Any safe smart relays for heaters/AC? (17A+ / 240VAC)

I’d like to replace my wall-mounted heater’s rheostat with a smart switch. It would be a simple task if I could find a safe device. Itead’s POW Sonoffs are rated for 16A max and they’ve had recalls over PCB meltdowns at even that load (they also haven’t kept track of their bad batches).
My heater is 16.7A / 240VAC / 4000W. I plan to have homemade temperature sensors around the house and would love it if the heater knew when to turn itself on.

Has anyone retrofit a heater/AC like this? I’m asking partly because I’m wondering if I’m playing with fire (literally).

Use a standard relay to switch a higher powered contactor, such as this

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can you only power up the whole heater, or has it a switch to set it at half or less?
i ask because we got some 2400 watt heaters here and they have a switch to use 1200 or 2400 watt.

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I did what @accyroy suggested for my hot water heater.

Good thought. This wall heater does work on a rheostat which cuts the wattage, but I’m interested in bypassing that rheostat with my own switch. I’m assuming that my on/off bypass will draw the full 2400 watts while on. My space heater has an option to cut the voltage though and I have been considering an Itead POW Sonoff in a metal enclosure for that.

Thanks guys. I’m pretty new to all this stuff. I like this idea and could definately see myself putting a small 10A sonoff on an extension cord into a metal box with the relay, then connecting the sonoff to the relay, then connecting the relay to my wall heater’s main line. That would definitely do the trick. I could even wire up baseboard heaters and my in-wall AC unit the same way.

I’ve been looking into laws for my area and realized I’d have to hire a contractor to work on or inspect any wiring that wasn’t “like-in-kind replacement” of any already installed component (I don’t want to burn the place down and have insurance negated due to wiring that wasn’t up to WA code). I appreciate the help! I’ll get a relay and see what a contractor would cost.

not sure what your setup looks like but there are some zwave systems out there.


i had the option to use 2 switches for the heater. i took a relayboard with 2 relays(10A) and hooked that up.
if you could splitt the input to 2 lines you could connect 2 relays also.
the other option is just 1 relay from 20A.

i simple board like this one:
and then connect that to a ESP or arduino.

cut 1 of the powerlines to the heater and stick the relayboard between it.
that can be put anywhere you can reach the powerline, and for safety in a box.

Hi Wheezy,

Sure you’re thermostat is switching the full 17Amps?
Interestingly, I’ve started also looking into replacing our wall mounted thermostat as well.
Current thermostat is a Siemens rev16 runs from 2 x AA batteries.
On removing the wall plate, has two wires entering the unit, which measures 240Vac

The thermostat shorts out the two wires via a small internal relay this in turn must then trigger a larger relay within the heater.

Which makes sense, why run a massive copper cable all the way out to your thermostat?

Keep us posted on which thermostat you end up with, my initial investigation I’ve only found smart thermostats that support 24Vac ie RadioThermostat.


I was looking for more information online and found out that you’re absolutely right. The wall heater is actually two separate 2000W heaters wired in series.

I’d have to slightly alter the wiring to install a Sonoff POW onto each heating element, so, if I do this, I’ll talk to an electrician / contractor / any other inspectors to check my work before I turn it on. I’ll have to monitor the Sonoffs for a week or so to ensure they’re from a good batch too. But that should do it!

Other design/safety notes: The heater’s internals/rheostat are also already self-enclosed in a metal box with good air flow for safety, so the sonoffs can just join the encased internals. Also, importantly, I realized that I may need to test wiring the sonoff behind the rheostat instead of replacing the rheostat entirely (and get another rheostat to install on the heating element that doesn’t have one since both are currently wired in series and under the control of just one rheostat). This way, I can check to see if the rheostat prevents the heater from over-heating itself. I wouldn’t want my HASS server to lose wifi connection for some reason and forget to regulate the Sonoffs controlling the heater.

EDIT: After more consideration, not modifying the hardware or using dodgy Sonoff POW’s seems like the more responsible route even if it is technically feasible. Installing the Z-wave 40A switch on the main line seems like a lot safer option from a liability standpoint. The Z-wave switch and a Z-wave dongle would be pricey, but I think it would be worth the peace of mind. And, again, I’d have it inspected before turning it on if I went that route.

Thank you guys

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I don’t actually have an HVAC/thermostat installed, but I see how my initial post could sound misleading. Instead, all my heating/cooling comes from a wall-mounted space heater and a wall-mounted AC unit