Interlock: avoiding simultaneous heating and cooling in the same room

What is service C-1f?

C-1f is a smart-ready service within the Cooling domain of the Smart Readiness Indicator (SRI). The SRI is a voluntary European assessment framework that measures how well a building's technical systems work together.

Service C-1f checks whether a building has an interlock. An interlock is a technical safeguard that prevents heating and cooling from being active simultaneously in the same room. Without this safeguard, systems can waste energy by working against each other.

What levels are there?

The SRI methodology defines three functionality levels for C-1f, numbered 0 to 2. Each level describes a different degree of control over simultaneous heating and cooling.

• Level 0: No interlock is present. The building management system (BMS) does not intervene if heating and cooling are active at the same time.
• Level 1: A partial interlock is in place. The risk of simultaneous heating and cooling is reduced, for example by sliding setpoints that widen the temperature margin between heating and cooling.
• Level 2: A full interlock is in place. The control system technically prevents simultaneous heating and cooling from occurring.

Where is this service weighted?

C-1f is part of method B of the SRI, the comprehensive assessment method with 54 smart-ready services. In method A, the simplified variant with 27 services, C-1f does not appear.

The score on this service counts within the Cooling domain, together with scores on other services in that domain. Each domain contributes via a weighting factor to the building's total SRI score, expressed as a percentage and a class.

Why is this relevant for building owners?

Simultaneous heating and cooling wastes energy and increases the energy bill without improving comfort. An interlock is typically a relatively simple adjustment in the control technology of the building management system (BMS).

Buildings with high heating and cooling capacity receive particular attention here. The EPBD (Energy Performance of Buildings Directive) already requires building automation above 290 kW since the end of 2024, and above 70 kW from the end of 2029.