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How to use a smart thermostat with an electric heater

Smart thermostats are primarily designed to be used with central heating boilers, but what if you want to manage an outbuilding or somewhere else with an electric heater? Here, I’ll show you how to use a smart thermostat with an electric heater.

This is the same set up that I use to test smart thermostats, as it gives me an easier way to switch out different review products. Before you start, it’s important to point out that this project involves working with mains electricity. As a result, you should seek help from a qualified electrician: think of this as a guide to show what you want to achieve.

Next, you need to have the right type of electric heater: one with a physical, manual on/off switch that can be left in the on position, so that when power is turned on, the heater comes on. Many, fancier, electric heaters turn on in standby mode, which is no good. I use a Honeywell HZ-510E1 Heavy Duty Fan Heater (£99).

Buy: Tado Smart Thermostat Starter Kit for just £199.99

This project is designed to work with a single radiator only, although an electrician could adapt the design so that it works with a full electric heating circuit.

Related: Best electric heater

1. Choosing the components that you need

First, you need to understand how the system works. Using a smart thermostat we want to be able to toggle the fan heater to come on and off. To do this we need to interrupt the live power cable to the heater, letting the thermostat toggle this on and off. Due to the power levels that electric heaters draw, we can’t do the switching through the thermostat.

Instead, we’ll use a solid-state relay instead (around £11). A solid-state relay is an electronic switch that turns on or off when voltage is applied across the control terminals. Effectively, it means that the switch can be controlled via a low-voltage control system, via the thermostat, toggling the high power live feed to the heater. The model that I’ve chosen is a 40A relay, which is more than enough for our heater.

Next, we need the control circuit and to power the heating system. For this project, we’re using a Tado Smart Thermostat with an Extension Kit (the wireless boiler controller). Nest thermostats have a similar heat link and most thermostats have something similar.

External boiler controls are usually powered from the boiler, using a 3A fuse. As we’re feeding in a 13A signal, we need to put in a 3A fused spur (around £7) with surface pattress box. You’ll need to swap out the fuse for a 3A model (£2.50).

We’ll use this to feed the connection to the boiler controls. For this, I used an AC to DC power supply converter (around £7), which is fed by the 3A fused spur, and outputs low-voltage (5V) power, which we use as our switching circuit.
You’ll also need some 3 Core, 13A cable (around £17) and a 13A plug (around £4).

To house everything, I used a junction box (around £11), and some cable glands (around £7). You’ll also need a 20A junction box (around £3). Finally, you’ll need some connector blocks: I had 5A ones (around £2).

For tools, you’ll need some wire strippers, a 20mm drill, an electrical screwdriver and some small screws for mounting components to the bottom of the junction box. For ease, you can find everything you need on my smart thermostat Amazon wishlist.

Related: Best smart thermostat

2. Wire up your starting circuit

First, you need to drill 20mm holes in the sides of the junction box, so that you can fit in the cable glands. These will hold the cables in place without them rubbing through. One on each short side is good, and you need one on one of the long sides.

Next, mount the 20A junction box to the floor of the main junction box. Cut the plug off of your heater, and strip back the rubber sheath. Feed this through the cable gland on the short side of the box, farthest from your 20A junction box. Remember to put the screw-in part of the gland in first, so you can tighten the connection.

Then you need to cut two lengths of flex: give yourself some flexibility in length, so 50cm or so should do it. Then, take another 50cm length of flex cable, but strip out just the brown (live) cable and strip it both ends.

Wire one bit into your 13A plug, then feed the bare end in through the gland closest to the 20A junction box. Finally, take the spare bit of flex cable and strip the cables at both ends.

Buy: Tado Smart Thermostat Starter Kit for just £199.99

We’re now ready to do the first bit of wiring. For the power cable coming into the junction box, run the live (brown), neutral (blue) and earth (green and yellow) cables into the 20A junction box. Connect up the 50cm bit of flex into matching terminals (this will run to our fused spur).

For the cable running to the heater, put the live cable (brown) into the second output of the solid-state relay. Then use the length of live-only cable from terminal 1 on the relay to the live junction on the 20A junction box. Finally, connect the heater’s earth and neutral connections to the 20A junction box. You should have something like the image below. You can now fit the cover on the 20A junction box, after you’ve checked that the cables are secure: give them a slight tug.

Smart Thermostat Electric Heating Initial Wiring

3. Fit the fused spur

Next, you need to fit the fused spur. This has a cut-out on the front, and you can also push the cable through the pattress box at the bottom. I took power in through the cut-out on the switch, and the power out through the bottom. Don’t screw the pattress into place yet.

First, you need to take the bit of flex from the 20A junction box and wire this into the feed side (check the wiring diagram to get this right). Then, strip another bit of flex cable, cutting enough to run out through the bottom gland to your heating control – 1m or so should do it.

With this bit of flex, wire it into the load side of the fused spur. There’s only a single shared earth connector. Also, wire in the live cables from the AC to DC converter: the one I have has two red cables for this, and it doesn’t matter which way you wire them in, and there’s no earth cable. Make sure that you run these cables up through the back of the pattress box.

Smart Thermostat Electric Heating fused spur

You can now screw the pattress box into place, an fit the switch, using the retaining fuse. Mount the AC to DC converter into place, too. This should leave you with the red and black cables from the AC to DC converter loose, and a power cable sticking out through the gland on the long side of the main junction box.

Smart Thermostat Electric Heating main components in place

4. Wire up the low voltage control circuit

Next, you need to wire up the low voltage controls for your heating system. To do this, we need to run the wires for the 5V control circuit. You can use more flex cable if you want: just strip out some 1.5mm cable from inside. I used the cabling from an old 5V USB power adaptor instead, stripping it down.

For ease, I ran the cables from the AC to DC power converter into a 5A junction box: red is positive and black is negative. Using my 5V power cable, I connected the positive connection to the positive connection on the control side of the solid-state connector.

I then needed two more lengths of the 5V power connector: enough to reach the external controls. Connect one cable to the negative side of the solid state’s control side. Connect the other cable to the negative connection of the AC to DC converter’s junction box. Run both cables out through the same gland as the boiler control’s power connector.

Tado Smart Thermostat connector block

5. Wire up the external box

Next, you need to wire up your boiler control. The Tado Extension Kit has a wiring diagram on the back of the box, which shows you what to do, and we’re using the Potential Free option, not Switched Live. If you’re using a different heating system, check the wiring.

A white Tado water heater's internal structure view

With Tado, the wiring is done through the block that connects to the wall. Hook up the flex cable to the Live and Neutral terminal, and the earth to the earth terminal. We then need to hook up the two flex cables. It doesn’t matter which one goes where, but you need to use terminals 1 (COM) and 4 (Normally Open or NO).

A white Tado water heater's internal circuit view,  attached to a wall

So, what happens is that when the smart thermostat wants heat, the boiler controls connect terminals 1 and 4, creating a circuit. This, in turn, runs voltage through the solid-state relay’s control side, which switches on the live cable to the heater turning it on. Put the cover on your heating controls.

6. Turn it on and hook up your heating

A white Tado water heater attached to a wall

Now, you can turn your system. Plug your plug into a wall socket and turn it on. Turn on the switch in the fused spur. You should see a status light turn on, plus a red LED in the AC to DC converter, and a light on your boiler controls.

You can then follow the instructions to add the smart heating using the app. Once done, test your system by turning the heating up and down: when you want more heat, the heater should turn on; when the room’s at temperature the heating will turn off. Fit the lid to your large junction box and screw it into place, and you’ve now got electric heating fully controlled by a smart thermostat.

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