Radiant Heat Night Setback Temperatures
Should you use a night setback? If so at what settings?

Bottom line: using a night time temperature setback for a radiant heat floor slab system might save energy costs but achieving significant savings may require a large setback that in turn risks occupant comfort if the radiant heat system takes too-long to return to comfortable temperatures the next morning.

While sources and studies report a 10-15% gas heat cost savings (as an example) for conventional heating systems, most of the independent, expert sources we reviewed were of the opinion that a night setback for radiant heat high mass floor systems is not useful and is not recommended!

Example: For a super-insulated home, the US DOE points out that (for typical hydronic heating systems)

Setback for super-insulated homes should be avoided. Because the heating loads are so small, the potential savings from operating in setback mode would be minuscule compared to the extra cost of the baseboard needed to recover quickly. - US DOE 2014

We recommend that you experiment with your building and night setback temperatures to find how much you can set back the indoor temperature at night for a given outdoor temperature. Notice how long it takes for your indoor space temperature to return to its comfort zone.

If you want to try for energy savings by using a night setback with an in-slab radiant heat floor system, to avoid an hours-long recovery period in the morning,

start with a small night setback of just 2-3 deg F for a radiant heated concrete floor slab.

This is much less than a recommended 16 deg. setback for more conventional heating systems, e.g. in the U.K.

And a very aggressive temperature setback, say from 66 deg F. down to 50 deg F. in a very cold climate may mean that the building temperature never recovers adequately in the following day.

A second risk for some radiant heat slab systems is that the operator overheats the slab to try to return to comfortable temperature quickly.

In our test cabin in northern Minnesota, when the cabin is unoccupied for weeks or longer we keep the indoor temperature in the high 40s F - just to prevent frost heave damage.

But on return of occupants in this poorly-built radiant heat slab it takes three to five days for the floor temperatures to warm enough for comfortable barefoot or socked-foot walking.

The optimum night temperature setback for radiant heat slab systems varies by building and climate conditions.

If the night time setback temperature drop for a radiant heat slab system is too great, it may take several hours to bring back comfortable indoor temperatures the next morning.

An automatic thermostat that learns the time necessary to return the building to its target daytime temperature can handle this task for you. The result, for a poorly-insulated slab or building will be a relatively-short night setback period and thus a smaller night-time energy cost savings.

A very well insulated slab and building may keep enough neat overnight that even with a thermostat set-back for night time hours the building temperature can drop very little. In this case the worry of wasting energy by causing the heater to work too hard for too long in the morning won't occur.

Radiant Slab Floor Heat Nighttime Temperature Setback Settings Research

• General heating systems setback advice is at
  
  THERMOSTAT SETBACK ADVICE
• Arena, Lois B., MEASURE GUIDELINE: CONDENSING BOILERS—OPTIMIZING EFFICIENCY AND RESPONSE TIME DURING SETBACK OPERATION [PDF] (2014) NREL, The National Renewable Energy Laboratory, by Steven Winter Associates, Inc. of the Consortium for Advanced Residential Buildings (CARB),
  
  Excerpt:
  
  4.1.2 Who Shouldn’t Use Nighttime Setback
  
  For any home with a hydronic heating system, if the answer to any of the following questions is “yes,” the project is probably not right for nighttime setback and should be maintained under constant temperature operation.
  
  1. Does the home have a boiler that cannot modulate its output?
  
  2. Is the home insulated to levels significantly better than the 2009 IECC?
  
  3. Is the heat emitter radiant flooring or radiant panels?
  
  4. Are the walls or floor of the conditioned space constructed with concrete? If yes, is that concrete uninsulated to the interior?
  
  5. Do the occupants expect/desire a recovery time shorter than 1 hour?Koenig, Kenneth. "Gas furnace size requirements for residential heating using thermostat night setback." ASHRAE J.;(United States) 84, no. 2 (1978).
  
  Abstract:
  The effects of night setback of thermostats and of furnace sizing on both energy savings and temperature recovery times were studied using a digital computer program to simulate in detail a system consisting of a residential structure, its environment, gas-fired hot air heating equipment, and thermostat.
  
  The results show that the optimum furnace size and level of setback depends on the relative importance placed on energy savings and short recovery.
  
  It is recommended that the use of night setback be encouraged as a means to reduce energy consumption.
  
  In applications where the furnace is fixed and greatly oversized, setback should be as high as possible until the morning recovery becomes objectionable.
  
  Alternatively, setback should be no more than 5/sup 0/C in cases where the homeowner may take undesirable action to eliminate prolonged recovery on very cold days.
• Meier, Alan, Cecilia Aragon, Therese Peffer, Marco Pritoni. THERMOSTAT INTERFACE AND USABILITY: A SURVEY [PDF] Lawrence Berkeley National Laboratory, Environmental Energy Technologies Division September 2010, (2011).
• NC SETBACK TEMPERATURE CONTROL [PDF] North Carolina DEQ, retrieved 2023/05/31, original source: deq.nc.gov/environmental-assistance-and-customer-service/ias-energy-efficiency/opportunities/setback-temperature-control/download
  
  Excerpts: DOE projects an energy cost reduction of 5% - 12% with a 3°F to 10°F setback and a 9% - 18% energy cost reduction with a 10°F to 20°F setback.
  
  Setbacks temperatures are often dependent on recovery time of the hvac equipment capacity to reestablish the normal occupied building temperature prior to people arriving for work or students for school.
  
  The worst case scenario is when a sustained extreme temperature excursion below (winter) or above (summer) the setback temperature for a short period of time (several days).
  
  Under these conditions hvac capacity will determine the transitional time to reach occupied building setpoint and thus probably an earlier start in the morning for ramping to the accepted room temperature.
• PM Magazine, BEAUTIFUL HEAT SETS BASELINES FOR RESIDENTIAL RADIANT HEAT EFFICIENCY IN CANADA [PDF] (2014) Plumbing & Mechanical Magazine, 2401 W. Big Beaver Rd., Suite 700, Troy, MI 48084 USA Tel: 248-362-3700 - retrieved 2023/05/31, original source: pmmag.com/articles/99852-beautiful-heat-sets-baselines-for-residential-radiant-heat-efficiency-in-canada
  
  Excerpts:
  
  Beautiful Heat, Canada’s not-for-profit association dedicated to educating Canadians about the option of residential radiant heat, recently engaged ICF Marbek to complete an Energy and Comfort Study, measuring energy consumption of radiant heat vs. forced-air systems.
  
  Based on the results from energy simulations of different home types in six regions of Canada, annual energy savings of up to 18% could be realized with radiant heat.
  
  

  It is important to note that energy savings as a result of radiant system multi-zoning in residential buildings are a direct function of occupant behaviour; and therefore, the calculated savings will vary significantly for each based on the thermostat setback setting.
  
  In order to capture the possible range of energy savings, ICF Marbek modeled each house with the following two setback setting scenarios: 

  • Minimum setback temperature: 1° C reduction during the night for “Living” and “Basement” zones and during the day for “Bedrooms” zone.  

  • Aggressive setback temperature: 4° C reduction during the night for “Living” and “Basement” zones and during the day- for “Bedrooms” zone.

  Energy savings for a radiant system multi-zone system in two-story homes in Toronto and Edmonton are expected to be in the range of 11 GJ to 26 GJ, depending on thermostat setting in which the occupant is continuously managing the thermostat setback temperature.
  
  This corresponds to a range of 10% to 18% of the total baseline energy consumption.

  Multi-zoning setup in two-story homes in Toronto and Edmonton can increase the potential energy savings resulting from the use of radiant heating system, by a range of 1% to 6% of baseline consumption, depending on the thermostat setting in the different zones.
  

• Radiant Floor Co. Radiant Floor DESIGN & INSTALLATION MANUAL 11th Ed. (2007) Radiant Floor Company Barton, Vermont Fax: 1-802-525-3940 1- 866-WARM-TOES 1-866-927-6863 1-802-525-1132 www.radiantcompany.com retrieved 2019/09/20 original source: https://www.radiantcompany.com/wp-content/uploads/radiantcompanymanual.pdf
• RADIANT HEAT MISTAKES - Additional radiant heat and tubing installation design criteria
• UK Underfloor Heating, SHOULD my UNDERFLOOR HEATING BE LEFT ON ALL DAY [PDF] (2016) U&K Underfloor Heating, retrieved 2023/05/31, original source: https://www.ukunderfloorheating.co.uk/should-my-underfloor-heating-be-left-on-all-day/
• US DOE, HYDRONIC SYSTEMS: DESIGNING for SETBACK OPERATION [PDF] (2014) U.S. Department of Energy, retrieved 2023/05/31, original source: energy.gov/sites/prod/files/2014/06/f16/ba_case_study_hydronic_setback.pdf
• Watson, Richard D., ADVANTAGES of RADIANT HEAT [PDF] (1992) - retrieved 2023/05/31, original source https://www.radiantec.com/about-radiant-heating/technical-description/ (this website page was reprinted from Fine Homebuilding)
  
  Excerpt:
  When applied to the sizing of a radiant system, conventional heat-loss analysis often includes a reduction in design temperature from 70°F to 65°F and a 10% to 25% reduction in building air infiltration, exfiltration, stratification and glass heat loss. The average 65°F radiant comfort temperature with 59°F day/night setback should reduce building heat load by 25% to 35% over convective systems.
• WBDG, RADIANT FLOOR HEATING, COMMERCIAL [PDF] NIBS, National Institute of Building Science, 1090 Vermont Avenue, NW, Suite 700 , Washington, DC 20005-4950 USA TEl: (202) 289-7800, WBDG, Whole Building Design Guide, - retrieved 2023/05/31, original source: https://www.wbdg.org/FFC/ARMYCOE/TECHNOTE/technote15.pdf
  
  Excerpt:
  
  High mass radiant floor heating systems do not typically respond quickly to a change in load due to the thermal mass of the floor. Therefore, night setback control is not feasible for high mass floor radiant heating systems, unless long durations of unoccupied spaces occur such as in a chapel.