Control a Braemar TB heater with a Nest thermostat by adding a Common (C) wire

I was fortunate enough to receive a Nest learning thermostat for Christmas 2014, despite them not being available for purchase in Australia at the time. I have a Braemar TB ducted heating system which was installed before we moved into this house, I’m guessing around the year 2000. It’s a fairly simple unit that does heat only, so the wiring setup was very basic.

I went onto the Nest web site to check if the thermostat would work for me and they had a handy compatibility checker to see if it would work with my existing wiring. I removed my old Honeywell thermostat finding only 2 wires – W1 and Rh. I entered that into the checker and it came back saying that it would work. Hooray!

However, I obviously had a few issues with the installation or this post wouldn’t exist. I’ll be addressing the 2 big issues I had – not enough power to the thermostat and a heater that has a normally closed (NC) circuit, meaning it comes on as soon as the circuit is completed and therefore could be running all the time without the thermostat to control it.

The Initial Installation – finding the problem

After labelling the existing wires I proceeded to remove the backplate from the old thermostat and mount the new one as per the instructions that came with the Nest. I connected the wires to the appropriate spots. Before I even mounted the thermostat part my heater suddenly came to life. Thinking that was a bit odd, I went out to my heater to have a look at the wires and found the wiring diagram below. In the top right corner under the word ‘Thermostat’ there is a picture of a relay indicating that the thermostat would have to handle the switching.

Braemar TB Wiring Diagram
Braemar TB Wiring Diagram

I went back inside and attached the thermostat unit to the back plate and turned the temperature setting right down, expecting that it would turn the heater off. Nope – didn’t happen. I could be wrong, but it seems that the Nest thermostat doesn’t have an internal relay, instead expecting that the relay will be part of the heating/cooling system it is attached to. That was problem 1.

The next thing I found was that the Nest unit constantly shut itself off, saying it needed to charge it’s battery. Through the unit’s information screen it showed that it was receiving 16V of power through the W1 and Rh wires. According to the Nest support pages, the unit requires between 20V and 30V AC to operate. That was problem 2.

Secondary Installation – implementing the solution

Thankfully the solution to both problems was fairly easy to find using the Google machine and not too difficult to implement. Thanks to a few posts from users in the forum (especially the user hqmhqm – massive thank you ) I found the solution I needed. Post #28 has a hand drawn diagram (below) that had everything I needed to know, so off to my local Jaycar to get the bits I needed.

Wiring diagram for adding a 24V power supply (for Common wire) and a relay to a Nest thermostat

Here’s what I bought at Jaycar:

The tools you will probably need:

  • Soldering iron and solder
  • Shrink tube/electricans tape
  • Wire cutters
  • Stanley knife
  • Power drill and drill bits
  • 3 strands of speaker wire all the same length – I used a heavy gauge wire
  • Serrated blade/plaster board knife
  • Screw drivers

First off, I labelled my 3 new wires on both ends. I then taped them to the original wires and pulled them through the wall cavity to where my power supply was going to be. For me this was conveniently in the air intake chamber directly below the thermostat, but was still the most difficult part of the whole setup.

To wire everything up I  just followed the diagram that hqmhqm drew. The result is super hacked and would probably look a bit nicer mounted to a PCB, but as everything was going to hidden I didn’t really care for aesthetics. I did a ‘dry run’ first to make sure everything worked as expected before soldering things in place. The final results are in the following pictures.

Relay and Bridge Rectifier
Relay and Bridge Rectifier – white wires from the heater attached to the C and NO terminals

Above we have the two white wires leading to the heater unit soldered to the relay. These are the old W1 and Rh wires – it doesn’t matter which way they are connected as they simply complete the circuit when the relay is triggered. These are connected the the common (C) and normally open (NO) points on the relay as per the data sheet  found on the Jaycar site.

Next I attached the power supply that would give the Nest the juice it needs. The bridge rectifier stops the relay from buzzing. I found this useful page for understanding how the bridge rectifier worked, particularly that “the input to the circuit is applied to the diagonally opposite corners of the network, and the output is taken from the remaining two corners“.

Dummy wiring for testing
Hooking up the bridge rectifier

The positive wire (red) from the power supply is attached to the new Rh wire leading to the thermostat and the join protected with shrink tube. The negative wire (white) from the power supply is joined to the new C wire leading to the thermostat. These are then attached to the ‘~’ terminal at the 9 o’clock position on the bridge rectifier. The new W1 from the thermostat is attached to the ‘~’ terminal at the 3 o’clock position.

The ‘+’ and ‘-‘ on the rectifier are then attached to the two terminals on the ‘coil’ end of the relay. It doesn’t matter which way these go as the power flowing through is what triggers the relay to close. This is what makes the heater turn on.

From another angle
Bridge rectifier attached to the ‘coil’ end of the relay

After soldering all the wiring in pace and shielding with shrink tube and electricians tape I did one final test with the power supply to make sure everything still worked properly before tidying up the cables in a more semi-permanent fashion.

Testing the power supply
Testing the power supply

I attached a powerpoint mounting block just inside the air intake vent, breaking away a hole in the side to feed all the wires through, with the relay and rectifier stuffed inside. I had to ensure nothing was exposed that could be dangerous and that there was sufficient clearance for the back of the powerpoint and its wiring.

Everything 'neatly' stuffed inside a mount for the power point
Everything ‘neatly’ stuffed inside a mount for the power point

A hole was then drilled through to underneath the house for the 240v electrical wire to be fed from the powerpoint to a junction box by an electrician. All that was left was to plug in the power supply, zip tie some cables and put the vent cover back on, hiding away all my hard work.

Power supply plugged in ready to hidden behind the vent
Power supply plugged in ready to be hidden behind the vent away from little fingers

Success! After connecting the Nest to wifi and letting it do an automatic firmware update I was pleased to find that it has Australia as one of it’s configuration options and can get weather data for my local area – no further hacking required. That’s toasty warm, ‘smart-controlled’ awesomeness.