Hi All
I decided to share my experience modifying my Harmon P68 to accept and be controlled by a Google Learning Nest Smart Thermostat with wireless room temperature sensors.
Our pellet stove is now controlled by the Nest Thermostat which can be controlled and monitored by our phones. The thermostats on / off temperature is taken from a different room other than where the pellet stove is located which is also where the main thermostat is located. We have our stove in the dinning room and we have the wireless temp sensor in living room, upstairs hallway and in one of the upstairs bedrooms. The two upstairs are only for monitoring. I learned the setup really is only feasible to have satisfy the temp sensor in the living room so the dinning room doesn't reach over 90 degrees. We do have fans circulating the air which helps a lot.
I would imagine it is possible to do this with most pellet stoves as long as it uses an external thermistor circuit or if there is some manual override but I don't know how all pellet stoves operate so if you want to attempt this, do so at your own risk. There is high voltage involved.
Many will not like this setup and that is ok. This works for our household. This will shorten the life of the ignitor on the stove as it will be turning on and off more often than if you just run it constantly. Maybe your house is well insulated and holds heat better than ours and the cycling wont be as much of an issue.
Parts Required for My Install
1. 120v AC to 24v AC Power Transformer
2. 24v AC Single Pole Double Throw Relay
3. 1x 3.6K Ohm Resistor (Cold Room)
4. 1x 1.5K Ohm Resistor (Hot Room)
5. Thermostat Wire. (Used Red, White & Blue conductors)
6. Google Learning Nest Smart Wifi Thermostat
7. Google Nest Wireless Room Temperature Sensors
Wiring up the thermostat is easy enough. I only used the Red and Blue Wire (R: Power / C: Common) and White (W1: Heat)
Short Explanation
R and C wires come directly from the transformer output terminals to the thermostat. Then the white where connects to the W1 terminal on the thermostat which gets sent power from the R terminal and energizes the relay. Technically, R and C are "power" terminals as it is AC power and polarity reverses multiple times a second.
The control board outputs a 5v reference voltage on the thermistor circuit which gets pulled down more or less as the resistance changes based on room temperature. So I did some quick measurements and found 3.6K ohms (3600 ohms) works for a cold room and 1.5K(1500 ohms) works for a hot room. When the relay is OFF, the 5v ref thermistor circuit is completed with the 1.5K ohm resistor to tell the stove to shutdown. When the thermostat calls for heat, it then energizes the relay then completes the thermistor circuit with the 3.6K ohm resistor telling the stove the room is cold and to start up. The onboard temperature control is set to 73*F. That is in between the ON and OFF resistances I picked. As far as the stove is concerned, it is trying to heat to 73*F. However it is actually being controlled by the thermostat.
The control board has diagnostic capability so there is a window of resistance which is acceptable to the control board otherwise the status light will flash 4 times indicating a problem with the external thermistor wire.
I can edit the photos for more clarity if need be. I just don't have a lot of time lately.
Wireless room temperature sensor
Temporary Wiring. Permanent wiring will be done eventually. I'll add in fused protection on the 24V circuits. External thermistor wire is not connected to anything. It is just there so it does not get misplaced.
Pink /Black wires are going to thermistor circuit terminals (Polarity does not matter)
I decided to share my experience modifying my Harmon P68 to accept and be controlled by a Google Learning Nest Smart Thermostat with wireless room temperature sensors.
Our pellet stove is now controlled by the Nest Thermostat which can be controlled and monitored by our phones. The thermostats on / off temperature is taken from a different room other than where the pellet stove is located which is also where the main thermostat is located. We have our stove in the dinning room and we have the wireless temp sensor in living room, upstairs hallway and in one of the upstairs bedrooms. The two upstairs are only for monitoring. I learned the setup really is only feasible to have satisfy the temp sensor in the living room so the dinning room doesn't reach over 90 degrees. We do have fans circulating the air which helps a lot.
I would imagine it is possible to do this with most pellet stoves as long as it uses an external thermistor circuit or if there is some manual override but I don't know how all pellet stoves operate so if you want to attempt this, do so at your own risk. There is high voltage involved.
Many will not like this setup and that is ok. This works for our household. This will shorten the life of the ignitor on the stove as it will be turning on and off more often than if you just run it constantly. Maybe your house is well insulated and holds heat better than ours and the cycling wont be as much of an issue.
Parts Required for My Install
1. 120v AC to 24v AC Power Transformer
2. 24v AC Single Pole Double Throw Relay
3. 1x 3.6K Ohm Resistor (Cold Room)
4. 1x 1.5K Ohm Resistor (Hot Room)
5. Thermostat Wire. (Used Red, White & Blue conductors)
6. Google Learning Nest Smart Wifi Thermostat
7. Google Nest Wireless Room Temperature Sensors
Wiring up the thermostat is easy enough. I only used the Red and Blue Wire (R: Power / C: Common) and White (W1: Heat)
Short Explanation
R and C wires come directly from the transformer output terminals to the thermostat. Then the white where connects to the W1 terminal on the thermostat which gets sent power from the R terminal and energizes the relay. Technically, R and C are "power" terminals as it is AC power and polarity reverses multiple times a second.
The control board outputs a 5v reference voltage on the thermistor circuit which gets pulled down more or less as the resistance changes based on room temperature. So I did some quick measurements and found 3.6K ohms (3600 ohms) works for a cold room and 1.5K(1500 ohms) works for a hot room. When the relay is OFF, the 5v ref thermistor circuit is completed with the 1.5K ohm resistor to tell the stove to shutdown. When the thermostat calls for heat, it then energizes the relay then completes the thermistor circuit with the 3.6K ohm resistor telling the stove the room is cold and to start up. The onboard temperature control is set to 73*F. That is in between the ON and OFF resistances I picked. As far as the stove is concerned, it is trying to heat to 73*F. However it is actually being controlled by the thermostat.
The control board has diagnostic capability so there is a window of resistance which is acceptable to the control board otherwise the status light will flash 4 times indicating a problem with the external thermistor wire.
I can edit the photos for more clarity if need be. I just don't have a lot of time lately.
Wireless room temperature sensor
Temporary Wiring. Permanent wiring will be done eventually. I'll add in fused protection on the 24V circuits. External thermistor wire is not connected to anything. It is just there so it does not get misplaced.
Pink /Black wires are going to thermistor circuit terminals (Polarity does not matter)
