Every commercial HVAC problem starts as a symptom — a tenant complaint, a monitoring alert, a strange smell, an energy bill that doesn’t make sense. The gap between symptom and cause is where most facility managers get stuck, and it’s where most bad repair decisions get made.
This guide walks through the 10 most common commercial HVAC problems we see across our service area in New Hampshire, Southern Maine, and North Shore Massachusetts. For each one, you’ll see what’s typically causing the symptom, how urgent it is, whether it’s a repair you can defer or one that will escalate, and what to do next. The goal isn’t to turn you into a technician — it’s to help you make the right decision about which calls to make and when.
KEY TAKEAWAYS
- Most commercial HVAC problems fall into a small number of well-understood patterns. Knowing which pattern you’re seeing tells you how urgent the call is.
- Roughly 60% of the emergency service calls we run could have been caught by preventative maintenance in the weeks before the failure.
- The most expensive words in commercial HVAC are ‘we’ve been meaning to look at that.’ Small problems compound quickly in this equipment.
- When in doubt, call. A five-minute phone diagnosis is free; a Sunday emergency call after the equipment has failed catastrophically is not.
1. Some Rooms Are Too Hot, Others Too Cold (Uneven Temperatures)
This is the single most common tenant complaint in commercial buildings — and one of the most misdiagnosed. Most facility managers assume it means the unit is undersized or aging. Usually, neither is true.
What’s typically causing it: Airflow imbalance. Dirty filters, restricted return-air paths, closed or blocked diffusers, dirty coils, or a control system that’s stopped compensating for actual zone conditions. In multi-zone buildings, a stuck damper or a failing VAV box is often the specific culprit.
How urgent: Not an emergency, but don’t sit on it. Ignored imbalance drives up energy costs meaningfully and creates a slow drip of tenant complaints that eventually escalate.
What to do: Have a technician run a balance check and inspect the airflow path — filters, coils, dampers, VAV boxes, and controls. This is exactly the type of problem that a maintenance plan catches before tenants do.
2. The System Runs Constantly and Never Reaches Setpoint
A system that runs 24/7 without ever satisfying the thermostat is almost always signaling one of two problems — and the more benign one is the more common.
What’s typically causing it: Low refrigerant charge (usually a leak), a failing compressor, a dirty condenser or evaporator coil, or a control setpoint that has drifted or been overridden. In refrigeration equipment, a stuck defrost cycle can produce similar symptoms.
How urgent: Same-day service. Continuous run time destroys compressors quickly, and the energy costs of the runaway operation add up fast.
What to do: Call for service. If the unit has a visible service disconnect, you can shut it down briefly to prevent further damage while you wait — but don’t cycle it on and off repeatedly, which is worse for the equipment than either leaving it off or leaving it on.
3. Short-Cycling (The Unit Turns On and Off Repeatedly)
Short-cycling is what technicians call it when a system starts and stops in rapid succession — running for a few minutes, shutting off, restarting a few minutes later, and repeating. It’s more damaging to the equipment than almost any other failure mode.
What’s typically causing it: An oversized system for the current load, a failing pressure switch, an oversized or miscalibrated thermostat, refrigerant issues, or (in commercial refrigeration) a fouled condenser causing high-pressure trips.
How urgent: Immediate. Every short-cycle event stresses the compressor start winding. A compressor that would otherwise last 15 years can fail in months under sustained short-cycling.
What to do: Call for service today. This is one of the failure modes that most rewards fast intervention — and one of the most expensive to ignore.
4. Water Is Leaking From the Unit or Ceiling Below It
Water on the floor beneath an air handler or dripping from a ceiling below a rooftop unit is a problem that gets bigger every hour you leave it.
What’s typically causing it: A clogged condensate drain line, a cracked or full condensate pan, a frozen evaporator coil that’s melting, or (in worst cases) a refrigerant leak that has caused ice to form and then melt.
How urgent: Same-day. Water damage to ceilings, walls, and equipment below the unit compounds quickly, and standing condensate promotes microbial growth in the system.
What to do: Shut off the unit if you can do so safely, protect the area below with a bucket or plastic, and call. This is a common enough problem that most service techs carry the parts to resolve it on the first visit.
5. The System Is Louder Than It Used to Be
New noises in HVAC equipment are almost always meaningful. The nature of the noise usually points at the source.
What’s typically causing it: Grinding or squealing usually indicates a failing bearing or belt. Banging or rattling often means loose panels, a broken component inside the unit, or (in older equipment) a compressor slug event. A rhythmic thump can indicate a fan blade contact issue. In refrigeration, a new hum can signal a compressor problem.
How urgent: Depends on the noise. Grinding or metal-on-metal contact is same-day. A new rattle is worth scheduling in the next week.
What to do: Call and describe the noise clearly. A good dispatcher will triage it correctly based on your description alone.
6. Utility Bills Have Spiked With No Obvious Explanation
Energy costs that climb faster than utility rates or usage patterns explain are almost always signaling an HVAC or refrigeration inefficiency somewhere in the building.
What’s typically causing it: Dirty coils, dirty filters, low refrigerant charge, a stuck damper allowing conditioned air to escape, a failing economizer, controls that have drifted out of their setpoints, or (in older buildings) an outdated control strategy that never really matched the building’s actual use.
How urgent: Not an emergency, but the meter is running the whole time. A single un-diagnosed inefficiency can add thousands of dollars per year to a mid-sized commercial building’s utility bill.
What to do: Request a service visit specifically framed as an energy audit — bring copies of the last 12 months of utility bills and any equipment monitoring data you have. Many NH commercial operators are also eligible for NHSaves incentives on efficiency upgrades identified during this kind of review.
7. There’s a Strange Smell When the System Runs
Odors in commercial HVAC are diagnostic gold — most of them point at a specific cause.
What’s typically causing it: A burning smell in the first minutes of the heating season is usually just accumulated dust burning off — normal. A burning smell that persists suggests electrical issues or a motor problem. A musty or mildew smell almost always means moisture in the system (often at the coil or the drain pan). A gas smell — natural gas or a distinctly chemical odor — is never normal. Any smell of rotten eggs on a gas-fired system should be treated as an emergency.
How urgent: Gas smell — evacuate and call the gas utility and 911. Electrical burning smell — shut the system down and call for service same-day. Musty smell — schedule service in the next week.
What to do: Match the smell to the urgency above. When in doubt on a suspected gas leak, get people out of the building first and call second.
8. Humidity Is Off — Either Muggy or Excessively Dry
Commercial buildings that hold humidity poorly aren’t just uncomfortable — they’re expensive to operate and hard on both equipment and building materials.
What’s typically causing it: For high humidity: an oversized cooling system that satisfies temperature setpoint too quickly to dehumidify, a failed reheat coil, or ventilation bringing in unconditioned outside air. For low humidity: winter operation without adequate humidification, or excessive outside-air infiltration.
How urgent: Not an emergency, but persistent high humidity in commercial buildings promotes mold, damages equipment, and creates tenant health complaints that carry real liability.
What to do: Request a service visit that includes a humidity diagnosis — this is a longer diagnostic than a typical repair call and worth flagging in advance so the tech budgets the time.
9. Ice Is Forming on the Evaporator Coil, Refrigerant Lines, or Outside Unit
Ice on HVAC or refrigeration equipment is not a sign of good performance — it’s a sign of a problem, usually a serious one.
What’s typically causing it: Restricted airflow (dirty filter, dirty coil, blower failure), low refrigerant charge, a failed defrost cycle (in refrigeration), a stuck expansion valve, or a stuck TXV.
How urgent: Same-day. Ice on the coil damages the compressor by returning liquid refrigerant to it. Extended operation with a frozen coil can require a compressor replacement.
What to do: Shut the unit off, let the ice melt fully (typically several hours), and call for service. Do not run the unit again until a technician has diagnosed the underlying cause.
10. The Thermostat or Controls Aren’t Responding Correctly
Control problems in commercial HVAC often masquerade as mechanical problems. When the system ‘won’t listen’ — won’t switch modes, won’t hold setpoint, won’t respond to a schedule — the issue is often upstream of the equipment itself.
What’s typically causing it: A failed thermostat or building automation system component, a drifted or corrupted schedule, a communication fault between the controller and the equipment, a sensor that’s reading incorrectly, or (in older BAS installations) an integration that has been slowly falling out of sync for years.
How urgent: Depends. If the system is stuck in a bad state (heating in summer, cooling in winter, running continuously), it’s same-day. If it’s just intermittently unresponsive, schedule within the week.
What to do: Call a contractor who can diagnose both the equipment and the controls. Not every commercial HVAC contractor is competent on building automation systems — ask about controls capability before dispatching.
When to Call and When to Wait
A useful rule of thumb: any problem involving water, ice, unusual noises, unusual smells, or continuous run time deserves a same-day call. Any problem involving comfort complaints, humidity, or unexplained energy use deserves a scheduled service visit within the week. And any problem that has been happening for a while and just hasn’t been addressed deserves a call now — because problems in this equipment compound.
The five most expensive words in commercial HVAC are ‘we have been meaning to.’ Small problems in commercial HVAC do not stay small.
The Meta-Answer: Most of These Never Happen on Maintained Systems
If you read through these 10 problems and noticed a pattern, you’re seeing what every commercial HVAC service manager sees every week. The vast majority of the emergency calls we run — likely 60% or more — are for symptoms that a preventative maintenance visit in the preceding weeks would have caught.
Dirty coils, dirty filters, drifted controls, low refrigerant, worn bearings, and failing capacitors are all problems that develop slowly and become emergencies suddenly. A properly designed maintenance program catches them in the slow-development phase — cheaper to fix, no downtime, no product loss, no tenant complaints. The math on this has been consistent for decades and it does not change: the cheapest emergency call is the one that never happens.
Something on this list happening in your building? Call (603) 433-1311 for 24/7 commercial HVAC service across NH, Southern ME, and North Shore MA. A technician will diagnose the actual cause, give you a written recommendation, and — where the answer is ‘this could have been prevented’ — help you get on a maintenance plan that keeps you off this list going forward.