Stella Maris Catholic School, Folkestone, Kent.

System Overview

The installation comprises two Hercules 85 air source heat pumps (ASHPs) connected to a 2,400-litre buffer tank that stores water at a mean temperature of 75°C. The new ASHP system is hydraulically separated from the existing building’s low-temperature hot water (LTHW) heating system via a plate heat exchanger.

The existing LTHW system operates with a 65°C flow and 55°C return temperature. From the buffer tank, the LTHW circuit also supplies a 500-litre domestic hot water (DHW) storage tank, maintaining stored water at a minimum of 60°C to ensure immediate hot water availability.

Operation Strategy

The heat pumps are programmed to operate between 2:00 a.m. and 6:00 a.m., taking advantage of off-peak electricity tariffs to preheat the buffer tank. This ensures the system is fully charged and ready for heating demand from 7:00 a.m. onwards, while the DHW tank remains at temperature for instant hot water delivery throughout the day.

Performance Feedback

The building caretaker reported to the school board that he was highly impressed with the new ASHP system, particularly its excellent performance during winter conditions (ambient temperature around 7°C) and its ability to maintain stable internal temperatures. He also praised the units for their remarkably quiet operation.