What temperature should I set my thermostat to?

We all have different personal comfort preferences.  The older we become, the warmer we typically like the temperature to be. 

Energy usage is another matter.  Most folks realize that the warmer we set the thermostat in the winter, the higher our heating bills will be.  And the cooler we set it in the summer, the higher our cooling bills will be.  In both cases we’re making the system run longer.  Our home is our castle, but we must remember that our castle is merely a box in the huge outdoor environment that is seldom the same temperature as the inside of our house.  Heat moves from hot to cold.  And the greater the difference between the two temperatures there is, the faster it moves.  Therefore, the greater the difference between the indoor and outdoor temperatures is, the more energy it takes to keep the house at a given temperature.

In the winter, we keep our houses warmer than the outside temperatures.  So we have “heat loss” from the house to the outdoors.

In the summer, we keep our houses cooler than the outside temperatures.  So we have “heat gain” from the outdoors to the indoors.


How does a conventional thermostat work?

The conventional thermostat’s job is to attempt to make the heating or cooling system run long enough to maintain a certain temperature in the house as heat is being lost from the house in the winter or gained into the house in the summer.  It does this in an automatic manner, by sensing the air temperature at the thermostat and turning on or off the system to try to maintain the temperature we have it set to maintain.

But most folks don’t know that it isn’t designed to keep the temperature at exactly the temperature they set in on.  There must be at least 1 degree of difference between the temperature it shuts the system off and the temperature it turns it back on at.  The thermostat does this automatically, and we just don’t realize it (when it’s working properly).

This number of degrees of difference in temperature between the on and off cycles is called the temperature differential. 

This process involves 2 things: Time and temperature.

Here’s where time comes in.  Remember that heat moves from hot to cold, and the greater the difference in temperature between them is the faster it moves from hot to cold.  So let’s say that it’s winter time and it’s 10 degrees outside.  We have the thermostat set to maintain 70 degrees inside.  We’re losing heat out of the house to the outdoors at a given rate per minute, because it’s 70-10 = 60 degrees cooler outside than it is inside.  But our furnace is capable of putting that lost heat back at a faster rate than we’re losing it at.  When the thermostat senses that the temperature has dropped a degree to 69 degrees, it turns the furnace on.  And the temperature in the house starts to increase over time. 

When that temperature reaches 70 degrees, it turns the furnace back off.

And with the furnace off, the temperature in the house slowly begins to drop to 69 degrees again.

And the process repeats itself over and over again.


If that differential (number of degrees between the on and off temperature) isn’t big enough, the system shuts off 70 degrees, but it kicks back on at 69.5 degrees.  So it doesn’t stay off long enough.

And if the differential is too big, like over 2 degrees, the thermostat shuts off the system at 70 degrees, but doesn’t kick the system back on until the house gets down to 68 degrees.  And the average human can feel about a 1.5 degree change in temperature in a short period of time.


Will my house heat up faster if I turn the temperature way up higher in the winter when I get home?


Will my house cool down faster if I turn the temperature way down lower in the summer when I get home?

Most of us have conventional type thermostats operating conventional type heating and cooling units. 

The systems operate at only one speedfull speed.  So the thermostat is merely turning them on or off. 

So the house will reach the actual desired temperature in the exact same amount of time.

And if you forget to set the thermostat back to the temperature you actually want the house to be, the furnace will keep heating the house past that temperature in the winter, or the A/C will keep cooling the house below that temperature in the summer.  And both are a waste of energy, because you’re making the system run longer. 

Some folks have more expensive multi-stage heating and/or cooling systems, and/or zoned damper systems all of which are more complex than conventional systems.  But the vast majority of systems in our area are of the conventional (single speed) type with no damper systems.


What is the lowest temperature I should set my thermostat for in the cooling season?

This varies a lot from system to system, and can’t be determined without checking numerous things in that given system.  But it generally isn’t a good idea to set the thermostat below 70 degrees in the cooling cycle.  I’ve seen systems where the airflow was so poor that I advised them not to set the thermostat below 78 degrees in the cooling cycle because it would cause the cooling coils to get too cold and ice up.   You want the cooling coil to be cold, but definitely not to freeze.  And generally, the lower you set the thermostat temperature in the cooling cycle, the lower the coil temperature becomes.  And generally, the less volume per minute of air you have moving through the coil, the colder the coil gets.  So you’re really asking for trouble if you set the thermostat too low and there are airflow issues.  And once the ice starts to form on the coil and begins to spread, you really have an airflow issue, because all of the air moving through the system has to pass through the open areas in that coil.  So it can ice up completely very fast when the ice itself is blocking part of the open coil area.


(IMPORTANT) You can also cause mold to grow inside the walls by setting the cooling temperature too low.  This typically occurs when warm, humid outside air enters the external wall cavities, and comes in contact with the much cooler temperature of the opposite side of the wall material inside the rooms in your house.  The warm, humid outside air then cools down to the point that it reaches dew-point and condenses on that wall material.  This provides an ideal environment for the mold, as the mold has the two things it needs to thrive – moisture and food (the wall materials).  Mold remediation companies must be called, and the walls must be opened to get rid of the mold.  And sometimes it reaches such an advanced point that it cannot be eliminated, and the house must be demolished.  Houses are not built like refrigerators.  Walls aren’t air tight.  Don’t court disaster by setting the cooling temperature too low.  You’re risking property and health damage by doing so.


Which kinds of thermostats are better?

Digital thermostats (the ones with digital displays) are typically better at maintaining a constant temperature and a constant temperature differential compared to analog thermostats (the old fashioned kind).  Until recently, there were 2 types of analog thermostats being manufactured – the mercury bulb thermostats and the bi-metal thermostats.  The mercury thermostats were more expensive and typically a bit more accurate IF they were properly leveled on the wall.  These are now being phased out because they contain mercury.  So the bi-metal thermostats are the only analog models currently available.  Both of these thermostats inherently respond very slowly to changes in temperature.  So they are equipped with heat anticipators (little heaters) to fool the thermostat into shutting off more quickly in the heating cycle, so it wouldn’t “overshoot” the temperature setting.  The digital models contain electronic sensors, i.e. thermistors, which don’t need to be fooled, because they respond faster to changes in temperature. 

So I prefer the digital models.

Most of the digital models are also equipped with an electronic time delay mechanism to prevent the thermostat from accidentally turning the A/C system off and back on too quickly, which is very hard on the outside unit.  So that’s another reason I like them.


Should I buy a programmable digital thermostat?

Some of the digital models are also available in programmable versions.  These models typically allow you to specify up to 4 times each 24 hour period you would like the thermostat to automatically change the temperature setting.  This is typically done to save energy during the heating season.

Here’s the programming strategy.  You don’t want the house to be as warm during one or two periods per day.

So if you were gone from like 9 to 5, the typical program would work like this:


Time                    Temperature Setting     Activity

7 00 AM                           70                         Waking up

9:00 AM                           65                         Leaving the house

5:00 PM                            70                         Returning home

11:00 PM                         65                         Bedtime


The overnight savings allow you to save energy by not heating the entire house to a higher temperature when you’re only in the bedroom(s).  Obviously you could do something similar manually without the programmable thermostat.  But what you can’t do (because your arms aren’t long enough) is to have the house temperature automatically turned back up to 70 degrees before you get out of bed, or before you get home. 


That schedule with two set-backs (drops in temperature) per day represents the optimal savings of any program you can use. 

But if you’re home all day, you would only want to use the overnight settings.  So you’ll save less. 


These thermostats are available in the following versions:

5-1-1.  You have one program for Monday through Friday, and one for Saturday, and one for Sunday.

5-2.              You have one program for Monday through Friday, and a separate program for the weekend.  (Saturday and Sunday share a program).

7 day    You can program each day of the week individually.


The amount you’ll save using either one or two cooler periods depends on how efficient your heating system is.  With a less efficient furnace, you can save up to 1/3 on your gas consumption. 


When not to set programs for.


In our area, it generally isn’t a good idea to program the thermostat for the A/C season, especially during the hottest part of the A/C season.  And if you have a heat pump system, you generally wouldn’t want a program in it for the heating season either.  There are, however some exceptions.

With the A/C system, the determining factors are how oversized the cooling system is for that house, what times of the day it will be operating, how long you’re gone, and how hot those days are, etc.

You don’t want to overtax the outdoor equipment or make it operate for longer periods of time at less than efficient outdoor conditions.  The rate of heat output from a furnace is fairly constant.  But the cooling output of the A/C decreases as the outdoor temperature increases.  An A/C system uses more power and delivers less BTU per hour of cooling the hotter it gets outdoors.  And when the unit is off for a long time during a setback period, the heat and humidity re-enter the house.  (Humidity is also heat that must be removed by the A/C system).  So if you utilize a setback program that allows the system to be off until 5:00 PM (the hottest part of the day), you’re forcing the system to work under some really strenuous conditions, because the outside unit can’t deliver as many BTU, and there is a LOT MORE BTU inside for it to remove.

So it’s taking a beating and not giving much back in return.  So your energy savings would be minimal, and you’re overtaxing the equipment.  It’s working harder per minute, and it has to run a lot of minutes to muscle the temperature down several degrees.


I really love riding my bicycle in the summer.  But I would never wait until it was 95 degrees outside to ride it.  And I wouldn’t have a huge feedbag full of food strapped on my face gobbling it down while I was in a 20 mile race during that time of the day either.  That’s what we’re asking our A/C to do when we set the temperature back and force it to operate during the hottest part of the day in that hot outdoor environment and at the same time feed it a big plate of heat and humidity from inside the house.

And as more people do this, it creates greater problems with the “peak demand” the power companies are seeing. 

The scenario in the previous paragraph causes each individual system to use more electricity per minute at a similar time of day.  And that strains the power grid, i.e. the source of everyone’s electricity.  And in the worst-case scenario, this can cause entire cities to lose their power and have “blackouts”.  This is one of the reasons why the electric companies were giving away those “radio controlled” digital setback thermostats when you signed a contract allowing them to remotely control (limit) your AC usage during the hottest times of the day.  It allowed them to minimize the peak demand by shutting off your AC system for x period of time during “peak demand” periods like that.  So it isn’t just hard on your system – it taxes theirs too.  And even if they have the total power capacity to produce enough power to supply everyone’s needs during those periods, the entire delivery system must be considered, like the transformers located near your home that become weakened and blow out when they’re overloaded.


I’ve seen the results of studies that were conducted to determine the savings from using summer setback strategies.  The scope of these was too small and/or they were conducted in climates that were different from ours, or they were done decades ago when A/C sizing strategies were much different (oversizing was more common) and the “average” home was much different than today.  None of these studies addressed the negative impact on the system.  They were focused solely on electrical consumption.  So factors like wear and tear on the system and comfort levels were beyond the scope of the studies.  But one of them did mention the “peak demand” problems it could cause.


I’ve also seen and dissected numerous calculators and spreadsheet applications designed for the purpose of estimating energy savings for A/C setback strategies.  Unfortunately, without exception, the “assumptions” used by them were too faulty and incomplete to yield even “ballpark” results for a given actual scenario.  Some of this could be improved upon by changing many of the assumptions to input questions in order to form a more accurate basis for the calculated savings.  We as individuals are more concerned with the results on our system, rather than an average of the results from other systems.  So the more input questions, rather than assumptions there are, the more accurate the results will be.


Update 7/29/12.  The latest wrinkle in all this is that the electric companies in some areas are abandoning the plan of attempting to control peak usage by giving away those thermostats that they can remotely control, i.e. to turn off everybody’s AC in a certain area in that city.  That hasn’t proven effective enough in reducing peak demand, obviously because so few of those thermostats were installed by them.  So the new plan is to create a higher rate per kWh during the peak demand hours of the day, i.e. when everyone comes home. 

The effect of that is far reaching, because the AC isn’t the only appliance that people tend to use more of at that time of day.  They turn on the TV’s, ovens, clothes dryers, washing machines, computers, and start opening the refrigerator a lot more after they get home.  So the increase in the kWh rate for that time of day could really add up. 

And most thermostat setback strategies would cause the AC system to run non-stop during that time of day.  So it pretty much puts a big dent in any savings you’d get from using the setback.





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