When a commercial building has comfort problems or high energy bills, the instinct is often to blame the equipment — the chillers are too old, the AHUs are undersized, the boilers are inefficient. In the majority of cases Alpha Controls investigates across London and the South East, the plant itself is perfectly capable. The problem is how it is being told to operate.
HVAC controls are the intelligence layer of any mechanical system. A modern, well-sized chiller running on a poorly configured sequence will consume far more energy and deliver worse comfort than an older machine with a well-tuned control strategy. Controls determine when equipment starts and stops, at what capacity it operates, how zones interact, and whether energy recovered in one part of the system is used before another part has to generate fresh energy from scratch. Get the controls wrong and every piece of capital expenditure on plant is working against itself.
This article sets out the control sequences and integration principles that Alpha Controls specifies and commissions for HVAC installations across commercial, public sector, and mixed-use buildings throughout Gravesend, London, and Kent.
Air handling units are central to most commercial HVAC systems and offer more opportunities for both energy waste and energy recovery than almost any other piece of plant. The following sequences are the baseline Alpha Controls implements on every AHU project.
A fixed supply air temperature setpoint — for example, always delivering 16°C into the ductwork regardless of conditions — is one of the most common and most costly control errors we encounter. During mild weather or low occupancy, this forces the AHU to cool or heat air far beyond what any zone actually needs, wasting energy and frequently causing overcooling complaints.
The correct approach is a supply air temperature reset strategy based on the aggregate demand signal from all served zones. The BMS polls every zone controller and identifies the zone calling for the greatest cooling or heating. If the most-demanding zone is only at 60% of its cooling valve stroke, the supply air temperature can be allowed to rise — reducing the cooling load on the AHU coil. Only when a zone reaches 95–100% of its control range does the supply air temperature reset back towards its minimum design value. This single measure routinely reduces AHU cooling energy consumption by 15–25% in part-load conditions, which describes the majority of operating hours for most UK commercial buildings.
Outdoor air is expensive to condition. Bringing in cold January air at 2°C and heating it to 19°C consumes significant energy, and over-ventilating unoccupied or lightly occupied spaces is pure waste. Demand controlled ventilation (DCV) using CO₂ sensors in the return air stream or at zone level allows the BMS to modulate the outdoor air damper in direct proportion to actual occupancy.
A CO₂ setpoint of 800–1,000 ppm is typically appropriate for UK office environments. When concentration is below setpoint, the outdoor air damper is held at its minimum hygienic position. As CO₂ rises with occupancy, the damper opens progressively. This approach can halve ventilation-related energy costs in buildings with variable occupancy patterns — meeting rooms, lecture theatres, open-plan offices with flexible working arrangements.
Where an AHU is fitted with a heat recovery device — rotary wheel, plate heat exchanger, or run-around coil — the control sequence must establish heat recovery as the first stage of conditioning, with mechanical heating and cooling only enabled once recovery is exhausted. This sounds obvious but it is frequently misconfigured, particularly after a controls upgrade where the commissioning engineer has not correctly mapped the recovery device into the staging sequence.
The BMS should monitor recovery efficiency in real time. A dirty or degraded heat wheel operating at 50% efficiency when the design value is 75% represents a measurable energy penalty and should trigger a maintenance alert rather than silently accepting the shortfall and compensating with additional mechanical energy.
Frost protection is a safety sequence that must be integrated into the AHU controls from day one, not added as an afterthought. The heating coil frost thermostat should be set to 5°C and wired to close the outdoor air damper, stop the supply fan, and open the heating valve to full before any freeze damage can occur. The BMS must log every frost protection activation as a fault event — repeated activations during mild weather indicate either a sensor fault, a damper not closing fully, or a heating valve that is slow to respond, all of which need investigation.
Buildings with multiple chillers or multiple boilers must operate them on a lead/lag rotation strategy. Running the same machine as the permanent lead plant accumulates running hours unevenly, shortens the life of the lead machine, and means the lag plant may fail to start when it is actually needed because it has been sitting idle for too long. Alpha Controls programs automatic rotation on a time or hours-run basis — typically every 168 hours of operation — so that all plant accumulates hours at a comparable rate and remains exercised.
Where chillers have different efficiency profiles — for example, an older screw chiller and a newer magnetic-bearing centrifugal — the sequencing logic should favour the more efficient machine at the loads where it performs best. Modern centrifugal chillers typically have excellent part-load efficiency but less competitive full-load COP compared to screw machines. The BMS should calculate the current system load and select the machine or combination of machines that minimises combined energy input, not simply start the next machine in a fixed sequence.
Chiller efficiency improves as condenser water temperature falls. During cooler weather, the BMS should reset the condenser water temperature setpoint downwards — typically from a design value of 29–30°C towards 24–26°C when ambient wet-bulb temperature allows. For every 1°C reduction in condenser water temperature, chiller COP typically improves by 2–3%. This reset strategy requires close coordination between the cooling tower fan speed control and the chiller sequencing logic, which is why it is often absent on systems that have been installed by separate sub-contractors without integrated controls commissioning.
Four-pipe FCU systems — with separate heating and chilled water circuits — give individual zone control regardless of the season and are the standard choice for high-quality office fit-outs. Two-pipe systems are simpler and cheaper to install but require the building to commit to either a heating or cooling mode, which is a significant limitation for perimeter zones in shoulder seasons. The BMS changeover sequence for two-pipe systems must be carefully designed: changeover based on a single outdoor temperature sensor risks switching the entire building to cooling while north-facing perimeter zones still need heat.
In four-pipe systems, the control sequence for each FCU must include a dead band between heating and cooling setpoints. A typical office zone might have a heating setpoint of 20°C and a cooling setpoint of 23°C, creating a 3°C band in which neither valve is open. Without this dead band, minor temperature fluctuations cause the heating and cooling valves to open simultaneously, conditioning against each other and wasting energy directly. Alpha Controls has audited buildings where simultaneous heating and cooling was responsible for 20–30% of the entire HVAC energy consumption.
FCU fan speeds should modulate based on zone demand — high speed when the temperature deviation from setpoint is large, low speed when the zone is near setpoint. Running all FCU fans at high speed continuously creates unnecessary noise and energy waste, and draws more primary air than the ventilation design intended.
Variable speed drives on AHU supply and extract fans are standard on modern systems, but they are frequently commissioned incorrectly — set to a fixed speed or a fixed duct static pressure setpoint and left there. This defeats most of the energy benefit of installing the VSD in the first place.
The correct strategy is a duct static pressure reset: the BMS monitors the position of all VAV dampers in the system and resets the duct pressure setpoint downwards until the most-open damper reaches approximately 90% open. This ensures the fan is running at the minimum speed required to satisfy demand rather than maintaining an arbitrary pressure setpoint that may be far higher than needed at part load. On a typical office building this strategy reduces fan energy consumption by 30–50% compared to fixed-speed or fixed-pressure operation.
Learn more about how Alpha Controls approaches BMS commissioning to ensure VSD strategies are correctly implemented and verified.
Good setpoints are the foundation of every control sequence. The following values reflect Alpha Controls' standard recommendations for commercial office environments in the UK, informed by CIBSE guidance and adjusted for practical operation:
Setpoints should be reviewed seasonally — at the very minimum at the start of the heating season and at the start of the cooling season. Buildings change: occupancy patterns shift, heat gains from IT equipment increase, glazing gets added or changed. A setpoint that was correct at commissioning may be wrong three years later.
When Alpha Controls carries out a controls audit — whether as part of an energy reduction project or a comfort investigation — the following faults appear repeatedly across buildings of all ages and sizes:
Carrying out a thorough controls audit on a live commercial building requires careful preparation. The goal is to understand what the sequences are actually doing — not what the as-built documentation says they should be doing — without causing comfort complaints or equipment damage in the process.
Alpha Controls follows a structured approach to audit work:
If your building's HVAC controls have not been formally reviewed since commissioning, or if you are experiencing unexplained energy costs or comfort complaints, Alpha Controls offers structured controls audit services for commercial buildings throughout London, Kent, and the South East. Get in touch to discuss your site.
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