2 rooms, 1 heat pump.

• Initial Conditions
• Sauna and freezer both start at ambient: 14 °C.
• Heat pump is off.
• Phase 1: Internal Cycling Begins
• You turn on the single reversible heat pump.
• It begins transferring heat: From freezer → sauna.
• Since both rooms are initially at 14 °C:
• It is most efficient to cycle heat between the two rooms (small delta, high COP).
• No energy is wasted to ambient.
• Phase 2: Divergence Reaches Trigger Point
• Sauna climbs past a threshold (e.g., 35–40 °C).
• Freezer drops toward 0 °C or below.
• The temperature gradient between:
• Freezer and sauna becomes large (lower COP).
• Freezer and ambient is now smaller.
• Sauna and ambient is now greater (more efficient to dump heat to ambient).

At this point, a controller triggers a valve system to switch modes:

• Phase 3: Switch to Ambient Loops
• Freezer circuit starts dumping its heat to ambient air.
• Sauna circuit now draws heat from ambient instead of freezer.
• The internal cycle is effectively broken and replaced with independent ambient-sourced loops:
• Heat pump acts as a cooling unit for the freezer, expelling heat outside.
• Heat pump acts as a heating unit for the sauna, drawing heat from outside.

🧠 System Behavior Summary

Phase Heat Source Heat Sink Efficiency (COP) Trigger

1 Freezer Sauna High Default start

2 Freezer Sauna (↑temp) Dropping Sauna ≥ ~40 °C

3 Ambient air Sauna Higher (again) Valve triggered

Freezer Ambient air Higher (again) Valve triggered

🔧 Implementation Requires:

Temperature sensors in all 3 zones (freezer, sauna, ambient).

Thermostats with logic to trigger a solenoid or motorized valve.

A 4-way reversing valve (if refrigerant-based).

Air dampers (if air-duct based).

Optional: microcontroller (like ESP32/Raspberry Pi) or smart HVAC controller.

Use 2 systems if:

You want consistent, reliable extreme temperatures.

You prioritize energy efficiency and longevity.

Use 1 system with smart ambient crossover if:

Budget is limited.

You accept some performance trade-offs and complexity in exchange for lower hardware cost.