How do I set my programmable thermostat for optimum fuel cost savings?
I have an in floor heating system. Laars boiler heating water in 2"
concrete slab. I need to know how long to turn the heat down for at
night and in the day when we are away. I am concerned that if the slab
cools down too much, it may cost more to heat it up than was saved by
lowering the temperature in the first place.

canoehead...

I knew I'd read the answer to this somewhere...it just took
me awhile to find it on the Salt River Project website. 
SRP delivers power to Arizona, and is highly rated by
JD Power and Associates. A page from their site, which
has recommendations for businesses in regard to thermostats
says:

"Thermostat Adjustment"

"Begin by adjusting your thermostat to a temperature no
 lower than 78°F for cooling and no higher than 68°F for
 heating."
 
"Contrary to popular opinion, it never costs as
 much to "heat up" or "cool down" a building to the
 desired temperature over a period of time."

"Turning down your thermostat just 3° in the winter,
 for instance, will save approximately 9% on your fuel
 bill; a 7° reduction will save approximately 20%."
http://www.srpnet.com/businessenergymanager/library/HVAC004.asp

Obviously, you can set it even lower when you are away
or sleeping, and reap even more savings. A consideration 
for longer periods away include the presence of pets,
including fish - and older, arthritic, cats and dogs,
who appreciate warmer temperatures. 

The question then becomes "How long of a warmup period
do I need to restore the space to a normal temperature?"

The same page, cited above, notes:

"Optimal Start Capability"

"An optimal start capability may accompany the installation
 of an occupied/unoccupied control system. This allows for
 varying start-up time in the morning depending on outside
 temperatures. Cooler exterior temperatures will require a
 longer warm-up time."

Occupied/unoccupied control systems are also known as 
'Setback Thermostats':

"Setback Thermostats"
"Setback thermostats, also called occupied/unoccupied
 controls, automatically lower (or increase in summer)
 temperatures when the building is unoccupied, thereby
 reducing heating or cooling needs."

"A 24-hour programmable thermostat can be programmed
 to automatically change thermostat settings ("setpoints")
 two or more times a day. A seven-day programmable
 thermostat can be customized for different days of the week."

So, obviously, a longer startup time would be required
to warm the space after a long period away, set for a 
lower temperature, than after a period of sleep set at
a smaller decrease. And, the colder it is outside and 
the larger the space, the longer it will take.

The exact time for each situation will best be determined
by trial-and-error with your particular space and system.


Please do not rate this answer until you are satisfied that
the answer cannot be improved upon by means of a dialog with
the researcher through the "Request for Clarification" process.

sublime1-ga


Searches done, via Google:

"programmable thermostat" "maximize savings"
://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q=%22programmable+thermostat%22+%22maximize+savings%22

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Request for Answer Clarification by canoehead-ga on 12 Jan 2003 14:27 PST

Some good information here but I am looking for information specific
to infloor heating. I am concerned with the mass of the concrete floor
and how the heating up and the subsequent cooling down affect energy
use. Are you saying that it will always cost less in energy $$ to turn
the heat down when vacant and reheat for occupancy even when heating a
concrete slab?

Steve

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Clarification of Answer by sublime1-ga on 12 Jan 2003 18:09 PST

canoehead...

Yes. It is the contention of the experts that it is always
less expensive to lower the heat as far as possible, and 
reheat the space upon your return (short of causing your 
pipes to freeze, of course), rather than maintaining a 
the heat at a higher level. The latter will allow you a 
quicker return to the preferred temperature, but will not
save you as much in energy costs. Most people choose to
do the latter simply because they don't have a programmable
thermostat, and they want to quickly bring the space to a
comfortable temperature (or because they have pets).

With a programmable thermostat and no pets to concern you,
you can program the unit to go as low as you like (short
of freezing your plumbing), and, as long as you program
enough lead time in the return to normal temperature, you
can come home to a comfortable environment having saved a
good deal of money.

There is, nonetheless, a myth that it takes more energy to
bring the room back from a lower temperature because of the
need to reheat the objects in the room, as well as the air.

I read that this was not true in the newsletter for my power
company some time ago, which is why I sought to verify this
for you by searching on the SRP website - and there it was!

While it may consume more energy to bring the space back
to preferred temperature due to it being set much lower, the
amount used doesn't begin to compare with the amount saved.

If you require further clarification of this answer, please
don't hesitate to ask.

sublime1-ga