The supply air handling unit is running. Where is the building still losing energy?

The easiest faults to identify are those that trigger an alarm. Inefficient operating modes are harder: the unit runs, the temperature is more or less maintained and there are few complaints, yet some energy is being used without useful effect. The absence of alarms is not a measure of energy efficiency.

Unnecessary operating hours

Two fans consume 6 kW in total. An outdated schedule keeps them running three hours longer over 250 days. The fans’ electricity use alone during those hours is:

E = 6 × 3 × 250 = 4500 kWh per year

This does not yet include heating and cooling the outdoor air. But calling these hours ‘unnecessary’ requires evidence: cleaning, process requirements or moisture removal may require operation before people arrive and after they leave.

One system heats what another cools

If supply air is unnecessarily heated to 3 °C above the agreed setpoint at an airflow of 3000 m³/h, the excess heating duty is 3,06 kW. Over 1000 hours, this amounts to 3060 kWh of heat. If the rooms are being cooled at the same time, this creates an additional load on the cooling system.

Q = 0,34 × 3000 × 3 / 1000 = 3,06 kW

Not all reheating is a mistake: it may be part of dehumidification or justified zone control. Conclusions must be based on the purpose of the operating mode, temperature and humidity.

Excessive pressure

The fan maintains the pressure required for peak operation even though most zone dampers are partly closed at partial load. The setpoint can be adjusted to suit ductwork demand, but airflow in the most heavily loaded zones must first be verified. This is a system tuning task, not necessarily a reason to replace equipment or introduce AI.

An unnoticed actuator failure

If the heating valve command is zero but the air downstream of the heat exchanger is noticeably warmer, check the valve, sensors and transient behaviour. Apply the same analysis to the heat recovery unit, outdoor air dampers and free cooling. The BMS screen shows a command, but does not always confirm the physical response.

Calculate the effects of measures sequentially against a single baseline. Savings from a new schedule cannot be added to savings from reduced speed if both were calculated using the previous full operating hours. Assess electricity, heating, cooling and cost separately.

The aim is not minimum consumption at any cost, but eliminating energy use that does not help the building operate as required.