I built a double-insulated garden shed/workshop several years ago and installed a 750-watt fan heater to keep the inside temperature above freezing. The fan heater was regulated with a simple analog thermostat using a bi-metal strip. Unfortunately, the thermostat crapped out and needed replacement.

I used an older Honeywell digital thermostat, a 24-volt Omron relay, a 22-volt transformer and a W005G bridge rectifier for this project, but. you can use a different thermostat and other components to suit your requirements and budget.

Because my 22-volt transformer has a much larger capacity than necessary to operate the relay, I may look for a 12-volt relay and use a 10-volt AC transformer. The transformer rating needs only to be 300mA.

Step 1: Workshop Heater Control

First, I glued the bridge rectifier & relay to the back of the thermostat using BONDO plastic epoxy. Then I soldered wires to the relay coil and the bridge rectifier. The connections are insulated with heat-shrink tubing.

You may find it easier to solder the wires to the components before gluing them to the thermostat.

NOTE:- The original furnace circuit used 24 volt AC to switch the furnace relay on and off. I decided to use a 24 volt DC supply to switch my relay on and off but soon discovered that the relay would not release due to what appears to be ‘leakage’ current when the thermostat ‘is off’. Because of this, I used a 22-volt transformer and a bridge rectifier for the relay.

Also, because my 22-volt transformer has a much larger capacity than necessary to operate the relay, I may look for a 12-volt relay and use a 10-volt AC transformer. The transformer rating needs only to be 300mA

Step 2: Workshop Heater Control

I have "photo-shopped' the schematic to the first photo here and added text to other photos to help you understand my construction progression.

Step 3:

The 'system' has been operating for a week and the shed/workshop temperature has been a constant 10 degrees C despite the outside temperature going below zero at night.