Water heater relay (and a contactor if needed)
Up to 2500 W the Shelly Pro 1PM relay (or the plain Pro 1) switches the water heater directly; for a bigger heater it switches a contactor, and the contactor switches the heater. The heater's own protection stays either way. An electrician's job, about 30 minutes.
A short video of this guide is coming.
Is a contactor needed?
Check the heater's rating plate and its breaker. Not needed if the heater is single-phase, at most 2500 W (about 11 A) and on a breaker of at most 16 A; most 80–120 litre heaters are like that. Needed above 2500 W (e.g. 3 kW), for a three-phase heater, or on a breaker above 16 A.
Why 2500 W?
The Pro 1PM (and Pro 1) contact is rated 16 A, but the heater draws full current for hours inside a closed enclosure. Up to 2500 W (about 70% of the rating) contact wear and heating stay well within limits; above that a contactor gives a longer life and headroom.
Direct wiring (up to 2500 W)
The heater's phase conductor runs after the breaker through the relay contact (I → O); neutral and earth go straight to the heater. The relay supply (L, N) can come from the same breaker. The heater's thermostat and safety cut-out stay in the circuit unchanged.
With a contactor (above 2500 W)
A 2-pole (3–4-pole for three-phase), 25 A installation contactor with a 230 V coil in the heater's supply, after the breaker. The relay output (O) goes to coil A1, A2 to neutral: the relay itself carries no heater current.
Controlled night circuit
If the heater is on the controlled night circuit, it has no power in the daytime. Daytime supply from the normal circuit must follow the utility's rules; agree it with the customer.
Test
Switch the relay on in the app: the heater heats (if its thermostat allows); with a contactor it pulls in too. Switch it off: it releases. With a Shelly Pro 1PM the heater's power shows in the app too.
