Hooking up Any Smart Wifi Thermostat to Harmon P68

[jgray152]

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)

 

[jgray152]

Being able to control the pellet stove from my cell phone is a big win. Easily see when the thermostat is ON or OFF and see the history of when it is running and how often is nice to see as well.

Naturally we have saved pellets by not having this stove run all the time on "constant stove temp"

 

[nh-jjyy]

nice, we are trying to do the same thing, but either of us is handy enough....

 

[jgray152]

This setup has been working great. We use about 1 bag in a 24 hour period. I have it turn up to 74 at night with a fan blowing up stairs and 70 during the day. No issues at all.

 

[mtnbiker727]

Why not flip the switch to manual? The stove will trickle out pellets when heat is not being called for, and it will ramp up when heat is called for. Then you have a constant heat source like a wood stove, but increasing temp when needed, and not use the igniter except starting it up for the first time. This mode will make the whole house feel warmer, because it'll warm up the floors and walls. It's pretty frugal with pellets too, because it doesn't have the big slug on startup over and over again throughout the day.

 

[bogieb]

People worry too much (IMO) about the igniter life. Both my stoves are usually kept on room temp / auto so they only use pellets when the house needs added warmth (I only heat with the pellet stoves) There are times that I end up putting one, or both, in a constant burn mode, but that is only when it is single digits or if it is super windy and in the teens. So far this year, I only did that a couple of days for a few early morning hours for the P43. Neither stove has lost an igniter yet (KOW).

I have them each on a wired, remote basic thermostat that is battery operated. I wired in a bit differently, using and "in-line" method to the room temp probe.

Looks like you put together a system that works well for you and gives you the info you desire 👍

 

[rickwai]

People worry too much (IMO) about the igniter life. Both my stoves are usually kept on room temp / auto so they only use pellets when the house needs added warmth (I only heat with the pellet stoves) There are times that I end up putting one, or both, in a constant burn mode, but that is only when it is single digits or if it is super windy and in the teens. So far this year, I only did that a couple of days for a few early morning hours for the P43. Neither stove has lost an igniter yet (KOW).

I have them each on a wired, remote basic thermostat that is battery operated. I wired in a bit differently, using and "in-line" method to the room temp probe.

Looks like you put together a system that works well for you and gives you the info you desire 👍

100% agree. Let the stove do what it is designed to do. Mine runs in auto 24/7 all winter and goes thru start up many times a day for months at a time and my igniter is several years old at this point. OEM igniters last for a long time it is worth the extra money. . Years back we use to replace igniters all the time, now I have not replaced a Harman igniter this season yet. And I have been on probably 50 service/cleaning calls.

 

[hlthy]

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.

View attachment 304811
View attachment 304815

Wireless room temperature sensor
View attachment 304817
View attachment 304814

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.
View attachment 304822

Pink /Black wires are going to thermistor circuit terminals (Polarity does not matter)
View attachment 304820
View attachment 304821
View attachment 304816 View attachment 304819

Thank you for pointing me in the right direction i have a PF120 which also uses a millivolt thermostat i was able to wire in the relay setup pictured using just one 3.6k on "off" leg and straight short on the "on" works with any powered thermostat now no 4 blink errors