Most cheap space heaters have only an analog knob to coarsely set the temperature; even the fanciest models only allow you to set them to turn off automatically after a preset number of hours.
This project allows you to set the room temperature according to the time of day and day of the week, giving you much needed flexibility to save energy and avoid waking up to a freezing house! You can save energy by programming the thermostat to lower the room temperature at night, but still get out of bed to a toasty room in the mornings.
Step 1: Materials and safety warning
- A digital programmable thermostat. I found a used one on ebay for about $15. It is a Bryant and was originally used in a commercial building. Commercial thermostats typically don't have battery backup, something to keep in mind if you plan on moving the heater around and don't want to reset the clock. Commercial thermostats are also typically cheaper than name brand consumer models. Make sure you get one that is programmable, many digital models are not, shocking considering the minimal effort in adding the feature and the energy savings in a typical home!!
- A relay with a 24VDC coil voltage and around 700 ohms coil resistance. The contacts should be rated to at least 15 or 20A at 110VAC minimum. $3-$5 at your local electronics surplus store.
- A 110V to 24VAC transformer. My transformer was rated at 36VAC, 65mA on the secondary, and maintains about 20VAC under load between one end of the secondary and the center tap. 20VAC seems to be within the input supply range of the thermostat, the exact voltage is not critical. Another electronics store find - $3.
- An enclosure, power cord and AC receptacle. I gutted a fax machine power conditioner and got all three for about $2.
- Some parts you might already have in your junkbox - a 1k resistor, 1n4001 diode, 100uF capacitor. A terminal strip or some perfboard.
- And I almost forgot - a space heater. Mine is a Bionaire MicaThermic Convection Console Heater - about $40 (in store price) at Costco.
Space heaters typically consume on the order of 1500W, or roughly 15A at 110VAC. All wiring needs to be sized appropriately to handle these currents. Undersizing the wire gauge used or poor connections could lead to a fire! Also, operating a space heater while you are not home is probably a bad idea. I recommend unplugging the space heater before leaving it unattended for an extended period of time. Be safe!