Save Energy With a Setback Thermostat

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

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

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.

EDITOR’S NOTE: Thermostats do differ in their
specifications. In particular, electric baseboard controls
often operate on 115V current and can be wired directly to
a timer of suitable load capability. Also, when you
purchase an auxiliary thermostat, check to be sure that the
unit has a load rating similar to that of your main

Need Help? Call 1-800-234-3368