HVAC Thermostat Setback Settings

How to set or adjust the heating or cooling thermostat setback in buildings & how much will we save on cooling or heating costs?

Save Heating Cost & Air Conditioning Costs by Thermostat Setback Adjustments

Substantial heating or air conditioning cost savings can be achieved by manually setting back your thermostat when the building is unoccupied or when occupants are asleep.

The thermostat is set several degrees (or more) cooler during heating season or warmer during cooling season.

The settings of thermostat "mode" switches into "heating" or "cooling" and the use of other thermostat switches to control an air conditioning blower fan or heating system blower fan are discussed in detail

at OTHER SWITCHES on a ROOM THERMOSTAT.

Lots of websites offer calculators that will tell you how much you'll probably save in energy costs for every degree you set back your heating or cooling thermostat.

Just don't set the thermostat down so low that you suffer costly damage from freezing pipes in winter or mold from high humidity in summer.

More details about heat transfer rates are at

HEAT LOSS R U & K VALUE CALCULATION 

Where an InspectPedia reader, himself a physicist, asked:

How do I Explain the Effectiveness of Insulation Variations under a Heated Driveway?

This link may be the start to resolving my problem, but I need more help.

I'm trying to understand the "effectiveness" of various configurations of insulations underlying a heated driveway.

I have instrumented a heated driveway with 18 calibrated temperature sensors at various depths--both above and below the insulations--and four insulation configurations: no insulation, 1-layer of a bubble-foil insulation (R=1.6), 2-layers of the bubble-foil insulation (R=3.2) and a 2" closed-foam insulation (R-10).

The data clearly shows the markedly different thermal response as the boiler heats the driveway but other than the temperature data, I'm at a loss in trying to explain the data in more quantitive terms.

Any help readers of this site can provide me leading to a meaningful analysis of the data will be appreciated. (I'm a retired physicist, but with no experience in thermodynamics.) - Dennis Douglas · Feb 5, 2021

Daniel Friedman Replied: Without enough insulation under your driveway (or under a radiant heated slab) you'll never get ahead of the heat sink effect of Mother Earth

Thank you for the posting, Dennis, that's great work and quite interesting.

The very basic RUK formulas are at Formulas and an explanation of how we use R U or K values to determine the rate of heat loss at a building (or heat gain if we are cooling it)

at HEAT LOSS in BUILDINGS , within that article at a section titled Formulas to Calculate the Rate of Heat Loss Per Hour for a Building Using it's "R" Values or "U" Values.

I hope you'll also take a look at the empirical discussion at RADIANT HEAT MISTAKES 

where I note that the installing contractor had the "alternative fact" view that we could heat up mother earth enough that eventually we'd stop losing heat down through the inadequately-insulated radiant heat slab floor into the earth below;

The empirical point is that when the un-wanted heat transfer rate down into the soil exceeds the desired heat transfer rate up into the radiant-heated floor (or in your case driveway) the heating source may never be able to pump enough BTUs per hour into the floor (or driveway) to make it comfortably warm.

In the case of a heated driveway we have the greater transfer rate down through soil, impeded by insulation successfully if there's enough insulation material, and the slower heat transfer rate into cold air above the drive, or on occasion, into snow and ice forming on the drive surface; thence the energy needec to melt those off needs to be included in the calculation.

warmlyyours[dot]com offers

Square footage of heated area x 50 watts = Total wattage

Total wattage ÷ 1,000 = Total kilowattage

Total kilowattage x kWh rate = Hourly operating cost

but that does not express heat loss rates, ignores R-values of insulation under a drive, and fails to accomodate the observation that the rate of heat transfer between materials increases exponentially as the temperature between them increases.

Dick Chu, (perhaps listed as Richard Chu), a research Engineer at IBM, working probably at IBM's East Fiskhkill NY research lab and chip fabrication plant (where I too, worked in the 1980s) and perhaps at IBM Yorktown Research, did some of the seminal work on heat transfer that you might want to research.

DEFINITION of HEATING, COOLING & INSULATION TERMS 

and at

DEFINITION of HEATING, COOLING & INSULATION TERMS 

Supporting Research on Heat Transfer Rates & Dr. Richard Chu

• Chu, Richard C., Robert E. Simons, Madhusudan Iyengar, and Lian-Tuu Yeh. "Thermal management of flip chip packages." In Advanced flip chip packaging, pp. 413-469. Boston, MA: Springer US, 2013.
• Chu, Richard C., Robert E. Simons, Michael J. Ellsworth, Roger R. Schmidt, and Vincent Cozzolino. "Review of cooling technologies for computer products." IEEE Transactions on Device and materials Reliability 4, no. 4 (2005): 568-585.
• Chu, Richard C. "The challenges of electronic cooling: past, current and future." J. Electron. Packag. 126, no. 4 (2004): 491-500.
  
  Abstract:
  
  This paper represents my personal recapitulation of my 4 decades of continuous involvement in all phases of electronic cooling, from conceptual design, through engineering development to product implementation.
  
  The cooling designs that we applied successfully in the past are reviewed chronologically.
  
  The challenges we are currently facing are also discussed and an attempt is made to forecast the challenges that will confront the electronics cooling community in the near and distant future.
  
  The paper includes a summary of IBM sponsored research spanning a period of 25 years at 12 universities on a wide range of topics related to electronic cooling technology.
• Yeh, Lian-Tuu, and Richard C. Chu. "Thermal management of microelectronic equipment: heat transfer theory, analysis methods, and design practices." (No Title) (2002).
• Chu, Richard C. "A review of IBM sponsored research and development projects for computer cooling." In Fifteenth Annual IEEE Semiconductor Thermal Measurement and Management Symposium (Cat. No. 99CH36306), pp. 151-165. IEEE, 1999.
• Chu, Richard C. "Heat transfer in electronic systems." In International Heat Transfer Conference Digital Library. Begel House Inc., 1986.
• Campbell, Levi A., Michael J. Ellsworth Jr, Madhusudan Iyengar, Robert Simons, and Richard Chu. "Experimental Investigation of the Heat Transfer Performance of Arrays of Round Jets With Sharp-Edged Orifices and Peripheral Effluent: Convective Behavior of Water on a Heated Silicon Surface." In Heat Transfer Summer Conference, vol. 47349, pp. 629-636. 2005.

Thermostat Setback Short Cycle Warning

Watch out: setting back the boiler control high limit during warmer seasons when the heating requirement is less has long been touted by some heating professionals as a way to reduce hot water heating boiler short cycling.

That's because short-cycling (a short "on" cycle) is an inefficient way to run a hot water boiler since the boiler never runs long enough to reach its efficient operating temperature.

That is to say that when a cool or cold hot water heating boiler first turns on, because the boiler's internal temperatures are low, the oil sprayed into the combustion chamber is partly cooled and is not fully consumed. You're wasting some oil.

On a longer oil burner on cycle, say 8-10 minutes or longer, the boiler is spending more of its "on time" at full operating temperature and the oil waste is less so the overall system's efficiency improves and its operating cost to heat the building is less.

Here's a quote from an expert on this question:

Conventional wisdom among heating professionals says boilers exhibit a setpoint effect due to short cycling, which results in lower seasonal boiler efficiency during seasons when the system load is lower.

Because the boiler is said to be less efficient during some seasons, energy-savings measures can then be portrayed as not beneficial. This article introduces and examines seasonal boiler efficiency (setpoint effect) and the effect of short cycling on boilers. (Hoback 2025)

And

Short cycling is a related issue. If a boiler runs only a short time because of reduced load, then there would be expected to be more energy losses related to the cycling on and off of the boiler.4 It is conventional wisdom that short cycling would be more likely when the demand is lower. (Hoback 2025)

And from the article's conclusions:

The data collected for discussion in this article indicate energy conservation measures are very unlikely to have a significant effect on boiler efficiency. (Hoback 2025)

• Alan S. Hoback, EXAMINING BOILER SETPOINT EFFECT [PDF] ASHRAE Journal, March 2022, pp. 36-44 - local copy saved as Boiler-Setpoint-Hoback.pdf
• Eoff, D. 2008. “Understanding fuel savings in the boiler room.” ASHRAE Journal 50(12):38.
• McDonnell & Miller's HEATING BOILER BASIC OPERATION [PDF] McDonnell & Miller, retrieved 2019/11/06 original source: controlcomponents.com.au/Open-File/557/Boiler%20control%20basics.pdf - can also be downloaded free from InspectApedia.com
  
  This article explains why using an automatic water feeder to keep the proper water level in a steam boiler can avoid burner short cycling.
  
  Excerpt:
  
  A boiler attendant might put too much water in a steam boiler. He doesn’t have an automatic feeder and he’s tired of checking the water level every day ...
  
  Without enough room to break free of the water, the steam will now carry water up into the system piping. This leads to higher fuel bills, uneven heating, water hammer, scale formation in the boiler and burner short-cycling. ...
  
  Automatic water feeders help you avoid these problems.

How much will be saved on heating or cooling costs by setting back the thermostat?

How much can I save on heating costs by using a manual or programmable thermostat set-back?

According to the U.S. Department of Energy,

By turning your thermostat back 10° to 15° for 8 hours, you can save 5% to 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. - U.S. DOE (2014)

How much can I save on cooling costs by changing the air conditioning thermostat settings?

The U.S. DOE has similar advice about savings on cooling or air conditioning costs:

In the summer, you can follow the same strategy with central air conditioning by keeping your house warmer than normal when you are away, and lowering the thermostat setting to 78°F (26°C) only when you are at home and need cooling.

Although thermostats can be adjusted manually, programmable thermostats will avoid any discomfort by returning temperatures to normal before you wake or return home. - U.S. DOE (2012)

The settings of thermostat "mode" switches into "heating" or "cooling" and the use of other thermostat switches to control an air conditioning blower fan or heating system blower fan are discussed in detail

at THERMOSTAT SWITCHES, EXTERNAL

You can use a special calculator like one provided by Warmair.Net ( warmair.net/html/Thermostats.htm ) to compute how much money you're likely to save by setting back your thermostat.

For example, if your normal thermostat setting is 70 °F. and you set it back to 60 °F. and if during that time the outside temperature is hovering at 40 deg .F., you will use about 33% less energy during that period.

Best Thermostat setback advice for heat pump systems are different from other heating systems

Watch out: the U.S. DOE gives some important advice for heat pump thermostat settings to save money.

The bottom line is that unless you are using a special programmable thermostat that is designed to work with heat pumps, the best thermostat setting to save heating cost is to keep a moderate temperature setting - say 65°F rather than dropping the heat pump thermostat back to a much lower setting during the setback period.

Best thermostat setting for heat pumps in cooling mode:

Programmable thermostats are generally not recommended for heat pumps. In its cooling mode, a heat pump operates like an air conditioner, so turning up the thermostat (either manually or with a programmable thermostat) will save energy and money.

Best thermostat setting for heat pumps in heating mode:

But when a heat pump is in its heating mode, setting back its thermostat can cause the unit to operate inefficiently, thereby canceling out any savings achieved by lowering the temperature setting.

Maintaining a moderate setting is the most cost-effective practice.

Recently, however, some companies have begun selling specially designed programmable thermostats for heat pumps, which make setting back the thermostat cost-effective.

These thermostats typically use special algorithms to minimize the use of backup electric resistance heat systems. - U.S. DOE (2012)

How many degrees should we set the thermostat back ?

Watch out: Lots of websites offer calculators that will tell you how much you'll probably save in energy costs for every degree you set back your heating or cooling thermostat. Just don't set the thermostat down so low that you suffer costly damage from freezing pipes in winter or mold from high humidity in summer.

For details on how to avoid frozen pipes in buildings

see WINTERIZE A BUILDING.

Also see HEAT WON'T TURN OFF - Stop Unwanted Heat explanation of why heat may continue to come out of radiators or baseboards even though you have turned down the thermostat. Stop wasted heat by making sure you are not delivering heat when it is not needed or not wanted.

If your setback temperatures are taken too low this advice may not provide the most comfortable approach in all cases, nor the smartest. For example, in using electric heaters to warm a slab on grade cabin in Northern Minnesota, the subject of our

article

RADIANT HEAT MISTAKES,when heat was left off completely in zero and sub-zero weather the concrete slab was so cold that even after air temperatures in the cabin reached 62°F, heat had to be kept at high levels for two to three days to warm the floor slab sufficiently for the building comfort to stabilize.

And even where freezing pipes and water damage are not a concern (the cabin above has no plumbing), allowing building temperatures to reach zero and sub-zero levels can invite cracks and damage to ceramic tile floors or drywall walls and ceilings.

More modest setback temperatures that keep the building at above-freezing levels are fine however.

To really save on heating costs 

see HOW TO REALLY CUT HOME HEATING COSTS

and see HEATING TUNEUP PRIORITIES

For example, at AQUASTAT HI LO DIFF SETTINGS we explain that in addition to improving boiler and heat transfer efficiency by using the optimum settings on a boiler aquastat control, you may want to completely disable the LO limit (AQUASTAT LO & DIFF DISABLED ) on combination aquastats that are installed on boilers that do not include a tankless coil for which that control is intded.

What is the Effect of Having to Overcome Thermal Mass Losses when Temperatures have Been Set Up or Down?

According to our own US DOE source,

A common misconception associated with thermostats is that a furnace works harder than normal to warm the space back to a comfortable temperature after the thermostat has been set back, resulting in little or no savings.

In fact, as soon as your house drops below its normal temperature, it will lose energy to the surrounding environment more slowly.

The lower the interior temperature, the slower the heat loss.

So the longer your house remains at the lower temperature, the more energy you save, because your house has lost less energy than it would have at the higher temperature.

The same concept applies to raising your thermostat setting in the summer -- a higher interior temperature will slow the flow of heat into your house, saving energy on air conditioning. - U.S. DOE (2012)