Lennox Elite Series Thermostat Settings Optimization for Air Source Heat Pump Energy Efficiency

My new house came equipped with a Lennox X4147 Elite Series thermostat - a fairly high end touchscreen unit that can control up to three stages of heating and two stages of cooling. Last weekend I upgraded the Fan Coil unit's blower motor to an Evergreen IM electronically commutated motor to help increase the efficiency of the system. While testing the system, I was noticing how quickly the thermostat would call for emergency / auxiliary heat. If I increased the setpoint by 1/2 degree Celcius, the thermostat would shut off the heat pump demand, and call for emergency 2nd stage heating - in my case electric strip heating in the fan coil unit. This heat will cost 2 or 3 times as much as heat from the heat pump will cost (equivalent to the Coefficient of Performace of the heat pump performance at a particular exterior temperature, interior temperature, and airflow across the indoor evaporator coil). So - I started looking into the thermostat settings to see how I could avoid the emergency / auxiliary heating from kicking in so soon.

A little research lead me to the balance point settings on thermostats for heat pump systems. In order to be able to set the balance point - the thermostat needs to know the exterior temperature - to be able to know when to lock out the emergency heating, and when to lock out the thermopump. Setting these lockout temperatures can allow the thermostat to control when the emergency heating is engaged - so you're only using pure electric (or gas / oil) heat when the temperature is too low outside for the heat pump to make up the entire heating demand. This will improve the overall system efficiency - especially in the start and end of the heating season, when the exterior temperatures are still warm enough for the heat pump to deliver sufficient heating to the house.

There are a few ways that a thermostat can detect the external temperature. In the case of the Lennox X4147 - you need to install an external temperature sensor - the X4148 pictured below:

The Lennox X4148 Temperature Sensor and Bracket

The other method is to use an internet enabled thermostat, that can get the external temperature from an internet data source. The Nest thermostat is an example of a thermostat that doesn't require an external temperature sensor.

Since I already had a decent thermostat, I picked up the temperature sensor on Ebay for a reasonable price, and did the installation this morning. The instructions recommend that you use a separate, shielded cable for the temperature sensor installation, in order to minimize interference with other cables. It took me about an hour to route a signal cable out to a foundation wall through the basement.

Pulling a second, separate cable for the temperature sensor.

The temperture sensor pulled outside. I'll have to spend some time pulling this to a better location, out of direct sunlight and where the sensor could be buried with snow in the winter time. 
The final steps were to connect the cables, and program the thermostat to recognize the external temperature sensor.

The temperature sensor connects to the S1 and S2 terminals.
 Once the sensor is installed, wired, you can reinstall the thermostat face, and power up the system. Prior to engaging the heat pump or heating system, you'll need to program the thermostat to recognize the temperature sensor. You'll need to get into the setpoint programming interface - you can download the installer manual from the Internet for the full instructions.

Change installer setup number 340 to "2" in order to recognize the external temperature sensor and use it to control heat pump lockout settings. 
You can then  adjust your lockout settings for electric heat and the thermopump. Some caution is warranted here - you should check the specifications for your heat pump, and find a reasonable balance point based on the heat pump performance curve, and how well insulated / sealed your home is. You'll have to note the performance of the system in order to verify you have the correct balance point set. If you note when in ambient temperatures around the balance point you've selected your heat pump is not making enough heat for the interior temperature to meet the thermostat setpoing, you may need to adjust the balance point higher. You can research "Setting Heat Pump Balance Point" on the internet to find some further information on the thermodynamics on this. The Nest thermostat webside has a very good description on the relationship between balance point, and energy consumption.

I'm looking forward to reporting how this upgrade works with respect to energy consumption and performance. I'll provide updates over the coming weeks. 

Sources and Links

I hope you found this post useful. Feel free to ask questions in the comments section below. I answer all questions.

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