CommercialLight IndustrialMechanical
Rooftop units, make-up air, ventilation, duct distribution and building controls for warehouses, shops, offices and light industrial spaces.
01 / Service categories
Most commercial work falls into a small number of systems. Each category below covers the equipment involved, the measurements that matter, and the documentation you should expect to receive.
Split systems, packaged units and unitary air conditioners serving offices, shops and small industrial spaces. Diagnosis starts with a temperature split across the evaporator coil, static pressure at the registers, and a refrigerant charge check by weight.
Rooftop units combine an evaporator, condenser, supply fan and heating section in one cabinet. The most common findings are a failed belt or sheave, a packed condensate pan, a sticking economizer damper, and a dry burner pressure switch.
Gas and electric heat in packaged units, along with separate furnaces, unit heaters and make-up air furnaces. Heat exchangers are inspected for scale and cracks, and combustion is verified with a flue draft and carbon monoxide reading.
Exhaust fans, louvers, curbs and rooftop relief hoods move air out of a building. Make-up air brings replacement air in, and the two must be balanced so exhaust does not pull negative pressure on the envelope.
Commercial thermostats, zone controllers, sensors and a generic building control architecture that links units to a head-end over a low-voltage network. When schedules, sensors or points drift, symptoms look mechanical but start in the controls.
Duct sizing, static pressure and duct leakage decide whether a system works. Flexible duct runs, branch taps and VAV boxes are common trouble spots, and TAB is the standard method for proving airflow after any adjustment.
02 / Equipment types
Equipment is usually labelled with tonnage, input capacity and airflow. Reading those tags correctly tells you how the unit was designed to be used.
Self-contained units mounted on a curb, holding the evaporator, compressor, condenser and supply fan in one sheet metal cabinet. Two-pipe units use an economizer damper to introduce outside air when the coil can handle the load.
Air handlers move air without producing heat. Sections typically stack a return air mixing box, pre-filter, cooling coil, supply fan and heat section, and they pair with a separate compressor or boiler that handles the load.
VAV boxes hold back air to keep duct static pressure steady while a central unit supplies the peak load. Reheat coils, damper actuators and damper position feedback are the parts that usually fail.
When a building pulls air out faster than it brings air in, doors slam, dust enters and combustion appliances can starve. Make-up air units add heated or tempered replacement air, and relief openings equalise the pressure.
Commercial thermostats read the space instead of a single coil. Zone controllers, temperature sensors and a generic building control network sequence the equipment and hold schedules, and a low-voltage trunk carries points back to a head-end panel.
Replacement means matching the new unit to the load, the ductwork and the existing curb, then verifying the result. Existing buildings often need a manual J selection and a duct redesign because the original layout was not designed for the equipment installed.
03 / Diagnostics
Commercial calls tend to cluster around a short list of symptoms. The table separates the complaint from the components a technician checks first.
A setpoint of 72 to 74 degrees may simply be asking too much of a loaded unit. Other causes are a failing economizer stuck wide open, an oversized condenser with a starved airflow path, or a low refrigerant charge that is icing the evaporator coil.
Loaded filters, a loose blower belt, a failing blower motor or undersized return air grilles all raise duct static pressure. When static runs too high, the fan loses airflow and the coil loses the capacity to pull heat out of the air.
Water at a supply register is usually a dirty evaporator coil and condensate pan. A plugged drain line, a failed float switch, a sloping pan or a frozen line set can also push water back into the cabinet.
If supply air is set to 100 percent recirculated, a crowded space can hold elevated carbon dioxide and humidity without any thermostat complaint. Fresh air measurement can confirm whether the outdoor air damper is delivering what the design called for.
A zone that never reaches setpoint is usually a control problem. A stuck VAV damper, a failed zone temperature sensor or a schedule change left in manual can leave one area of the floor running all day.
Exhaust without enough make-up air puts the building under negative pressure. One unit cools, another reacts to the pressure change, and combustion equipment downstream can be pulled toward a draft-induced rollout risk.
A burner that will not light, or lights and then drops out, points at the gas valve, the ignitor, the flame sensor or the pressure switch. A cracked heat exchanger is also a safety item that is checked rather than guessed at.
Short cycling is usually low refrigerant charge, an oversized unit, a failing low-pressure or high-pressure control, or a low airflow condition that freezes the coil. Compressor short cycling shortens compressor life.
Field note Before scheduling a visit, check filters, verify the thermostat is in schedule, and look for a tripped disconnect or a breaker that has been switched back on. Clearing those two items resolves a surprising share of commercial calls that never need a technician.
04 / Maintenance
A maintenance program is a list of repeatable tasks tied to the equipment on the roof, not a generic visit. Every task on this page is one that can be verified with a reading.
Filters are checked against a pressure drop, and coils are inspected for frost, dust loading and biofilm. Evaporator and condenser coils are cleaned with the correct detergent and rinsed so airflow stays where the fan curve expects it.
Belts are checked for cracking and correct tension, sheaves are checked for groove wear, and bearings are checked for noise and play. Blower motors are tested for amp draw, and capacitors are replaced as they age out of tolerance.
Condensate pans are cleaned, drains are flushed and traps are checked so water leaves the building instead of sitting in the cabinet. Float switches are tested so a blocked drain stops the unit before it causes water damage.
Superheat and subcooling are measured and logged, amp draw is recorded at startup, and contactors, capacitors and pressure controls are inspected. Readings from one visit become the baseline the next visit is compared against.
Burners are cleaned, heat exchangers are inspected and the flue is checked for draft. Combustion is sampled with a flue gas analyser and safety cutoffs are tested with the door switch removed.
Thermostats are checked against a calibrated instrument, schedules are reviewed with the site contact, and TAB work verifies airflow at diffusers and grilles against the design CFM. Trending over time replaces the guesswork argument.
05 / System controls
Every point in a building control system carries an address: controller, point, then device. When a zone drifts, those addresses tell you whether to chase a sensor, a wiring run or a schedule.
Wall-mounted and zone controllers set a space to a schedule rather than a single dial. Inputs include a temperature sensor, a setpoint, an occupancy method and a fan command, and the schedule itself is the most common thing to get wrong.
A zone controller maps one control loop to the equipment that serves it. It holds the control loop, manages the minimum ventilation position, and reports whether it is actually following the setpoint instead of only showing a number.
A building control system is a distributed architecture: field controllers on each unit, a low-voltage trunk, and a head-end panel or server where schedules, trends and alarms live. A point list documents the whole chain in a format other contractors can read.
Control circuits carry far less current than power circuits but fail more often. Thermostat cable, 24 volt power, communication wiring and shielded sensor leads each need their own segregation from line voltage to stay reliable.
06 / Installation
A commercial installation is a mechanical, electrical and control sequence that has to stay in order. Each phase is verified before the next one starts.
Room size, glazing, occupancy, internal heat from people and equipment, and the local design temperature range all feed the calculation. The result is the tonnage and airflow the space actually needs.
The unit is matched to the load rather than to a price point, and the match between indoor and outdoor sections is confirmed before the equipment ships. Efficiency values such as SEER2 and AFUE describe the equipment, not the building.
A manual J calculation produces the design load, and duct sizing follows from the airflow that load requires. Doing the duct design first is what keeps static pressure under control after startup.
Curbs are set level and flashed, line sets are brazed with nitrogen and evacuated to the manufacturer specified vacuum before the refrigerant is charged. Rigging plans keep the roof deck and the curb within their rated load.
Duct is fabricated, supported and sealed, then TAB is performed to set and verify airflow at each outlet. Sealing joints and supporting flex duct properly decides whether the measured numbers stay where they were set.
The low-voltage wiring is terminated and checked for polarity and shorts before power is applied. Startup then verifies refrigerant charge by weight, superheat and subcooling, airflow, and economizer operation, and the readings are handed over in writing.
Before the job is closed out, these readings should exist in writing:
07 / Equipment gallery
A look at the installed equipment and mechanical spaces these systems occupy.
08 / Contact
Send the equipment tag numbers, the building address and what is happening in the space. If a unit has shut down, include whether water is present and whether the disconnect is accessible, because that changes how the visit is prepared.