When the mechanical timer of a Harvia PC90 sauna failed, the goal wasn’t just to repair it — it was to rethink how the sauna could be controlled entirely. Instead of simply replacing the timer or buying a modern oven with built-in Wi-Fi, this project transformed the existing setup into a fully smart, connected sauna system powered by Shelly.
From a Broken Timer to a Connected Solution
The original mechanical timer (SPZSK-510) was the weak point of the system. Replacing it with the same part would have been costly and limited, and upgrading to a brand-new Wi-Fi-enabled Harvia oven would have meant replacing perfectly good hardware.
Instead, the project took a DIY approach: swapping the broken timer for a Shelly-based system that adds smart scheduling, app control, and temperature monitoring — all while keeping the sauna’s built-in safety features intact.
The Smart Hardware Behind the Upgrade
To bring the sauna into the smart home world, several Shelly devices were combined:
Shelly Pro 1 – The heart of the system, controlling the oven over Wi-Fi or manually via a button.
Shelly Plus 1 – Adds an extra layer for monitoring and integration.
Shelly Add-on Plus + DS18B20 Sensor – Provides real-time temperature readings directly from the sauna.
GV2ME06 Motor Protection Switch – Ensures safe operation by protecting the power line to the sauna’s relay coil.
Manual push button (TRU Components) – Allows local control without needing the app.
Status indicator light – Shows instantly whether the sauna is running.
How It Works
The sauna’s old mechanical timer was removed and replaced with Shelly Pro 1. Now, the sauna can be switched on in two ways:
✅ Manually – Pressing the wall-mounted button starts the sauna for four hours. Pressing it again stops it instantly.
✅ Remotely – The sauna can be turned on or off via the Shelly app or web dashboard, from anywhere.
A status light provides a quick visual indicator, while the DS18B20 sensor constantly measures the temperature, sending data to the Shelly system for added safety and control.
Safety First
Even with the new Shelly setup, safety was never compromised. The original Harvia thermostat and overheating protection remain active, ensuring the sauna can’t overheat. Additionally, all wiring was carefully routed outside the sauna’s heat zone, with only high-temperature silicone cables inside.
Why This Upgrade Works
Cost-efficient: Instead of spending over CHF 700–800 for a new oven, the entire DIY smart upgrade was completed for around CHF 216 in components.
Future-proof: The sauna is now part of the home’s smart ecosystem — easily controllable from the Shelly app or via integrations like Alexa or automation platforms.
User-friendly: A simple button for local control means anyone can still use it without needing to open an app.
The Result
The Harvia PC90 sauna went from analog to intelligent. With a single button press or app tap, the sauna now runs safely for four hours, can be shut off anytime, and displays its status instantly. Temperature monitoring adds peace of mind, and the setup is fully customizable for future smart home integrations.
