■ FIGURE 3a
disconnect and wire it to a standard
15-amp grounded plug (Figure 2).
Wire a small stub of grounded
14-2 from the original service disconnect to a standard 15-amp grounded
outlet box. Screw or nail the outlet
box to a stud (Figure 3a). Use a cable
strain relief or make sure the box has
one — the plastic boxes usually
do. Finally, wire in a 15-amp outlet.
■ FIGURE 4
storm, blizzard, or power blackout,
but you should run a few tests
to make sure this is really going
Shut the power off to your
furnace at the main breaker box.
Generally, a furnace also provides
power to its associated thermostat
through the control wires, so when
the power is off, the thermostat will
also lose power. Programmable
setback thermostats have backup
batteries and may or may not operate
without AC power, so you must verify
proper thermostat operation when
running on an inverter.
Our air handler fan is a 1/3 HP
unit. We measured 6.5A for two
seconds at startup, then 2.3A when
running with the amprobe. This
■ FIGURE 3b translates to approximately 715W to
start and 250W to run. Inexpensive
inverters are generally available
in 250-watt or so incremental
capacities, so we tested several
different inverter units.
The first inverter we tried was an
unmodified 520W UPS with internal
batteries. It was unable to start
the furnace fan. We knew this, based
on our calculations above. Next,
we tried an inexpensive 750W
modified square-wave output UPS. It
was able to start the furnace fan.
Next, we ran on a 1,200W UPS
modified to run on an external
lead-acid battery. (See my December
1997 Nuts & Volts article on how to
You could wait for the next ice do this). Finally, we plugged in
a 2,400W trace
inverter in our van and
PARTS LIST ran the furnace with no problems on that.
QTY. The trace has the
❑1 added advantage of
load sensing so the
❑ 1 standby power draw is
❑ 1 negligible.
Make sure you observe proper wiring
Black — Hot (Narrow Spade)
White — Neutral (Wide spade)
Green — Ground
Before plugging your air handler
back in, turn on the breaker and
recheck to make sure your wiring and
polarities are correct. A plug-in neon
light polarity tester will be handy in
performing this test (Figure 4).
Trying Out the Inverter
“Carlon” or other single outlet
15 amp grounded plug
3” (or more) of 14-2 (with ground) wire
Adequately sized UPS or inverter with
external or internal battery (see text)
Other materials may include wire stripper,
wire nuts, wiring polarity tester.
The answer to this
question depends on