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Old 12-20-2004, 10:49 AM   #1
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Digital wall thermostat for A/C and furnace

The Coleman Delta TX 6769 Air conditioning unit in my trailer has a rotary knob, uncalibrated, to set ambient temperature. This, even if it worked, would be unsatisfactory. These units can also suffer from rapid on/off cycling as the air intake and temperature sensor are close to the cold exhaust. After consulting Coleman tech support I decided to deal with these issues by installing a digital wall thermostat to control the A/C, as well as the furnace. I purchased: (1) A Hunter Just Right Digital thermostat, Model 42999B from Wal-Mart for $19.97. (2) A 12volt, 30amp Auto relay from Radioshack, part number 2750226 for $6.74 (3) 10ft of 2 wire lamp flex from Ace Hardware, plus a pack of female spade connectors (4 needed), for about $5.
The relay uses the existing 12 volt supply for the furnace thermostat to energise its coil. The relay is marketed to use a 12v coil supply to switch a 30amp 12volt line. I am comfortable with using this device to switch a 120v AC line carrying 16amps continuous, 20 amps on start-up. That's my responsibility. If the relay fails it will not be warranted. If I locate a 120v AC/12 v coil 20 amps relay I will keep it as a spare.
I installed the system as follows:-
(1) I examined the wiring diagram for the air conditioner on the Coleman website,which holds wiring diagrams for many of its A/C's. ( I established that the purple wire to the control panel on the A/C was the 120v power to the compressor.
(2) I turned off the 120v AC supply to the trailer and removed the plastic cover from the A/C unit by sliding the 2 knobs off the rotary controls and undoing the two securing screws.
(3) I located the purple wire by sliding back the steel tube covering the bundle of wires to the control panel, and pulled a couple of inches of slack from the roof end. I cut this wire and bared the cut ends.
(4) I fitted the relay to the roof of the trailer, where it would be concealed by the A/C cover, about 6" back from the purple wires, using one self-tapping screw.
(5) I cut off about 8" of the lamp flex, bared all four ends, and crimped two spade connectors to one end. These connectors went on the spade terminals, one marked 87 and one marked 30/51 on the relay, and the other ends of the short flex were connected with wire nuts to the cut ends of the purple wire.
(6) I bared the 4 ends of the rest of the flex and put two spade connectors on one end. One spade connector, which connects to ground, goes on the spade terminal marked 85 on the relay. The other connector, which goes to the digital thermostat, goes on the spade terminal marked 86 on the relay.
(7) My previous analogue wall thermostat for the furnace was on the bulkhead by the refrigerator. Removing this thermostat revealed two blue wires. A volt meter revealed that one wire was 12 volt live, and the other was at ground potential. I labelled the live wire. I used this to power the thermostatic control of both the furnace and the A/C. I turned off the 12v supply at the isolating switch behind the couch.
(8) I fed the long flex from the A/C through a cupboard for the microwave, and down into the refrigerator compartment. I connected the wire from the relay terminal marked 85 to the ground at the 12v supply to the back of the refrigerator, as this was conveniently accessed via an outside hatch. The other wire from relay terminal marked 86, I fed back out from the refrigerator compartment through the back of the hole where the two blue wires emerged for the furnace thermostat, which I had removed and discarded.
(9) There are five connectors inside the digital thermostat. These are labelled G,W,Y,Rh,Rc.
G is for Fan Control. I did not use this as my furnace board does not have this feature.
W is for the furnace valve. I connected the "non-live", zero potential blue wire to this.
Y is for the A/C compressor. I connected the flex wire from relay connector 86 to this.
Rh and Rc I connected together (jumpered) with a short piece of wire, and then connected the 12v furnace thermostat "live" blue wire to Rh.
(10) I re-installed the A/C cover and control knobs, and set the cooling on the A/C control to its maximum cooling position.
(11) I switched on the 12v isolating switch and the 110v supply.

The digital thermostat has a manual slide switch with three positions:- "heat, off, cool". It has a number of useful facilities, including a centigrade/fahrenheit option, and the ability to opt for a switching differential (or span) of plus or minus 1,2 or 3 degees.
On the Coleman AC control panel is a "low power" red warning light. This warns when voltage drops to 86v AC, plus or minus 6v AC, but it also warns if the compressor has failed to start within 0.9 to 1.9 seconds. This will illuminate when "cool" is selected at the digital thermostat, but the ambient temperature is such that the thermostat does not call for cooling. It will extinguish when cooling is called for.
The system for both heat and cooling has been working efficiently for several weeks. It has considerably improved our comfort in temperatures varying from 26F to 90F, and has been a great investment for $30 and a morning's work. We are two happy bunnies!
I am describing this not as a recommendation for anyone's action, but solely for information. Please do not attempt such modifications to potentially lethal systems unless you are confident, competent, and willing to accept responsibility for the consequences.

Nick Crowhurst, Excella 25 1988, Dodge Ram 2500 Cummins Diesel. England in summer, USA in winter.
"The price of freedom is eternal maintenance."
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Old 12-21-2004, 06:59 AM   #2
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Nice system Nick. Now to integrate that Fantastic Vent with an outside air sensor and a logic board to select the best environmental response to excessive or insufficient interior temperature.

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Old 12-21-2004, 08:03 AM   #3
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I've just updated the URL for the Coleman wiring link at post #1, and it now works. Nick.
Nick Crowhurst, Excella 25 1988, Dodge Ram 2500 Cummins Diesel. England in summer, USA in winter.
"The price of freedom is eternal maintenance."
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