Setback thermostat

[manu4ever]

Hi,
I am having difficulty in understanding how setback saves energy?
What's the difference in terms of energy savings if I shut down my system instead of setting it at a setback.
And how correct is this statement :
"By turning your thermostat back 10°–15° for 8 hours, you can save about 5%–15% a year on your heating bill—a savings of as much as 1% for each degree if the setback period is eight hours long"

Is there a published paper or article about it?

Thanks
M

[browntigerus]

>I am having difficulty in understanding how setback saves energy?

With todays furnaces, and AC set as close as possible to hourly gains and losses you probably won't save much at all, as you will need energy and long running times to recover.

The way I see it, they are for comfort. In the winter can have a little bit warmer in the morning, cooler during day, warmer at nigh, and colder when we sleep.

In the summer I also like to run low fan at night.

I know some tstats claim up to 33% ( but this is probably on abandoned house mode)

[dan sw fl]

Hi,
I am having difficulty in understanding how setback saves energy?

And how correct is this statement :
"By turning your thermostat back 10°–15° for 8 hours, you can save about 5%–15% a year on your heating bill—a savings of as much as 1% for each degree if the setback period is eight hours long"

Think about the lack of REAL benefit ...
..... 6% of $500 ... a $30 per year savings !

[amd]

Heat loss is directly proportional to the temperature differential between the indoor and outdoor temperature.

Using a setback thermostat reduces the rate of heat transfer through the building envelope and therefore energy consumption.

The attachment summarizes a study done which proves that programmable t-stats save energy.

[amd]

Think about the lack of REAL benefit ...
..... 6% of $500 ... a $30 per year savings !

Has it occured to you that most houses cost $1000+ per year to heat with natural gas?

Reductions in peak summer electricity demand also have to be considered.

A basic programmable t-stat only costs $70-$150 unless it's purchased with a huge markup.

[Kevin O'Neill]

Has it occured to you that most houses cost $1000+ per year to heat with natural gas?

Reductions in peak summer electricity demand also have to be considered.

A basic programmable t-stat only costs $70-$150 unless it's purchased with a huge markup.

Installed cost depends on where you are, rspecially contractor labor rates. Heating and cooling costs vary by utility and climate also. There is also the size and insulation levels of the house. setback thermostats also make more sense with oversized equipment. With a properly sized system, you may not have capacity for recovery at design conditions.

Be careful of making blanket statements.

[dan sw fl]

Has it occured to you that most houses cost $1000+ per year to heat with natural gas?

A basic programmable t-stat only costs $70-$150 unless it's purchased with a huge markup.

A good HNY programmable thermostat can be procured for $32.

http://www.lowes.com/lowes/lkn?actio...033&lpage=none

If you have daytime temperature set at 65'F with setback to 55'F at night (i.e. 6 pm - 6 am), one can save ~ $65 per year in Detroit or Minneapolis.
( using an average gas rate of $1.20 / therm)

Many people would not consider a 10'F set-back after already lowering their high set-point to 65'F and the average savings might be in the $40 range.

Model house:
2,000 Square Feet single story with 210 square feet of U-0.55 windows, R-13 walls, R-30 ceiling.

Minneapolis $1.20 Detroit $1.20
____ ____ Therm ____ /Therm Savings ____ Therm /Therm Savings
____ 70 ____ 1036 ____ $1,243.20 _ $(62.40) ____ 916 _ $1,099.20 $(68.40)
____ 65 ____ 984 ____ $1,180.80 _ _____ _______ 859 _ $1,030.80 $-
____ 60 ____ 932 5.3% $1,118.40 _ $62.40 ______ 802 _ $962.40 $68.40
____ 55 ____ 880 10.6% $1,056.00 _ $124.80 ____ 746 _ $895.20 $135.60

[ampulman]

..... With a properly sized system, you may not have capacity for recovery at design conditions.

Would the fact that a new furnace might have a lower temperature rise than a 20 year old unit, also account for this? Likewise, would newer A/Cs being derated (3 ton unit actually 35,000 BTU) similarly affect recovery?

If this is true, would it be economically pheasible to have an 11 degree heating setback with a new furnace? My old clunker (oversized) can recover through most of the winter in 40 minutes or less. Would a new, properly sized unit take vastly longer to recover, thereby eliminating (any part/all) savings?

AM

[Kevin O'Neill]

Would the fact that a new furnace might have a lower temperature rise than a 20 year old unit, also account for this? Likewise, would newer A/Cs being derated (3 ton unit actually 35,000 BTU) similarly affect recovery?

If this is true, would it be economically pheasible to have an 11 degree heating setback with a new furnace? My old clunker (oversized) can recover through most of the winter in 40 minutes or less. Would a new, properly sized unit take vastly longer to recover, thereby eliminating (any part/all) savings?

AM

Temperature rise is affected by heating capacity and air flow, not age or efficiency. An oversized furnace has higher standby losses than a properly sized furnace with gravity flue.

A properly sized AC or furnace takes actual heating or cooling capacity into consideration. Some 3 ton systems will give 37000, some 34000 BTUs, some in between. It depends on what indoor coil you match it to. It has been that way as long as I can remember, about 30 years.

[cmajerus]

A good HNY programmable thermostat can be procured for $32.

http://www.lowes.com/lowes/lkn?actio...033&lpage=none

If you have daytime temperature set at 65'F with setback to 55'F at night (i.e. 6 pm - 6 am), one can save ~ $65 per year in Detroit or Minneapolis.
( using an average gas rate of $1.20 / therm)

Many people would not consider a 10'F set-back after already lowering their high set-point to 65'F and the average savings might be in the $40 range.

Model house:
2,000 Square Feet single story with 210 square feet of U-0.55 windows, R-13 walls, R-30 ceiling.

Minneapolis $1.20 Detroit $1.20
____ ____ Therm ____ /Therm Savings ____ Therm /Therm Savings
____ 70 ____ 1036 ____ $1,243.20 _ $(62.40) ____ 916 _ $1,099.20 $(68.40)
____ 65 ____ 984 ____ $1,180.80 _ _____ _______ 859 _ $1,030.80 $-
____ 60 ____ 932 5.3% $1,118.40 _ $62.40 ______ 802 _ $962.40 $68.40
____ 55 ____ 880 10.6% $1,056.00 _ $124.80 ____ 746 _ $895.20 $135.60

sorry but I have switched 2 of these out in the last week because the pins don't make contact for the heat circuit. ain't that a kick in the teeth, both paid for just a service call this summer because they could buy the stat cheaper and install it themselves, then they call me now for no heat, pay another service call, and buy my stat.
That's why I recommend getting ours, those honeywells are from China.
the vision pros and focus pros are from USA, but assembled in Mexico(same assemblers anyway)

[cmajerus]

this one is much better, not too much more. http://www.lowes.com/lowes/lkn?actio...056&lpage=none
we have used the Trane version of this one and no problems.

[manu4ever]

Thanks for nice insight. Especially the technical report about Canadian houses.
Of course savings can be achieved from setback, but what this number "8 hrs" from where it is coming, why the setback has to be 8hrs or more to maximize the savings?
I understand that why setback is preferred over shutting down because of the recovery time, it will take forever to reach back to comfort level temperature am I right? So basically I can save more energy if I shut down the system when not in home but I will be compromising my comfort. What about the difference in energy savings, is the system going to consume more energy if started from shut down state to reach a comfort state as compared to a system which is on a setback and reach a comfort state?

thanks
M

[sskzekeman]

This is fairly complicated and will give good results for homes that aren't well insulated. It also depends on the average outside air temp. If you set back say 10 degrees in a poorly insulated home, then the house temp will be lowered by the 10 deg very early in the evening, say 2 hours and the average savings would be
10*6/24/(Ti-To)
or for Ti=70 assumed inside setpoint temp
T0=20 assumed outside temp
I get 5% savings
Now if it is better insulated it may take the full 8 hours to lose the 10 degrees so the average loss is 5 degrees and the formula is then
5*8/24/Ti-To) or
3%
Now if the home is extremely well insulated it may not reach the 10 degree loss and the savings would be even smaller.
So, if you experiment, you substitute the average loss during the night, say dT,and the formula is
dT*8/24/(Ti-To)

[Carnak]

Keep your house a little colder in the winter or a little warmer in the summer