Revised March 2013
Would it save more energy to only set back our heater (a heat pump)’s thermostat to 60 degrees at night (rather than to 55,) since then the heating system wouldn't have to work so hard to get to 70 degrees in the morning?
The answer in general is "no." Increasing the amount of setback increases the amount of energy saved. By reducing the internal temperature of your building during the heating season, you always reduce heat loss. (Heat loss is proportional to the temperature difference between inside and outside temperatures.)
When the setback period is over and the heating equipment begins the warm-up period, the equipment will have to run slightly longer to restore the space to your desired daytime temperature (70 degrees.) However, the heating system will have been off more during the setback period, resulting in less overall energy use.
In the case of gas-fired or district heating systems, an added bonus is that the equipment’s overall efficiency improves because it has to cycle less often during the setback period (reducing the number of pre- and post-purge cycles.)
However, there are some situations where increasing the amount of setback may not save energy or money. For example:
- If your home is heated by electricity, your utility may have "time-of-day" or "peak-demand" rates. If your warm-up period is during a high time-of-day rate or increases peak demand, you will pay a higher rate or demand penalty for that use.
- Although still more efficient when the thermostat is setback to 55, older heat pump heating systems with backup electric resistance heating coils may use the electric resistance coil instead of the heat pump coil. When replacing one of these heat pumps, consider one that can meet the heating requirement with compression heat alone. There are heat pumps that can operate down to -20 degrees at 75% of rated capacity.
But, in the meanwhile, for your existing equipment, if the backup comes on, the efficiency of the heat pump is reduced (from its normal coefficient of performance) to that of straight electric resistance heating. However, this can be fixed by installing an energy code required thermostat that stages the warm-up process (also called ‘optimal-start’) to force the heat pump heater to come on early enough to meet the required temperature at the right time. This new thermostat should also include the high temp lock out feature to prevent the electric resistance heater from energizing when the outdoor air temperature is above 35 degrees.