Where the energy is, and how it comes back.

A traction lift is counterweighted at roughly the car weight plus 45–50% of rated load. That means the machine motors in some directions and generates in others: a lightly loaded car travelling up, or a heavily loaded car travelling down, drives the motor as a generator. In a conventional installation that energy is dumped into a braking resistor in the machine room, where it becomes heat the building's air conditioning then has to remove — a loss counted twice.

A four-quadrant drive with an active front end returns it to the supply instead. In a 40-storey tower with eight machines running an intensive duty cycle, the recoverable fraction is typically 20–40% of lift energy, and it arrives at exactly the hours the lifts are busiest — which are also the building's peak demand hours.

A 40-storey tower pumps its entire water demand to a roof tank and then gives all of that potential energy back on the way down. Because 128 metres of static head would burst the fittings on a low floor, every downfeed riser is broken into pressure zones by pressure-reducing valves — components whose entire function is to convert head into turbulence and heat.

HERS replaces the PRV station with a turbine-generator that produces the same downstream pressure and takes the difference as electricity. The turbine is a pressure-regulating device first and a generator second; a bypass with a conventional PRV holds the setpoint if the machine is offline, so water supply never depends on the generator.

The same principle is applied to greywater and rainwater downpipes feeding a basement recycling plant. We do not tap raw soil or waste stacks: those are vented systems and interfering with them conflicts with drain-waste-vent requirements.

This is the pathway that tropical buildings get backwards. In a temperate climate you recover heat from warm drain water to pre-heat cold mains. In Kuala Lumpur the mains arrive at around 29 °C and shower drains leave at 34 °C — there is almost nothing in that gap. Meanwhile the chiller plant runs all year and rejects several megawatts of heat to a cooling tower, while electric calorifiers a few floors above buy heat back at full tariff.

HERS takes the heat from where it is abundant. A desuperheater on the condenser circuit intercepts the hottest part of the refrigerant discharge and raises domestic hot water in a stratified calorifier before the remaining heat goes to the tower. Separately, collected greywater at ambient temperature is used as a pre-cooling medium for condenser return water, which lowers condensing temperature and lifts chiller efficiency by roughly 2–3% per kelvin.

The two effects compound: the DHW load falls, and the chiller that serves the building gets slightly better at its day job.