Shop Heat Thermostat

A quick write up of a recent project that turned out to be pretty handy.

My workshop/garage is heated with a forced air overhead furnace that's not very efficient so it's kept pretty low when not in use.
In fact it's normally kept at just below freezing by using a "variable calibration" with an old school mercury thermostat.
i.e. The thermostat is mounted on a lever that allows changing from a normal (calibrated) level orientation minimum 40F operation, to an angled ~25F (re-calibrated) minimum temperature.
One of the reasons I'd never installed a smart thermostat was nothing went low enough. At least nothing in my price range.
So this is the alternative solution that allows a basic automation of the furnace.
The costs were minimal since all this was sitting on a shelf.

An old Thermostat from a previous renovation.

Donor Tstat

1 CCC tstat

A Sengled bulb that I'd picked up for cheap thanks to @smarthomeprimer and his Hack thread.

Bulb/ZigBee Module

A high level trigger relay assembly that was left over from another project.


And a few wires, cables, etc.
The end result is a ZigBee controlled thermostat installed in parallel with the old re-calibrated thermostat that is either:
a. active and acts as a high temperature setpoint thermostat, or
b. inactive and the furnace is controlled by the old sub-zero calibrated thermostat.

The process was straightforward and relatively quick to implement.

  1. ZigBee module.

Pair the Bulb to Hubitat
Name it: "Shop Heat Thermostat"
Switch the driver to "Generic Zigbee Switch" to eliminate any on/off ramping.

1a. Harvest the ZigBee module from the bulb.
Again, thanks to @smarthomeprimer for his efforts.


  1. Install module and relay in old thermostat using an old USB cable and some wires and velcro.

  2. Mount the thermostat in your shop/garage wire in parallel with the old unit and plug into a 5V USB adapter.




No more walking into a cold shop and waiting for the heat or forgetting to turn off the heat.

This worked so well I did another one for someone without a hub using a Wi-Fi relay, works great as well.


Clever idea to just rotate the thermostat. For the last 10 years I've been using a digital Honeywell thermostat that went down to 40 as the minimum. Always a pain when I had to bundle up, walk to the shop, crank up the heat, and strip off layers as the temp came up. (I know..."First World Problems") Last year I wired a zigbee Plug to a relay, and the contacts of the relay were in parallel with the thermostat. If I wanted to warm up the shop I asked Alexa to start the preheat which triggered a rule to turn the plug on (run the heater) for 15 minutes. Better, but still not perfect.

Just today I wired up a Centralite 3156105 zigbee thermostat. Factory set for a low limit of 44 but configurable anywhere from 0 to 99. I changed it to a low limit of 40. Now when I want the shop warmed up I have a rule to Heat things up to 65. The rule waits until midnight and sets back to 40. And the thermostat was less than $22 on eBay!

That bulb hack looks pretty useful though... have to bookmark that for sure.

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Yup, have to thank my father for that. A long time ago I went out and bought a heater for his garage. He modified the thermostat with a scrap of plywood to keep the snow and ice from melting while vehicles were parked inside.
Warm enough to heat up quickly after departing but not so warm that the floor was a mess.

Unfortunately these deals end up costing quite a bit more by the time they get to my place... Cheapest at the moment ends up over $50 in the end.
Not outrageous, but quite a bit more than the ~$8.00 in parts for this project.

Was that thermostat hooked to a coal furnace? :grinning:

Nope, that one had a longer handle, we called it the "shovel". :wink: