Wiring Lennox Thermostat

[Zita Lifland]

Confusedby the wiring, hope someone can help me out. At the furnace there's only four wires in the cable (R, B, G, and W), but at the other end, a yellow wire appears. It's not reflected in the picture of the thermostat wiring, but the blue wire is there, behind the plate. I figured it was the C wire and connected it to the Ecobee, but that didn't work. Opening up the furnace, I can see why, but I'm not sure how it should be wired. I understand that the diagram attached makes it pretty easy, but the previous thermostat wiring doesn't make any sense with the wiring at the furnace. For instance, the green wire isn't attached to anything at all! The blue wire is attached to the furnace, but unless it's tied to another color wire on the way to the thermostat, it's also not connected to anything. I have zero idea what the yellow wire is attached to. Do I just need to get a multimeter and test each wire??

The issue was that the C and Y wires apparently went nowhere. I traced them as best I could, but couldn't figure it out. Fortunately, there was a deprecated humidifier control box right next to the thermostat with two wires. It was cut close enough to the furnace that I could remove some staples, stretch it, and get it to the control panel. I used those wires as the C and Y wires and everything is fine.

Hello Seton,

Thank you for the clear graph and it really helps a lot. I have the exact furnace + AC combo as you do. Couple question regards to your graph and set up,

-From the SLP98V manual, for a 2 stage Heat/ 2 stage cool set up, the graph is missing the G wire from the control board to the Nest. Do you leave it out on purpose ?If so how did it go without the fan control?

-I know you need to set the board to non-communicating mode, may I know how exactly you do it? Which DIP switch you need turn on/off, and anything else you did on the board for it to work?

I want to thank you all. I have a Lennox ML180UHE furnace, with an AC condenser outside. After reading these posts I got brave and went up to the attic. My furnace has your standard W-C-G-Y-R plugs, and they had wired those 5 wires out to another box, which then turned the signal into the C-R-I+,I-, requiring the communicating thermostats.

The issue I did have, I must not have been too careful with the power, because I took off the thermostat after it was working, hung it on the wall, and plugged it back in, and the entire system was dead. After an hour of searching and cursing, reinstalling the old setup to no avail, I figured out it was the fuse, and popped in a new fuse and it was good to go.

My iComfort thermostat shuts off at night. I want to replace it with a Nest 3rd generation thermostat. I checked the connection of the indoor and outdoor units. The wiring ports look the same as what was posted by @Seton_Carmichael. I have some questions:

I have no experience in HRV, just some simple DIY weekend stuff. Any advice on how to tie in the 3 wired HRV to all of this, and the functionality difference between the 2 wiring diagrams?

@bade011 any feedback?

My icomfort wifi display just died. I also have a single blower system. Are new wires necessary? Could you install a new Nest thermostat with the existing 4 wires by disconnecting those wires from the Lennox control box and rerouting them to the furnace board directly?

WARNING

Improper installation, adjustment, alteration, service or maintenance can cause property damage, personal injury or loss of life.

Installation and service must be performed by a licensed professional HVAC installer (or equivalent) or service agency.

CAUTION

This is a 24VAC low-voltage thermostat. Do not install on voltages higher than 30VAC.

Do not short (jumper) across terminals on the gas valve or at the system control to test installation.

This will damage the thermostat and void the warranty.

WARNING

Always turn off power at the main power source by switching the circuit breaker to the OFF position before installing or removing this thermostat. All wiring must conform to local and national building and electrical codes and ordinances.

Before beginning installation, note the type of equipment, number of stages, and any accessories being installed. This thermostat is a

24VAC low-voltage thermostat and requires a common wire to the thermostat to operate.

DO

Thermostat wiring connections with various units, including dual fuel, zone control, and applications that include the Humiditrol Enhanced Dehumidification Accessory (EDA). See figures 3 through 7. For whole home dehumidifiers, refer to the installation instruction for the dehumidifier.

SYSTEM SETUP

Sets the thermostat for operation with either a non-heat pump or heat pump and defines the number of compressor stages. Indoor unit settings include no heat, gas/oil or electric and number of indoor heat stages.

OUTDOOR SENSOR

Options are yes or no (default).

RESIDUAL COOL

Default is 0 seconds. This is the time, in seconds, that the fan runs after a call for cooling is satisfied in order to deliver any residual cooling ability from the coil and ductwork into the conditioned space. Options are 0, 30, 60, 90 and 120 seconds. Touch

LOW AND HIGH BALANCE POINTS

These balance points are for heat pump systems only with an outdoor sensor installed and enabled plus an indoor unit setting enable. Either gas/oil or electric will enable the balance point options in the installer settings list.

Default is ON. When ON, all stage delay timers (stages 2, 3, and 4) are enabled and will serve to bring on additional stage(s) of cooling or heating on a timed basis (default 20 minutes) in cases when the previous stage of heating or cooling will not raise or lower the room temperature to the set point in a given time.

When OFF is selected all stage delay timers are disabled. This means stages are changed based on the temperature and not their timer delays. Scroll to STG DELAY TIMERS; and touch to select the applicable stage delay timer. Touch Energy saving recommended set points for heating and cooling can help save energy. The time and temperatures reference in table 2 are pre-programmed into the thermostat to achieve energy savings. Scroll to ENERGY SAVING DEFAULT; touch to select. Read the message on the screen and to continue, touch CONFIRM.

SYSTEM TEST MODES

After the thermostat has been installed and set-up, the installer may run a system test function (accessed through the installer settings menu), to test all cooling, heating, emergency heating stages and FAN outputs. Select system test mode. A pop-up will be displayed indicating all equipment will be stopped. Touch confirm to continue. Pressing the OFF button next to the desired option will change the status to ON and will enable the relay for that terminal. Pressing again will turn OFF the relay. Touch the left arrow (

NOTICE

Risk of equipment damage. Can cause compressor failure. In dual fuel system applications, do not turn on heat pump and furnace at the same time in system test mode.

All HVAC components can be tested to confirm the signals between thermostat and unit are being sent and were received.

NOTES: After 5 minutes without a test being initiated, the test modes is disabled and system goes back to the normal mode (i.e. HOME screen). When in SYSTEM TEST MODE, the compressor minimum off timer is bypassed.

Humidification (adding moisture to air) is provided only when the thermostat is in heat mode. The humidification signal (H terminal) to the humidifier (off when the thermostat is in the COOL mode) controls humidification. When the thermostat is powered, the H terminal is normally inactive (open circuit) in any mode (HEAT/COOL, HEAT, COOL, OFF). When a humidification demand is present, H terminal and G terminal are energized (24V). The system must be in heat mode and had at least one heating call before humidification feature is active. Once humidification is active the humidification modes are as follows:

If Humiditrol is enabled in the installer settings, then this adjustment affects overcooling operation. Overcooling ranges from 2F below the cooling setpoint (MIN setting) down to 2F above the heating setpoint (MAX setting). Halfway between the two settings is the MID setting. If a Humiditrol EDA is installed and enabled, then the thermostat must be configured to properly operate the Humiditrol EDA as follows (see Figure 22 for the Humiditrol EDA operation flowchart): Check the HUMIDITY SETTINGS in user settings to confirm that the user has turned ON dehumidification setting.

A label on the back of the thermostat is visible through an opening in the back of base plate. This identifies the Lennox Catalog , Part Number and Serial Number. Separate the base plate from the thermostat to see additional manufacturing information.

The thermostat stores all the information concerning its programming (state, mode, program information, last temperature measured) in a nonvolatile memory. This function avoids the loss of the state of the thermostat when a power-down occurs. The only thing that might be lost is the clock and date information, however, a super capacitor will remember clock / date information for as long as it has a charge (approximately 24 hours). When power down occurs (due to a power outage) the thermostat is able to shut off heat relays (Y1, Y2, W1, W2) when in heating mode and W1 and W2 when in cooling mode. The O and B relay will maintain their last state. When power is restored the thermostat will be in heat /cool mode so either mode can run to re-satisfy the temperature setting in the home. Day and time (schedules) may be off due to battery loss.

Thermostat will stop the sys tem operation if input from re quires outdoor temperature in formation when any of these conditions, i.e. balance point control and 2nd stage lock in are applicable. System will de fault to mode of operation that does not required input from the outdoor temperature sen sor.

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