On Sun, 2 Dec 2007 09:55:59 -0800 (PST), Fred the Red Shirt
wrote:

On Nov 26, 11:48 pm, Kenneth
wrote:
Howdy,

This is way OT...(again), but:

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 systems?



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
normal.

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
ever.

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.

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
heard.)

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
nights.

Each morning, at the same time, I read the barn's electric
meter.

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.


KWH/DD WS

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,
--
Kenneth

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