Wednesday, March 13, 2019

NEST thermostat for my Rinnai monitor heater

I heat my home in rural Maine with a combination of propane and a large wood stove.  I have 2 Rinnai monitor heaters - one in the living area and 1 in the guest bedroom.  The wood stove in the basement is capable of heating the whole house, but I only fire it up when the temperatures drop below 20F which is often in the winter.  This strikes a balance for me of cost and performance - and carbon footprint.  In an ideal world I would replace the propane units with heat pumps, but that is beyond my budget for now.

I have been exploring different ways of controlling the big propane heater in my living room.

It's a 20 year old model that has a manual switch that you push-on/push-off and a simple slider to set temperature.  

A while back I built an interface that allowed me to use a regular programmable wall thermostat to operate it by replacing the power switch with a relay, and that worked relatively well.  But my schedule is erratic and I wanted the option of remote control.  The NEST thermostat E is a very appealing thermostat, but I had considered it too expensive until I found a used one one on eBay for about $80.  (The newer E model lists for $169, and the previous one is $249 and these prices are way more than a regular programmable thermostat that sell for $45 to $65.)

Here's the hookup info for the relay I used to control my heater.
I used a 24VAC power adapter that I got on Amazon that is sold specifically for home thermostats, and a 24VAC relay from Digi-Key (their part number: Z9722-ND ).  It was pretty simple to hook these items up with the 25ft. of wire that came with the adapter.  I did have to get inside the heater and install the relay in there, and it was not something I would recommend for anyone not experienced with wiring or electronics.

This setup works really well for me.  Here's what I like about the Nest in particular.  First, I can control the thermostat setting from anywhere using my phone.  The user interface is excellent and it is extremely easy to program a schedule.

I also like that I can review the history that shows hours of operation per day and the individual cycles.

And the main screen could not be simpler:

Since I have installed an Alexa echo dot, I can say: "Alexa, set the thermostat to 68" and she set's it instantly.  This is nice if I decide to get up earlier than the scheduled time because I can use the 2nd echo dot in my bedroom to turn the heat up before I go downstairs.  Or, similarly if I decide to knock off work early I can set it from my office using the app.

Another clever feature is that the Nest can be programmed to set the heat back if I leave home.  It uses the IFTTT app to geolocate my home and can tell when I leave the area.  Overall I could not be happier with the Nest and all its features.

In my next post I show how I built an external relay box to control a portable electric heater from the Nest E.


  1. Hi Gus, this sounds interesting as I have a similar heater. Question, I am guessing you don't really use the slider to adjust temperature, you just leave it on a setting that will turn on when the Nest demands heat. Is that correct? Thanks.

  2. Yes, I leave the slider on the Rinnai at about 80F equivalent so it turns on full blast when heat is called for.

  3. Where you able to find a mounting base for the relay noted in your description so that the relay is securely fastened? If yes, do you have a part number?

  4. I found one on Amazon that comes with a socket, search on: OMRON INDUSTRIAL AUTOMATION MKS2PI AC24 Power Relays, MK-S Series, Power - General Purpose, Non Latching, DPDT, 24 VAC, 10 A

  5. . . . but you can just solder onto the relay and then wrap it in tape and let it hang loose in the heater. That's what I did.

  6. I also live in Maine and I also have the same exact model rinnai haha. I'm just looking to hook it up to a normal wall thermostat. I'm no electrician so any advice? How complicated is it to do so?

    1. You definitely need electronic skills to do what I did. So try and find someone who understands what I did an they should be able to figure it out.

  7. Are you relaying the power supply, just switching the power on/off, or are you relaying the switch allowing the unit to appropriately cool down before turning off?

    1. The relay contacts are in parallel with the power switch, so this does allow the heater to cool down after the switch/relay is opened. This is been working perfectly for a year now!


I welcome all thoughtful comments and feedback!