I'm hoping some of you knowledgeable folks from the UK (and beyond!) can help me out with a bit of a smart home head-scratcher I'm experiencing with my bathroom fan setup.
Summary of the Problem:
In a nutshell, I've installed a Moes 2-gang smart switch module to control my bathroom fan and light. When Home Assistant turns on the fan due to high humidity (independent of the light switch), the bathroom light also comes on, which I don't want. I suspect this is due to some sort of backfeed on the switched live circuit.
The Setup & What I'm Trying to Achieve:
My goal is to have a smarter bathroom fan that operates as follows:
Nomal Operation: Turn on the bathroom light switch, and the fan comes on. Turn the light off, and the fan continues to run for a pre-set overrun period (say, 10 minutes). This is how the fan works without the smart module.
Night-time Override: I want to be able to disable the fan via Home Assistant so it doesn't come on with the light during the night (e.g., to avoid waking people up).
Humidity Activation: I want Home Assistant (which already monitors bathroom humidity via separate sensors) to be able to turn the fan on without the main bathroom light, if humidity gets too high.
I'm using a "Moes Smart Switch Module (2-gang)". This module has two switch inputs (S1, S2) and two switchable live outputs (L1, L2). It's all connected into Home Assistant.
My wiring is as follows (standard UK setup):
- Permanent Live to the module's L-in.
- Neutral to the module's N-in.
- The original light switch wire (that used to go to the fan's switched live) now goes to S1 (switch input 1 on the module).
- L1 (switched live output 1 from the module) goes to the fan's permanent live
- L2 (switched live output 2 from the module) goes to the fan's switched live input.
I should mention that everything within the 'green' box is assumed as it's hidden in the ceiling. I'm installing the smart module next to the fan just using the wires presented to the fan.
The issue arises with the "Humidity Activation" scenario. When Home Assistant triggers the module to send power out on L2 (to the switched supply of the fan), the light (connected to S1) also illuminates. The fan does come on as intended, but the light coming on is the problem.
What I've Tried & My Research:
- Replaced the Smart Module: My first thought was a faulty module, so I swapped it out for an identical new one, but the problem persists.
- Leakage/Backflow Hypothesis: I'm assuming there's some kind of electrical leakage or backflow from the fan's switched live (powered by L2) finding its way back to the light circuit via S1.
- The "Diode" Idea: If this were a 12V DC system, I'd immediately think of putting a diode on the fan's switched live output from the module to prevent any current from flowing back.
- Searching for an AC Solution: I've been looking for an equivalent for 240V AC. I did stumble across something called an "Anti Reverse Diode Module" online, but the descriptions seem very much geared towards DC, and I'm (quite rightly, I think!) very wary of using potentially unsuitable or non-compliant components with UK mains voltage. Burning the house down is not on my to-do list!
My Questions:
Has anyone encountered a similar issue with backfeed when using smart modules to independently control devices?
Is there a recognised, safe, and compliant component for UK 240V AC systems that acts like a one-way valve for current, similar to how a diode works in DC, to prevent this kind of backfeed to the light?
Are there any alternative wiring configurations or perhaps a different type of smart module that might be better suited to completely isolate the fan and light outputs to prevent this?
Any advice, recommendations, or pointers would be hugely appreciated! I'm comfortable with wiring as long as it's safe and makes sense.
Thanks so much for reading!
Edit: The fan is a Monsoon EMDTK, from TLC Electrical. The light is a generic/unbranded LED light from Amazon.
Edit 25 May: More complete diagram and clarified 'green box'
