularly whether the
issue of thermostat setback is actually any different for geothermal
could you help me understand the issues involved? thanks in advance
for any input! chuck b:-)
=============and i got this reply
Set-up of a Geo system temperature during the cooling time of year
should allow a Geo system to recover the cooling and dehumidify as
quickly and more efficiently than other air sourced cooling systems.
Set-back is not typically recommended during the heating time of year
due to 1) a slower recovery time for heating, 2) the potential for the
back up electric elements kicking in to boost the reheating rate but, at
an added electric expense to you, 3) dependent on if you use a straight
well water open loop or only a minimal to non freeze protected closed
loop fluid, the lack of normal flows may allow for a potential for a
loop to freeze up and 4) many people who own set-back thermostats are
easily confused by the instructions for operating them and re-setting them.
All of these can cause contractor callbacks, they hate callbacks.
We do have a few of our regional Geo system owners who do a slight
setback for heating at maybe 2 to 4 degrees F maximum for 4 to 6 hours
but, all must realize the potential results.
During AC season and the daytime hours of unoccupied homes, they might
also do set up to minimize On Peak energy charges when they choose Time
of Use electric rate options.
I hope this helps. Thanks for the inquiry.
From: Alliant Energy Geothermal Web Forms
Sent: Wednesday, November 28, 2007 9:29 PM
Subject: Alliant Energy Geothermal -- Contact Us Form
My thinking precisely. The response basically is a bunch of
weasel-words. The upshot basically is if the aux heat issue is taken
care of properly and one doesn't mind the recovery time, setback will
save. For a reasonable source capacity, our experience was that while
the outlet temperatures aren't equivalent to gas, they're sufficiently
high the air feels "warm enough" circulating as opposed to the and as
compared to air-air heat pump that felt cool...
If there's freeze-up w/ a few degrees setback so the unit runs a little
less at night, there's going to be freeze up on other days as well...
but on the other hand:
from your OP
"The folks who designed the heating system say that with
these systems, it is best to leave the set temp unchanged."
any system that is operated outside it's design criteria risks failure!
good luck with yours!
A few degrees lower is "outside the design" for a residential hvac
system? I still think there's been no rational basis for the
proscription given unless, as Ken says, there's a problem--and if
there's a problem, I think there will be a problem irrespective of the
It would seem that I have not communicated clearly...
Indeed, the design folks tell me not to use any setback, but
they seem unable to tell me "why." Their lack of a
meaningful explanation was the cause of my original question
about the setback issue.
All the best,
The rate at which your house loses heat to the environment
depends in an almost linear fashion on the temperature difference
between your house and the outside environment.
So as your house cools, the rate at which it loses heat will
decrease. Keeping the house at the higher temperature means
it will constantly lose heat at that higher rate, and all of that heat
lost must be mad eup to maintain the temperature. If you let
it cool down and heat it back up the 'stored' heat that is lost
to the environment while cooling is exactly equal to the extra
heat needed to heat it back up. But the heat loss to the enviornment
is less the whol time during which the house is cooler than
Ergo, it ALWAY will use less heat to let it cool down and
heat it back up than to maintain it at the higher temperature.
What comes into play is the cost of pumping that heat into
your house at the higher rate for the short period of time
during which it heats back up. If that is down with auxillary
electric resistance heat that MAY cost more or use more
energy overall than just keeping it warm.
The presumption that you have an auxiliary heating system
may be part of the reason why you get advice to the
contrary. Another concern may be that cycling the
temperature may result in persistent cold spots or
condensation problems that could lead to overall
dissatisfaction with the system.
But mostly I doubt you have ever spoken on the
phone with anyone who actually studied heat
transfer phenomenon or even took a physics course
The other possibility suggested is that extracting heat
too fast from the groundwater could create a pool
of cooled water underground with a resultant lower
efficiency of heat extraction. That would depend
largely on the groundwater environment and how
extensive the heat exchange area is underground.
I doubt that a definitive general answer can be given
regarding that last concern. It would be highly dependent
on the specific situation.
On Sun, 2 Dec 2007 09:55:59 -0800 (PST), Fred the Red Shirt
Hello to all (again),
Well, I am the OP on this "Will I save if I use a thermostat
setback on my geothermal system" thread, and I believe that
I now have an answer:
Part of the hassle I faced in experimenting with this was
that for some reason, I kept thinking only of my house. We
have a number of electrical appliances there that are used
(essentially) randomly, and their use would certainly throw
off any comparisons that I could make over a relatively
short period of time.
I commented on that to my wife, and she said "So do the
experiment in the barn." (She did not actually say "So do
the experiment in the barn, you idiot", but that is what I
Our office-barn is heated with exactly the same system as is
our house (water to air geo with no backup resistance heat)
and there is no variability of electrical consumption other
than the heating system for most of each day.
So, with that information, I did a very simple experiment. I
have run it only for six days but, as you will see, the
pattern seems quite clear:
I set the programmable thermostat to drop the "call heat"
temperature by 10 degrees F for 12 hours on alternating
Each morning, at the same time, I read the barn's electric
Finally, I got the degree days, and wind speed, from a
weather service site.
With that, I could calculate the ratio of KWH to Degree Day.
I have also included in the table below the reported max
wind speed for the day.
Day 1: 1.2 (setback) 14
Day 2: 1.6 (no setback) 17
Day 3: 1.0 (setback) 8
Day 4: 1.3 (no setback) 0
Day 5: 1.0 (setback) 12
Day 6: 1.2 (no setback) 3
So, on the days with setback, the mean KWH/DD was 1.06. On
the days with no setback, that mean was 1.36.
The resulting savings are approximately 22%.
I do remain baffled by the reasons the geothermal folks
(installers, designers, sellers) seem to be consistent in
suggesting that such setbacks are not of value.
All the best,
As at least one other poster noted, they're concerned w/ other factors
that aren't applicable in your case (primarily dominated by the use of
resistance electric heat in many/most systems)...
There are others including the potential freezeup, etc., that are
possible but imo they're mostly cya kinds of responses. Did you try the
Water Furnace people directly or contact the Okla State or some of the
other resources for other input?
The CYA analysis makes sense to me, but as you probably
know, there is no real "freeze up" danger at all. These
systems simply turn themselves off is the incoming water is
Also, as you may know "Water Furnace" is a brand name. Our
equipment is ClimateMaster.
I have communicated about all this at some length with the
ClimateMaster folks, with the geo folks from my electric
utility, and with the installer of the equipment. They all
have said "no setback" is best.
All the best,
The problem I've normally seen is on the once-through water exchange
systems (which is also what I think I recall being mentioned in one of
the earlier postings of a problem--whether it was yours or another I
don't recall) is the freezeup of the outlet when systems aren't running.
My opinion remains as I noted there is that if that's a problem for a
given system, it will be so whether there's a setback or not unless the
system is so undersized as to run continuously; hence my assessment of
that as a response as being in the "CYA" category.
Yes, I had thought that was who you had said earlier...I don't know
ClimateMaster; had a Water Furnace system earlier and was pretty
impressed w/ their factory rep service/technical support.
I think again all of those folks are addressing the general case still
rather than the specifics of a given installation and are still using
the answer that is easiest for them. It would be interesting if could
get to one of the actual research facilities that might address a
specific system rather than the general consumer response. If you were
still interested in pursuing it from that standpoint I'd again suggest
ORNL, TVA R&D (not power) or OSU might be more likely to answer a real
For me there are two (essentially unrelated) issues:
First, I am concerned with my system(s) and whatever savings
I might realize with the setbacks.
Second, I am a curious sort, and often enjoy understanding
this sort of thing.
Right now, my energies are focused on #1, and with my very
simple experiment, I do believe I have my answer.
#2 will have to wait a bit!
All the best,
The answer to the first is clear -- a lower setpoint is less total
integrated demand as compared to no setback so unless there are
mitigating factors such as the higher-rate aux heat (that you don't
have), then a setback will invariably be less input.
The other issues are also system-specific but the design issues have
been dealt with by the various research groups. I never had a
convenient water source so didn't pursue the logistics of them that much.
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