Space Heater Controlled by Digital Thermostat
Intro: Space Heater Controlled by Digital Thermostat
This instructable shows you how to use an off the shelf digital programmable thermostat to control an inexpensive space heater.
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.
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
You will need the following materials for this instructable:
- 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.
Read this!!
Safety warning:
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!
- 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.
Read this!!
Safety warning:
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!
STEP 2: Schematic
Here is a rough schematic of the circuit (also my first experience with Eagle!).
Notes:
Only the W1 output of the thermostat is used.
C is the common terminal, some thermostats may not have this. Mine uses it to power the backlight and digital functions since it has no battery.
R is return and completes a circuit with the W1 terminal when the thermostat activates the heater.
C1 should be rated ~50VDC. The exact value is not critical.
The thermostat needs to be on the unswitched side of the relay so that the thermostat always has power. The hot wire should be switched for safety. The ground wire is not shown and just passes through the box from the cord to the receptable. If the enclosure is metal (not recommended) it should be grounded.
Notes:
Only the W1 output of the thermostat is used.
C is the common terminal, some thermostats may not have this. Mine uses it to power the backlight and digital functions since it has no battery.
R is return and completes a circuit with the W1 terminal when the thermostat activates the heater.
C1 should be rated ~50VDC. The exact value is not critical.
The thermostat needs to be on the unswitched side of the relay so that the thermostat always has power. The hot wire should be switched for safety. The ground wire is not shown and just passes through the box from the cord to the receptable. If the enclosure is metal (not recommended) it should be grounded.
STEP 3: Assembly
Assemble the 1k resistor, diode, and 100uF capacitor on a terminal strip or perfboard. The purpose of this circuit is to convert the output of the thermostat, which is AC, into a DC signal to drive the relay. There is a fair amount of flexibility in this circuit - these are just parts I had in my junkbox.
The thermostat uses a triac to switch the heater on and off. Triacs only work with AC signals, they can't be used directly to switch DC because they will "latch" on and not turn off until power is removed. The 1k resistor in the circuit ensures that a small amount of AC current can pass through the triac and avoids the latching problem.
The thermostat uses a triac to switch the heater on and off. Triacs only work with AC signals, they can't be used directly to switch DC because they will "latch" on and not turn off until power is removed. The 1k resistor in the circuit ensures that a small amount of AC current can pass through the triac and avoids the latching problem.
STEP 4: Assembly Continued
Assemble the remaining parts and complete the wiring. The thermostat is attached to the top of the box by its original wall-mount with three screws, and can be popped off to make the necessary connections. Be sure to use heatshrink or put tape on all 110VAC connections to reduce the chance of electric shock or shorts!
The pinout for the relay can be determined by the diagram on the top of most relays, or with an ohm meter.
The pinout for the relay can be determined by the diagram on the top of most relays, or with an ohm meter.
STEP 5: Final Assembly and Testing
Before you close up the enclosure, do some preliminary testing. With nothing connected to the AC receptable, plug in the power cord. Verify that the thermostat powers up. A test lamp or small wattage light bulb connected to the outlet should be off.
Set the thermostat for heat mode and increase the set temperature above the room temperature as shown. Make sure the relay closes and 110VAC appears at the outlet, or the lamp turns on. If it checks out, test it with a real space heater, and allow it to run at least half an hour on your bench. Turn it off and inspect for any overheated wires or hot components.
If everything checks out, congratulations! You now have a digitally programmable space heater!
Set the thermostat for heat mode and increase the set temperature above the room temperature as shown. Make sure the relay closes and 110VAC appears at the outlet, or the lamp turns on. If it checks out, test it with a real space heater, and allow it to run at least half an hour on your bench. Turn it off and inspect for any overheated wires or hot components.
If everything checks out, congratulations! You now have a digitally programmable space heater!
83 Comments
globrite 10 years ago
pbennett9170 1 year ago
immaculatelation 9 years ago
Thanks for creating this! I'm using this to control the heat in an outdoor, insulated cathouse--or perhaps I should call it a cat-cabin!
manicmonday 4 years ago
RayB105 5 years ago
I took this project to a whole other level. Its purpose is to control a space heater but also to control a window fan during the fall and spring. I also have it running the ceiling fan as well. I Used a programmable Honeywell thermostat, 24 v transformer, 2- 24 relays dpdt and 30 amp 110v relay spst. Yes grounded and 10 amp internal fuse (medium heater setting only) With an extra outlet on the backside. Purpose is to heat only one room and not the whole house. Only slight fluke is the heat from coils and relays adds 4 degrees over actual room temperature. Also motion sensor shuts down the coil turning everything off when I am not there (2amp fuse on that) as well.
Virgil2U 6 years ago
I have a propane space heater mounted on the wall, is their any way to install a regular wall thermostat to this just like you did for the electric heater? The current thermostat is mounted on the heater but it does not evenly heat the whole cabin, I want something that will keep it running to heat the whole cabin. It does have a very small fan built into it. But not powerful enough to feel it blow.
immaculatelation 9 years ago
Are you saying that all digital thermostats use triacs?
H3xx 12 years ago
ssbt 14 years ago
ssbt 14 years ago
ssbt 14 years ago
mightyohm 14 years ago
Thanks for your comment!
- Jeff
manicmonday 14 years ago
You can take advantage of this idea without any modification of the circuit. You only have to make sure that you connected your hot wire to the relay so that the "Normally Closed" terminal has power when the relay is not activated. Then you would run that hot wire out to appliance #2.
Maybe you want a heater running if it's below the set point, and you want a fan running if it's above the same set point. Maybe in your greenhouse you want some bright (and hot) lights on if the temp is below the set point, or some not so bright (but cooler) lights running if the temp is higher than the setpoint.
manicmonday 14 years ago
mightyohm 14 years ago
The capacitor, resistor, and diode are part of a circuit that converts the AC control voltage from the thermostat into a DC voltage used to switch the relay. Most relays need a DC voltage on the coil and won't work with AC.
manicmonday 14 years ago
I know you can convert ac to dc using a bridge rectifier using 4 diodes. I guess just one diode will block 1/2 the voltage so that what comes through is 1/2 strength dc. What do the resistor and the cap in that circuit do?
Thanks for making that more clear for me.
mightyohm 14 years ago
A bridge rectifier would be more efficient but requires more parts. The single diode will output the same voltage but as a series of pulses instead of a constant level. The capacitor smoothes out the pulses into a DC level.
manicmonday 14 years ago
I just measured voltage from line 1 of the transformer, and then to both sides of the resistor. The unmodified reading was 21.4vac, and the reading across the resistor was 21.1vac. Does that sound right to you? If so, why do we need a resistor at all for such a small voltage change? Just wondering cause I thought the voltage change was gonna be a lot more.
Thanks
mightyohm 14 years ago
21VAC should be fine, as long as everything works, don't worry about it.
manicmonday 14 years ago
I've heard of voltage drops with resistors. I looked it up on wikipedia. It gave me a lot of info, but nothing to tell me how a resistor works. Like "What does 1.1 K mean?" "What does 1/2 w mean?" If I use different values how does it change the overal circuit?
You are right. If what I have works, then I shouldn't be asking any questions then, right?
Again, don't mean to be a bother.
Thantks