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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.

10 comments :

  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.

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

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  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?

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  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

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  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.

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  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?

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    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.

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  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?

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    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!

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I welcome all thoughtful comments and feedback!