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Mariner
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Join Date: Jan 2008
Posts: 9

Programmable Thermostat for RV Trailer

10/31/2010 12:36 PM

I have purchased a programable thermostat for my RV trailer, to replace the existing 2 wire unit for my LPG furnace. The furnace electonics run off the RV's 12VDC system. The furnace controls are a simple ON/OFF contact closure of the 2 wires to the manual set thermostat.

The programable thermostat wiring is more complex, as it allows for controlling the furnace burner and fan as well as a separate A/C unit that I am not connecting. I am using only the furnace functions of the thermostat. The thermostat electronics run off the integral AAA batteries.

The installation instructions refer to replacing a 5 or 6 wire system typically found in a household residence. I have only 2 wires to connect to the following 5 terminals common to most programable thermoststs. They are numbered:

O/B Y R G W/AUX

1. Which terminals should I connect the 2 funace control wires to?

2. Can anyone tell me what is the function of the unused terminals?

Mariner
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lyn
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#1

Re: Programable Thermostat for RV Trailer

10/31/2010 3:44 PM

From:

Thermostat Wiring Colors - Thermostats Wire - Installation Basics ..

Color Chart

Thermostat Wiring and Wire Color Chart

Thermostat Terminal Designation

Color of Wire and Termination

R â€" The R terminal is the power for the thermostat. This comes from the transformer usually located in the air handler for split systems but you may find the transformer in the condensing unit. For this reason, it is a good idea to kill the power at the condenser and the air handler before changing or working on the wiring at the thermostat. If you have a package unit then the transformer is in the package unit. Red for the R terminal. *Although be aware that this may have changed especially if the person who wired the thermostat didn’t use conventional color coding.
RC â€" The RC terminal is designated for the power for cooling. Some HVAC systems use two transformers. A transformer for cooling and a transformer for heating. In this case the power from the transformer in the air conditioning system would go to the thermostat terminal. It should be noted that a jumper can be installed between RC and RH for a heating and cooling system equipped with a single transformer. Red for RC terminal. *Although be aware that this may have changed especially if the person who wired the thermostat didn’t use conventional color coding. Most installers use the color coding as noted but be aware that some do not use the thermostat color coding.
RH â€" The RH terminal is designated for the power for heating. See RC above for an explanation. It should be noted that a jumper can be installed between RC and RH for a heating and cooling system equipped with a single transformer. Red for RH terminal. *Although be aware that this may have changed especially if the person who wired the thermostat didn’t use conventional color coding. Most installers use the color coding as noted but be aware that some do not use the thermostat color coding.
Y â€" This is the terminal for cooling or air conditioning and goes to the compressor relay. Typically a thermostat wire pull is made to the air handler on split systems and then this wire is spliced for the separate wire pull which is made to the condenser. Some manufacturers put a terminal board strip near the control board in the air handler so a splice is not needed. Yellow for Y Terminal. *Although be aware that this may have changed especially if the person who wired the thermostat didn’t use conventional color coding. Most installers use the color coding as noted but be aware that some do not use the thermostat color coding.
Y2 â€" This is the thermostat terminal for cooling second stage if your system is so equipped. Many systems only have a single compressor but if you have two compressors which should only operate off of one thermostat then you need the Y2 thermostat terminal for second stage cooling. *The most common color I’ve seen used for this terminal and wire designation is light blue but this varies and is completely up to the installer what color to use. Most installers use the color coding as noted but be aware that some do not use the thermostat color coding.
W â€" This is the thermostat terminal for heating. This wire should go directly to the heating source whether it be a gas or oil furnace, electric furnace, or boiler, White for W Terminal. *Although be aware that this may have changed especially if the person who wired the thermostat didn’t use conventional color coding. Most installers use the color coding as noted but be aware that some do not use the thermostat color coding.
W2 â€" This is the thermostat terminal used for second stage heat. There are gas furnaces with low fire and high fire and some depend on control from a two-stage heating thermostat with a W2 terminal. Heat Pumps use staging for auxiliary heat and need a W2 terminal. *The most common color I’ve seen used for this terminal and wire designation is brown but this varies and is completely up to the installer what color to use.
G â€" This is the thermostat terminal used for the fan relay to energize the indoor blower fan. On a split system the blower fan is in the air handler while with a package unit the blower fan is in the outdoor package unit. Green for G Terminal. *Although be aware that this may have changed especially if the person who wired the thermostat didn’t use conventional color coding. Most installers use the color coding as noted but be aware that some do not use the thermostat color coding.
C â€" This is the thermostat terminal which originates from the transformer and is necessary to complete the 24 volts power circuit in the thermostat but only if the thermostat consumes electricity for power. Many digital thermostats require 24 volts for power so the common wire is necessary. C stands for common and there is no universal color used for this terminal although black is the most common color I’ve seen.
O or B â€" These thermostat terminals are for heat pumps and the B thermostat terminal is used on for Rheem or Ruud and any manufacturer that energizes the reversing valve in heating mode for the heat pump. Most other manufacturers of heat pumps will utilize the reversing valve for cooling and the O thermostat terminal will be utilized for this purpose. This wire goes to outside heat pump condenser where the reversing valve is located. Orange for O and Dark Blue for B depending on the installer of the heat pump and the manufacturer. If you have a Trane, Carrier, Goodman, Lennox, Ducane, Heil, Fedders, Amana, Janitrol, or any other manufacturer other than Rheem or Ruud you will be utilizing the orange wire for reversing valve. Rheem and Ruud will usually utilize the blue wire for reversing valve.
E â€" This thermostat terminal is for heat pumps and stands for Emergency Heating. If for whatever reason the heat pump condenser fails and it is necessary to run the heat there is an option on heat pump thermostats for emergency heating. Basically this simply utilizes the back-up heat source many heat pumps have to heat the home without sending a signal to the condenser to run for heat. E â€" There is no universal color used for this thermostat terminal designation but this should be wired directly to the heating relay or the E terminal on a terminal strip board in the air handler or package unit if you have a heat pump package unit.
Aux â€" This thermostat terminal is for back-up on a heat pump and allows for auxiliary heating from the back-up heat source usually located in the air handler. Aux - There is no universal color used for this thermostat terminal designation but this should be wired directly to the heating relay or the Aux terminal on a terminal strip board in the air handler or package unit if you have a heat pump package unit.
S1 & S2 â€" Some thermostats have this terminal and it used for an outdoor temperature sensor. The wire uses for this should be special shielded wire and completely separate form the other thermostat wires. Using shielded wire prevents electromagnetic forces generated from other wires from interfering with the signal inside the shielded wire. A remote temperature sensor is a solid state device and the signal needed to get an accurate temperature is sensitive to electromagnetic forces from other wiring inside the structure.
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Mariner
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Join Date: Jan 2008
Posts: 9
#5
In reply to #1

Re: Programable Thermostat for RV Trailer

11/03/2010 12:19 AM

Thank you LynLynch for the wiring color code chart, which would be useful for a household combination heating/cooling system.

The question however was for an RV (furnace only) system, with 2 wires to be terminated on a self-powered (AAA bateries) thermostat with 5 terminals. Thanks for your input.

Mariner
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BruceFlorida
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#2

Re: Programable Thermostat for RV Trailer

10/31/2010 7:54 PM

When my house AC was replaced I asked the installation crew about the color codes. What they told me seems to match the posting by Lynlynch.

RED = power
WHITE = heater
GREEN = fan
YELLOW = compressor

The heater, fan and compressor are basically loads to ground. Apply power (red) and they will be activated. It is necessary to turn on fan for both heater and compressor (cooling). Fan may be turned on by itself if desired.

You need to carefully read and make your own decisions. Any risk of DAMNage, fire, etc. is your risk. Based upon my 30 seconds on this it seems like you should be considering using the red and white wires from the thermostat.

Please remember that I don't know much about heaters, I don't have any formal qualifications and I did not stay at a Holiday Inn Express last night. Read at your own risk.

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Mariner
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Posts: 9
#6
In reply to #2

Re: Programable Thermostat for RV Trailer

11/03/2010 12:31 AM

BruceFlorida, you might not know much about heaters or the Holiday Inn, but you have good intuition. With a little bench work, I discovered that the R and W/AUX terminals were the dry relay contacts that I needed to use to turn the heater ON and OFF. Thanks for your opinion.

Mariner
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Brave Sir Robin
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#3

Re: Programmable Thermostat for RV Trailer

11/01/2010 8:16 AM

I agree with Bruce. Thanks LynLynch for the good post. My heat only thermostat in my toy hauler has red and white wires for the switch closure control. (cheapest looking POS thermostat ever but it basically works). My unducted rooftop A/C unit just uses the panel controls.

The other thing you could do, is to put the battery in your programmable unit, hook an ohm meter up to the R and Wh terminals and program it to a high temp at some time in the very near future (next 10 mins maybe?) and see if you get your expected switch closure.

Good luck with your quest.

Cheers !
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Mariner
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Posts: 9
#7
In reply to #3

Re: Programmable Thermostat for RV Trailer

11/03/2010 12:47 AM

Thank you Brave Sir Robin for your courageous response. Being a fellow RV'er, I guess our exploring, adventurous minds think alike. While waiting for replies, I tested it the same way you proposed and ohm'd the thing out. I discovered that the R and W/AUX terminals were the dry relay contacts that closed when the T-stat called for heat. Thanks for your suggestion.

Mariner
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Icarus
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#4

Re: Programmable Thermostat for RV Trailer

11/02/2010 7:36 PM

I understand your problem

You need a programmable battery operated thermostat that has a dedicated and isolated dry contact for your furnace. The industry standard thermostat uses the R terminal with any available terminal to provide parasitic power, thus extending the battery life. The R terminal is intergrated along with the other terminals into the thermostats power supply.

You could start with line voltage thermostats. They are for baseboard heaters. There may be one that you can use. Try Honeywell and white Rodgers.

After that look at gas fireplace controls. the gas valves use a millivolt system. As a result most remotes control systems are battery powered. The receiver module has a isolated dry contact that you need. Many remotes are programmable for both temperature and time. The application is different but the results are the same. ON/ Off
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Mariner
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Posts: 9
#8
In reply to #4

Re: Programmable Thermostat for RV Trailer

11/03/2010 12:59 AM

Icarus my friend, you understand the problem OK but failed to grasp the question. I HAVE a programable battery operated thermostat. You were right though about the R terminal being the one to use to turn the furnace ON and OFF. Thanks for taking time to answer my question.

Mariner
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Icarus
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#9
In reply to #8

Re: Programmable Thermostat for RV Trailer

11/03/2010 9:24 AM

One of the things I was trying to say was that by using the R & W terminals of a standard battery operated thermostat, on a 12V DC system may cause problems. The R, W, Y and sometimes G, are tied in the the thermostats power supply. When your furnace is not calling for heat and the R & W are open and the thermostat will be looking for 24V AC. The R terminal is meant for 24V AC and the W terminal would pick up the Common via the gas valve as one side of the valve coil is connected to the C of the transformer.

What you are proposing will mix a 24V AC stat with a 12V DC power source.

It might work or not, or something might get damaged.
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Mariner
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Posts: 9
#10
In reply to #9

Re: Programmable Thermostat for RV Trailer

11/04/2010 12:51 PM

Icarus:

Thanks for your clarification. The unit has been running fine for about a week the way I hooked it up, (R and W/AUX) but I'll check it out.

Mariner
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