HVAC controls and control cabinet repair

The part of the unit that makes the decisions

Every piece of commercial HVAC equipment has two halves. The mechanical half moves refrigerant, water and air: compressors, fans, pumps, coils. The control half decides when each of those runs, and stops them when something is wrong. When the control half fails, a perfectly good 40-ton rooftop unit sits dead on the roof, or worse, runs when it should not.

Controls are also where a lot of expensive misdiagnosis happens. A compressor that will not start gets condemned, and the real fault turns out to be a pitted contactor, a failed start relay or a control conductor that rubbed through on a sheet metal edge. We test before we replace. It is the practical side of "service first, not replacement first": the cheapest repair is the one that finds the real fault the first time.

What lives in the control cabinet

Open the control section on a rooftop unit, a chiller or a boiler and you find the same families of parts, laid out differently by every manufacturer:

• Controllers and boards: the unit control board or programmable controller that runs the sequence, stages compressors and reads the sensors
• Transformers and fuses: the 24-volt or 120-volt control power everything else depends on
• Contactors and relays: the switches that start compressors, fan motors, pumps and heat stages
• Safeties: high and low pressure switches, freeze stats, flow switches, flame safeguards, high limits and overloads
• Sensors: supply, return, outdoor and water temperature sensors, humidity and pressure transducers
• Control conductors: the wiring between all of it, terminal strips, and the connections that loosen with years of vibration and heat

Our work stays on that HVAC control side, meaning the circuits inside and between the equipment we service. General building electrical work such as panels, feeders and lighting belongs with an electrical contractor.

Control faults we find most

What you see	What is usually behind it
Unit completely dead, no display	A failed control transformer, a blown control fuse, a tripped disconnect, or a burned conductor at a terminal.
Contactor chatters or will not pull in	Low control voltage from a weak transformer, a loose or corroded connection, or a contactor coil that is failing.
Compressor locked out	A pressure switch or overload doing its job because of a mechanical problem, or a safety or sensor that has failed and is reporting a fault that is not there.
Runs at the wrong times, or not at all overnight	A schedule lost after a power outage, a controller clock that drifted, or an occupancy override stuck on.
Short cycling	A sensor in a bad location or reading wrong, a setpoint band set too tight, or a loose connection opening and closing the call.
Drive fault on a fan or pump	Overcurrent from a failing motor or bearing, overvoltage, a ground fault, or a drive overheating in a hot cabinet.

We write down fault codes and lockout history before anyone clears them. The first fault a controller logs is usually the honest one, and the ones after it are the consequences.

Schedules, thermostats and building automation

Buildings run on schedules, whether they live in a programmable thermostat on the wall, a time clock in a cabinet or a building automation system (BAS) with a front end on somebody's desktop. When equipment runs all weekend in an empty building, or a warehouse sits cold on Monday morning, the schedule is the first place we look. We check what the equipment is actually being told, not just what the screen says, and confirm the unit responds to each command: occupied, unoccupied, heat, cool, economizer.

We also verify sensors against a calibrated reading. A space sensor that reads 3°F off will quietly run a unit harder than it needs to for years, and nobody notices until the energy bill or the repair bill says so.

Variable frequency drives

Many newer air handlers, cooling towers and hydronic pump systems use variable frequency drives (VFDs) to slow motors down when full speed is not needed. A drive saves energy, and it also adds another box that can fault. The common drive problems are heat and dirt in the enclosure, failing cooling fans on the drive itself, and motor problems that show up as drive trips. We read the drive's fault history, check the motor and its bearings, and look at how the drive is programmed before deciding what actually failed.

Old controls on good equipment

A lot of equipment in Middle Tennessee is mechanically sound and running on a control board that was discontinued years ago. When the board fails, the usual advice is "replace the unit." Often that is not necessary. We find a replacement board or a compatible controller, or rebuild the sequence with standard components, and keep the equipment running as long as repairing it makes economic sense. When it no longer does, we help you plan an equipment replacement instead of reacting to one. Control problems are a regular part of our commercial HVAC repair calls, and a maintenance agreement catches loose conductors and cooked contactors before they strand a unit. We do this work across our Middle Tennessee service area, from the plants in Murfreesboro to Lebanon.

Controls questions