DIY Digital Thermostat-controlled Space Heater

Finished Controller

The rain and dropping temperatures in San Francisco this weekend reminded me of a project I made in the winter of 2006.  This was long before existed, so I originally documented the project at instructables.  I’m not going to repeat everything here, but I wanted to share some pictures and provide a link to the instructable in case anyone else wants to try this at home.

The project involves using a digital programmable thermostat to control an inexpensive space heater.  The original motivation for this was that I wanted to lower the temperature of the heater at night, reducing my energy bill, while still being able to wake up to a toasty room in the morning by setting the heater to turn on full blast 30 minutes before I awoke.

Here’s a schematic of the simple circuit I made to interface the thermostat to the space heater.  The resistor/diode/capacitor circuit allows the thermostat, which is designed to control an AC load, to switch power to a 24VDC coil relay.  A 36VAC

Space Heater Controller Schematic

I installed the necessary components inside the case of an old power and telephone line filter, used to protect a fax machine or office copy machine from power surges.  The case came with a handy 110V outlet mounted on the front panel which I reused for this project.  The digital thermostat mounts to the top cover of the case.

I used a barrier strip as a way to simplify the wiring and mount the few loose components:

Space Heater Controller - Inside

I had to adjust the value of C1 to get reasonably clean DC to the relay while not having an annoying turn-off lag when the control line from the thermostat goes low.  100uF works well for the relay I used.

Here’s the finished product installed in my former bedroom:

Controller and Space Heater

I haven’t used it since I moved into a house with working central heating, but it sure came in handy during the cold winter I spent in a 100 year old farmhouse in Petaluma.  This solved the problem of the sub-50 degree mornings I was having nicely! Space heater controlled by digital thermostat

Comments?  Questions?  Leave a comment below!

6 thoughts on “DIY Digital Thermostat-controlled Space Heater”

  1. hey i really like your design i hate to sound like and idiot could u draw my a picture showing the thermostat connections the relay and terminal with components to a out let a scimatic for dummies

  2. Your point about cost is well taken, our solution to a similar problem was more than $150 for sure but we used a Venstar thermostat and insteon control modules to turn on and off our window AC units in the bedrooms.

    Even if you don’t use it anymore, it’s neat to look back on and say “hey, that was pretty cool”. 🙂

  3. Why dont you just buy a heater with a digital thermostat already on it, like the iHeater that I bought from for only $279. It heats my house, and it cut my heat bill almost in half. It only operates til it warms up then it shuts off.

    1. Well, for one thing, I already had the space heater on hand, so adapting it to my needs vs. buying another one made more sense to me. Second, the total cost of my project was under $150 including the space heater, half the cost of the cheapest heater on that site…

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