From: John De Armond
Subject: Re: Adding wall thermostat to roof air
Date: Fri, 23 Jun 2000 17:17:44 EDT
> I have a Coleman 13.5K roof air (non-ducted). It has two knobs, fan speed
> and temperature.
> Is it feasable, and if so, has anyone successfully accomplished rigging a
> wall thermostat to control the AC, both fan and compressor?
Yes. Funny, you read my mind. This is the project I have underway
right now as I come in for a Gatorade break.
Warning: This technique will cause apoplectic fits in those whose
minds have rigidized to the point that thinking outside the box has
become impossible. I suggest that if you're of that sort, go to the
next message and don't have a cow or a stroke or something.
I want the furnace and AC to work in my RV the same way it does in
my house - separate fan and compressor/heating element control,
"fan/auto" control and with an electronic thermostat, automatic
switchover from heat to cooling and vice versa. This is pretty easy
to do, the only complication being there being 12 volts DC on the
control circuit instead of 24 volts AC. With one exception, this is
of no consequence. Most 24 volt AC relays will work just fine on 12
Here's what you need:
A thermostat designated for gas/electric service. This type of
thermostat will have the capability of bringing out the heat and
cooling fan contacts separately. (not used for my install but handy
if you want to control the heater fan. If you buy an electronic
thermostat, make sure it runs on internal batteries and does not use
sneak current from the 24vac supply.
3 15 amp HVAC relays with 24vac coils. These are commonly used as
fan relays on furnaces and ACs. My local dealer carries MARS brand
but any sort will do. You can use industrial relays with 12 vdc
coils but they're lots more expensive, don't work any better and
draw more coil power.
A suitable metal box to mount the relays in. A NEMA 1 4X4X3 pull
box will do.
Misc wiring stuff. Wire, wire nuts, tape, etc.
5 wire thermostat wire. Four wire telephone inside station wire can
be used if you use the chassis as ground return.
Mount the three relays in the box and mount the box near the breaker
panel. Connect an incoming pole of each of two of the relays in
parallel. This is connected to 120 VAC from the breaker panel. The
two relays will be the compressor and AC fan relays. The third
relay will switch 12 volts to the heater in place of the existing
Follow the instructions that come with the thermostat to wire it
up. Supply 12 volts from your fuse panel on the red wire. The
other standard color codes are:
Terminal label Color Function Note
Y ORAN COMP Compressor Relay
G GREEN FAN Fan Relay
W WHITE HEAT Heat relay
R RED HOT 12 volts from the fuse panel
C YEL COMMON Chassis ground
Simply run each wire from the thermostat terminal to one coil
terminal of the appropriate relay and then ground the other coil
terminal of all the relays.
Inside the overhead air conditioner, remove the controls, making
note of how they were wired. Identify the neutral, the fan hot lead
and the compressor hot lead.
Here's where the wiring gets a bit hinky. We need 4 wires to the AC
- compressor hot, fan hot, neutral and ground. The problem is, most
rigs have a hunk of standard 12/2 with ground run to the AC and it
is almost impossible to replace. In my rig, the wire was run and
then the spaces were injected with foam insulation. The wire is
embedded in the foam. We can deal with this. We have 3 wires
running to the unit, a black, a white and a bare ground wire. We
simply redesignate the function of each wire. For my rig, I
designated the wires as follows:
black - compressor hot
white - fan hot
ground - neutral
chassis - ground
Obviously this is non-standard and will profoundly confuse the
average light bulb mechanic. Clear and attention-getting labeling is
a must. I labeled each wire using a Brother EZy Labeler and then
secured the label to each wire with clear heat shrink tubing. I
then printed a description of what is what on fluorescent yellow
stickers and stuck one inside the AC, on in the relay box and one on
the breaker panel.
Unhook the wire feeding the AC from the breaker panel. At the A/C,
connect the compressor hot lead to the black wire, connect the fan
hot lead to the white wire and connect the neutral to the bare
ground wire. Then bond the chassis of the AC to the RV chassis in
at least two places using green jumper wires and threaded holes and
screws and not sheet metal screws. The use of threaded screws in
tapped holes is required by the Code and is good practice. Sheet
metal screws in aluminum can loosen from differential expansion You
can buy the green screws at Lowe's, etc. You can also buy a hole
tap that looks like a screwdriver with a tap made onto the shaft
instead of a blade. For sheetmetal, you punch a hole using an awl
so that a good pucker sticks through the other side. This pucker
receives the threads.
At the breaker panel, remove the AC wire and run it to the relay
box. Connect the black wire to the compressor relay. Connect the
white wire to the fan relay. Connect the bare copper wire to a
neutral wire run from the breaker panel. Run a pair of wires from
the furnace thermostat input to the heat relay contacts. Check your
wiring, reassemble everything, power up and go.
You now have thermostatic control of both the furnace and AC. You
can have the fan run all the time as it does now by putting the fan
switch to the "ON" position. In "auto", the fan will start and stop
with the compressor, as the fan in your home AC does. Inside the
thermostat will be a "gas/electric" switch or jumper. If you leave
it set to "electric", then the overhead fan will run when the
furnace is on, when AC power is available, of course. This can be
desirable to help distribute the heat. Setting the switch to "gas"
will inhibit the AC fan when the furnace is running. If your
thermostat has built-in time delay to run the fan after the
compressor turns off and delaying the fan til the evaporator gets
cool, then these functions will work with your RV AC.
The thermostat I bought has the capability to automatically change
over from heat to cool and vice versa to maintain the temperature
setpoint. This is very desirable for dealing with those nights
where it gets very cool but then warms up in the AM before you
The only penalty is an additional, about 1/4amp 12 volt draw when
the heat is on. Of no consequence to me since I have plenty of