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Friday, March 15, 2019

Controlling a portable electric heater with a Nest Thermostat

Nest E thermostat with heater and control box

My love affair with the Nest E thermostat continues (see previous post).  I acquired a second one for about $80 for use in my bedroom that is heated exclusively by a portable electric heater. I'm using a 1500W Pelonis Safe T Furnace with a ceramic heater core that works very well.  Apparently these heaters are classics now - I have had mine since the 1980's! 
Pelonis heater and my relay control box
The relay box contains a 24VAC relay rated for 15 Amps and a 20 Amp circuit breaker.  I wired it with a 12 gauge power cord and a 3-prong grounded outlet for the output.
inside my thermostat relay control box
I put a neon indicator next to the outlet so I could be sure it was switching.

Here's the schematic:

This should work with any thermostat, but use at your own risk.  It is important to use 12 gauge wire in the box and power cord to carry the high load, and I felt better putting my own 15 Amp circuit breaker inside.
Among the things that I have learned to love about the Nest are that it automatically adjusts for daylight savings.  But also it can set back the temperature when I leave home by using the IFTTT app.  Plus it could not be easier to use, program and adjust.  And of course I have connected it to Alexa so I can adjust the temperature by voice command and from my phone from aywhere.

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.