BackPage Primer from Environmental Building News
November 1, 2009
Programmable Thermostats
Programmable thermostats can save a lot of energy—during both heating and cooling seasons—but to do so they have to be operated correctly. Unfortunately, they often aren’t, which is why some studies conclude that programmable thermostats aren’t worthwhile.
A thermostat controls a heating (and cooling) system, turning it on and off to maintain the set-point temperature; larger buildings have multiple zones with separate thermostats. With heating, the lower the setting, the less heat is called for and the less energy will be used. A common rule of thumb is that for every degree Fahrenheit you turn down the thermostat, you save about 2% on heating costs. So, if you normally keep a house at 70° and drop the setting to 65°, you’ll reduce heating costs by about 10% (5 x 2%).
If you lower the thermostat for a portion of each 24-hour day during the heating season, say at night, there will be proportional savings. For example, if your thermostat is set to reduce your house temperature by 10° for eight hours each night (one-third of the 24-hour day), your annual savings would be about 7% (10 x 2% ÷ 3).
The simplest thermostat has a single setting for the desired temperature. These can work fine for saving energy as long as users are willing to adjust them manually—for example, turning it down before going to bed or leaving the house.
Setback thermostats have two different set-points: one for the daytime and another for night. You can adjust the set-point temperatures separately for day and night and also the timing of the setback.
Programmable thermostats provide much more flexibility—but are also more complicated. They allow you to designate multiple setback periods each day and differentiate weekdays from weekends. For example, one might set a programmable thermostat to maintain 70°F (21°C) from 5:30 a.m. until 8 a.m. during the weekdays. Then, for homeowners working outside the home, the temperature can be allowed to drop back down—to, say, 55°F (13°C) from 8 a.m. to 4:30 p.m. An evening setting boosts the temperature back to 70°F, and then at night (say after 11 p.m.), the setting could be dropped to 50°F (10°C). On weekends the thermostat is programmed with different times and temperatures.
Most programmable thermostats have handy override features. Simple up and down arrows manually raise or lower the temperature (the thermostat re-sets to the programmed temperature at the end of the cycle). A hold feature on most models allows you to set the temperature manually to the desired temperature and keep it there until the hold is released; this works well if a homeowner is going away for a weekend, for example.
Note that if significant setback is used, the heating system needs to be able to bring the space up to comfortable temperatures quickly. With hydronic heating and baseboard radiators (convectors), this means that the boiler temperature has to be kept fairly high—even during swing seasons (spring and fall). Using an aquastat that adjusts a boiler based on outside temperature may not be satisfactory.
Finally, when replacing thermostats, be aware that most older models have mercury switches; these should be taken to a hazardous waste facility.
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