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Save Energy With a Setback Thermostat

You can save 12% on your annual heating bill and still wake up to a warm house with this setback thermostat design.

| November/December 1979

  • 060-Setback-Thermostat-01a.jpg
    This setback thermostat design uses two conventional thermostats—a main unit (left) and an auxiliary (right)—and a timer to switch between them.
  • 060-setback-thermostat-02-Diagram.jpg
    Diagram shows method of wiring up the setback thermostat.

  • 060-Setback-Thermostat-01a.jpg
  • 060-setback-thermostat-02-Diagram.jpg

On these long and frigid winter evenings, many sleepers enjoy the economy (and comfort!) of clicking their thermostats back to 60°F (or lower) and snuggling deep beneath an extra blanket or quilt. The cool night air against one's face—contrasted with the envelope of warmth provided by the additional bedclothes—seems to bring on the sleep of the guiltless (well, at least the "slumber of the thrifty").

But oh! those first few dripping-wet steps out of the next morning's shower (and into 60°F air) can be a rude shock. Until now the only way around the a.m. goose bumps—short of an energy-eating space heater—has been either to rise at 4:00 a.m. and kick the thermostat back up, or to purchase a $45 to $90 electronic setback control. MOTHER EARTH NEWS' technicians (as well as a number of our readers) weren't satisfied with either one of these approaches, so they've developed a simpler and less expensive setback thermostat to achieve the same results as are possible with a store-bought  thermostat.

Instead of controlling one thermostat with a timer, our researchers took a tip from reader Thorn Daoust and designed a unit that uses two thermostats and a timer to control an electromagnetic switch called a relay. When the relay gets power—by way of the 115V wires from the plug-in timer—it switches the 24V thermostat current to the auxiliary furnace regulator, but when the timer kicks back off (in the morning) the temperature control reverts to the main unit. There are eight posts on a DPDT (dual pole dual throw) relay: two for the 115V household line from the timer (Nos. 2 and 7), two for the 24V wires from the furnace (Nos. 1 and 8), and two more for each thermostat (Nos. 3 and 6, and Nos. 4 and 5).

And for those occasions when you want to stay up a little later than usual—and thus want the house to stay warm for an hour or so more—a manual override switch is helpful. Your timer may already be equipped with such a feature, but if not, just wire a toggle switch into one of the 115V lines. (If you include a pilot light in this circuit loop, you'll be able to tell at a glance whether you're on the auxiliary thermostat or not.)

As you can see, there's next to nothing involved in building this thermostat control. A couple of off-the-shelf parts and a little wire and solder are all that go into it. In fact, the only thing about MOTHER EARTH NEWS' setback thermostat that takes less effort than building the device is using it. 

Bill of Materials

1 heating thermostat (24v)       $6.20
1 small appliance timer           5.03
1 DPDT 115v relay                 5.88
1 pilot light (115v)              0.78
1 utility box                     1.59
Wire, solder, and standard plug   1.00
1 SPST 115v toggle switch         1.03

Note: These figures represent the cost for new parts. Every washing machine has a suitable 115v relay in it waiting to be scrounged—and building recyclers often hav a supply of used heating thermostats they'd b glad to part with for a couple of dollars apiece—so you can probably construct your unit for considerably less than $20.

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