Will a Nest thermostat work with Lennox EL23/SLP99?

[Trooker]

I’m getting bids for a new home HVAC system. Lennox EL23/SLP99 lists a Lennox S30 Communicating Thermostat for the modulating variable speed system. In my existing home, I have a Nest thermostat working with a Daikin FIT inverter air conditioner & modulating variable speed furnace. The Lennox thermostat S30 and S40 seem expensive, waste screen space and I prefer Nest. But the question I can’t get answered is whether a Nest thermostat can work with Lennox. Thanks for any advice and insight!

[CoolingWizard · Accepted solution]

@Trooker , the Lennox SLP99UHV series have an advanced control board that supports both an advanced digital communicating thermostat, such as the Lennox S30. In addition it also has a conventional HVAC connection terminal that the Nest will work with.  Make sure your contractor deletes the S30/S40:from the offer, and simply installs a 10-conductor, 18-AWG thermostat cable between the air handler and the thermostat location. You set up your Nest Learning Thermostat as a 2-Stage gas heating furnace and 2-Stage Air Conditioner.  
Know that your Contractors is going to tell you that if you use a Nest thermostat with this furnace, that you’re gonna lose some efficiency that’s designed into the system between the SLP99 and the EL23 outdoor unit, Now here is the truth, the EL23XCV outdoor unit has very sophisticated, built-in control sensors on the suction and discharge refrigerant pressures as well as temperature sensors for the ambient outdoor air, the suction return line temperature, and the liquid line temperature. As a result, the system will still operate extremely efficient even if you’re using a conventional thermostat system to control the whole thing, This information is provided courtesy of Lennox to its licensed contractors. So you do not have to use an S30/S40 thermostat in order to get the efficiency, the system has to offer. What you will actually lose is the ability for the system to self diagnose probable system failures and report them to the indoor unit and onto the S30/S40 thermostat and you will loose the WiFi remote configuration capability of the system.  This will require the contractor to do a little extra manual  configuring of the system for maximum operational performance.

AC Cooling Wizard

[CoolingWizard · Accepted solution]

@Trooker , the Lennox SLP99UHV series have an advanced control board that supports both an advanced digital communicating thermostat, such as the Lennox S30. In addition it also has a conventional HVAC connection terminal that the Nest will work with.  Make sure your contractor deletes the S30/S40:from the offer, and simply installs a 10-conductor, 18-AWG thermostat cable between the air handler and the thermostat location. You set up your Nest Learning Thermostat as a 2-Stage gas heating furnace and 2-Stage Air Conditioner.  
Know that your Contractors is going to tell you that if you use a Nest thermostat with this furnace, that you’re gonna lose some efficiency that’s designed into the system between the SLP99 and the EL23 outdoor unit, Now here is the truth, the EL23XCV outdoor unit has very sophisticated, built-in control sensors on the suction and discharge refrigerant pressures as well as temperature sensors for the ambient outdoor air, the suction return line temperature, and the liquid line temperature. As a result, the system will still operate extremely efficient even if you’re using a conventional thermostat system to control the whole thing, This information is provided courtesy of Lennox to its licensed contractors. So you do not have to use an S30/S40 thermostat in order to get the efficiency, the system has to offer. What you will actually lose is the ability for the system to self diagnose probable system failures and report them to the indoor unit and onto the S30/S40 thermostat and you will loose the WiFi remote configuration capability of the system.  This will require the contractor to do a little extra manual  configuring of the system for maximum operational performance.

AC Cooling Wizard

[CoolingWizard · Accepted solution]

@Trooker ,  my congratulations for upgrading such highly efficient Air Conditioner and Furnace.  When you install your Nest Learning Thermostat you will should have the following wires:

Rc - Power 24VAC from SLP99UHV

Y1 - Stage 1 Cooling 

Y2 - Stage 2 Coling

W1 - Stage 1 Gas Heat

W2 - Stage 2 Gas Heat

G - Blower Fan

C - Power Common

Your HVAC a contractor will need to make the necessary configuration changes to the SLP99UHV control board. The Nest supports the needed 2 Heat / 2 Cooling stages for what Lennox describes as a non-communicating thermostat. There is a bridge wire labeled W915 on the control board that will need to be cut so the Nest can control both stages of cooling. 

AC Cooling Wizard

[Trooker]

This is great news! Thank you, @CoolingWizard , for the detailed response and solid answer. The remote wifi control isn't an issue as I work primarily from home office. The remodel includes sealing all air leaks after replacing the sheathing and insulation, hoping to mitigate the potential loss of HVAC efficiency.

[CTopher]

I am being told that if I revert to a simple thermostat like my Nest Learning will only operate at 70% or 100% of its capacity like a two speed unit. Is this correct? Is there a way to program the unit to operate at say 30-50% and 100%?

[CoolingWizard]

@CTopher , The Nest Learning Thermostat can support up three fan speeds. L, M,H.  

AC Cooling Wizard 

[CTopher]

I am considering a Lennox EL23XCV with a SL280 Furnace.

What specifically do I need to request wire side as well as configuration of the unit and the Nest? 
Does the Nest 3rd speed = dehumidifier function?
Will I be able to set AC fan speed option something like 30%, 70% & 100%?
Do you have a recommendation of speed settings for most efficiency in Texas heat/humidity?
Will the Nest handle the 2 speed heat as well as the 3 speed AC?

Thank you for the help and feedback.

CTopher

[CoolingWizard]

The Nest Learning Thermostat is a programable thermostat.  It can support multiple functions however there are limits. Take the * terminal. This can programmed to be either Stage-3 Heat (W3), Emergency Heat (E), Humidifier/Dehumidifier(HUM/DHUM)  or 3rd Speed Fan. It cannot be programmed to be a Humidifier and a 3rd speed fan at the same time.  Basically there are limitations.

• If you have a humidifier that requires direct control, then you will need the * terminal to be that control, if your humidifier has a built in humidistat, then the control of the humidifier is not going to be the Nest Thermostat. The blower speed for humidify and dehumidify is somewhat irrelevant since the humidistat deals with that.  The SL280 furnace uses a variable speed ECM motor that is directly controlled by the furnace controller and is not able to be controlled by the thermostat.  
• 
  Make sure your contractor installs a 7 conductor thermostat cable between your thermostat location and the furnace.  When it comes to dehumidification the air conditioner is a natural dehumidifier. If you wish to increase in dehumidification , the typical approach is the slow the blower speed down to increase the condensation rate of the evaporator coil. The SL280 cannot do this since it uses an ECM motor. The ECM is the more energy efficient blower motor when compared to a PSC 3-speed motor.  
  
  The Lennox EL23XCV is an highly advanced variable speed air conditioner and to take advantage of the ECM motor capability of the furnace, you must use an iComfort S30 ultra smart thermostat to activate it. Otherwise, you are wasting your money. If you want the Nest Thermostat to be the controlling device, you should use a less capable HVAC equipment which is less expensive such as the EL17XC1 air conditioner and an The Nest thermostat does have a built in dehumidification 
  
  AC Cooling Wizad