How To Test a Thermostat
Before you assume your furnace or A/C is malfunctioning, try these
simple steps to make sure it isn't a problem with your thermostat.
Thermostats are simply a switch; they turn the furnace on and off.
The only big difference from a light switch is that they have a mechanism
that measures the temperature and when it reaches a predetermined point,
If you turned on the thermostat and the heater (or A/C) didn't come
on, it may be the thermostat and not the furnace. First things first,
make sure that all the settings on the thermostat are as they should
be. Make sure the "ON" switch is on, make sure it is set to "HEAT"
or "AC" as appropriate, make sure there are no programming overrides.
I've turned up the heat and had nothing happen because I failed
to notice that someone turned it to "AC".
If your thermostat is battery operated, try installing a fresh battery.
Even if the lisplay is lit, a weak battery may be a culprit.
If the thermostat is properly set and you have a fresh battery, then
the next easiest thing to check is best done with two people. One person
operates the thermostat, turning it slowly from a low temperature to
a high temperature. The second person is stationed by the furnace (even
if the problem is with the AC, test the furnace). When the thermostat
is turned on, it usually makes some sort of clicking sound which indicates
a signal is being sent. At the furnace, when a signal is received,
it should make a sound. If it makes a sound, any sound, it is almost
certainly getting the signal and the problem is with the furnace. If
it is dead silent, then the problem is most likely in the thermostat
or connective wiring. If you don't have a helper, try using a walkie
talkie with the talk button taped to the "talk" position
or a baby monitor or even use a cell phone to call your home phone.
Leave the transmitter near the furnace and listen from the receiver
for a sound.
The next test is a little more difficult, but not too tough. Your
thermostat is a switch, right? Well, let's bypass that switch and see
what happens if we connect the wires together. That will do the same
thing as turning on the thermostat. In order to conduct this test you
will have to access the sub-base. Most thermostats have an outer shell
and beneath it is the sub-base. Some models simply pop off when pulled
straight off. Other types may require a twist, depress a tab or may
be held in place with one or more screws.
Before proceeding, turn off the power to the furnace. The thermostat
control is low-voltage and poses little risk, but best practice is
to turn off the power to avoid injury or damage to equipment. The sub-base
is screwed to the wall. You will probably see two or four wires coming
from a hole in the wall and connected to screw terminals on the sub-base.
If you have four wires, this indicates wiring for both a furnace and
central air conditioning. If you see just two wires, those are for
Before you disconnect anything
write down which wire connects where. Sure, you think you will remember,
but 15 minutes from now you will be thinking, "is it the red wire or
the green wire". Even more important, if you disconnect any wires,
secure them so them don't fall back through the hole in the wall.
To test the heater you will need to connect the "R" (or "Rh")
wire and the "W" wire. This bridges
the two wires and has the same affect as turning on a switch.
Turn the power back on and listen again to the furnace for any sounds.
If the furnace starts up, then the problem is almost certainly the
thermostat and you should replace it. Do not leave the furnace running
like this for an extended period of time, it will not shut off and
could overheat or cause a fire. If the furnace is completely silent,
make sure the furnace has power. Check the circuit breaker and use
a tester to make certain there is power at the furnace. If it has power
but does not start, then the problem could be a break in the wire running
from the thermostat to the furnace or with the furnace itself.