- A packaged rooftop unit combines cooling, heating, supply fan, and often an economizer in one weatherproof cabinet, so one failed part can take out comfort for an entire tenant space.
- Most RTU emergencies trace back to a short list of wear items (capacitors, contactors, belts, condensate drains, clogged coils), nearly all of which show up during a proper PM visit first.
- A broken economizer rarely triggers a complaint, but it quietly wastes energy and can fail Title 24 requirements. Check it every season.
- Coastal Orange County sites should specify protective coil coatings and plan coil cleaning more often than inland sites.
- Repair-vs-replace is a lifecycle decision: age, refrigerant, compressor condition, and heat exchanger integrity matter more than a single repair quote.
What a rooftop unit actually is
A rooftop unit (RTU) is a self-contained, factory-assembled HVAC system installed on a roof curb or, less often, on a ground pad. Everything lives in one cabinet: the evaporator coil, compressors, condenser coil and fans, supply blower, filters, and a heating section (gas heat exchanger, electric strips, or a heat pump reversing valve). Air returns through the roof curb, is conditioned, and is delivered down through ducts into the space.
That packaging is why RTUs dominate low-rise commercial construction in Southern California: strip centers, restaurants, medical office, schools, warehouses, and light industrial. It is also why they fail the way they do. All the electrical controls, refrigerant circuit, and airflow components share one weather-exposed box, and every service call happens on a roof.
| Size (tons) | Typical use | Notes |
|---|---|---|
| 3–7.5 | Small retail, offices, dental/medical suites, restaurant dining rooms | Often single-compressor, sometimes split-system condensers instead |
| 10–20 | Mid-size retail, schools, restaurants, light industrial | Two-compressor units allow staged cooling and part-load efficiency |
| 25–50+ | Big-box retail, warehouses, large office floors | Multiple circuits, often VAV or variable-speed supply fans, crane required for replacement |
The components that matter for service
You do not need to be a technician to manage RTUs well, but knowing which parts do what makes proposals easier to evaluate and failures easier to describe to dispatch.
- Compressors (scroll or reciprocating): The expensive part. They rarely fail first. They fail because something else (low airflow, low charge, electrical issues, liquid slugging) was ignored.
- Run/start capacitors and contactors: The most common no-cool callouts. Cheap parts, but they degrade with heat and cycling, and a weak one stresses motors and compressors.
- Condenser fan motors and blades: Seized bearings and failed capacitors cause high head pressure and compressor trips on hot days.
- Supply blower, belt or direct-drive: Belt-driven units need tension and alignment checks; worn belts reduce airflow before they snap.
- Evaporator and condenser coils: Dirt, grease (near kitchens), and in coastal areas, salt corrosion. Restricted coils drive up pressures and kill efficiency.
- Condensate drain pan and trap: Clogs cause overflow into ceilings. Water sensors help; a cleaned drain is better.
- Heat exchanger (gas units): Cracks or rust-through create carbon monoxide and safety risks. Inspect with a combustion analysis and visual or camera inspection.
- Economizer (dampers, actuator, sensors): Brings in outside air for free cooling when conditions allow. Frequently stuck or miswired.
- Controls: Thermostat or building automation connection, safeties (high/low pressure, freeze stat, limit), and any fault-detection logic.
Common failure modes, by symptom
The table below is how we triage a rooftop call. The first check is cheap and fast; the root-cause check is what prevents a repeat visit. If your current provider keeps swapping parts without resolving the complaint, read our page on the second opinion diagnostic.
| Symptom | Often the quick fix | Root-cause question |
|---|---|---|
| Unit runs but space is warm | Dirty filters, failed condenser fan, tripped high-pressure switch | Why did pressure climb: coil fouling, airflow, overcharge, a non-condensable in the circuit? |
| Unit won't start | Blown fuse or breaker, bad contactor or transformer | What caused the electrical failure: a shorted motor, a loose lug, a failed capacitor stressing the circuit? |
| Ice on the evaporator coil | Dirty filter or blocked return | Low airflow, low charge, or a bad metering device. Thawing it is not the repair. |
| Water through the ceiling | Cleared condensate drain | Trap design, pan slope, missing overflow switch, or restricted coil causing excess condensation |
| Short cycling | Thermostat placement or settings | Oversized unit, failed pressure control, or a safety tripping on limit |
| Heating works, cooling doesn't (or vice versa) | Reversing valve or contactor | Low refrigerant from a leak, or a failed compressor on one stage |
| High bills, no complaint | None. This is usually unnoticed | Economizer stuck, fan running continuously, failed staging, or worn coil efficiency |
Economizers: the quiet energy and code issue
California's energy code (Title 24, Part 6) requires economizers on most newly installed commercial cooling systems of meaningful size, along with fault detection and diagnostics (FDD) on many of them. An economizer opens outdoor-air dampers when the outside air is cool enough to offset mechanical cooling. In much of Orange County, with its mild coastal evenings and mornings, a working economizer can carry a large share of cooling hours.
The problem is that a failed economizer is invisible. A damper stuck closed just makes the compressor run more. A damper stuck open pulls hot afternoon air into the building and overworks cooling. Neither produces a complaint call, so the failure runs for years. A seasonal PM that cycles the actuator, verifies damper travel, and checks the changeover sensor is one of the highest-value items on a rooftop checklist.
When an RTU is replaced as an alteration, current energy-code requirements generally apply to the new unit, including economizer and acceptance-testing documentation where triggered. See Title 24, permits, and HVAC replacement.
What proper RTU maintenance includes
Maintenance scope varies by site, but a complete rooftop PM covers airside, refrigerant side, electrical, and safeties. Frequency depends on duty cycle: a restaurant kitchen make-up unit working 16 hours a day is not a small office unit on a weekday schedule. See how often RTUs need maintenance for the frequency reasoning.
- Replace or clean filters; verify filter rack seals (bypass air defeats the filter).
- Inspect and clean evaporator and condenser coils; rinse coastal units to remove salt deposits.
- Check blower: belt condition and tension, sheave alignment, bearing noise, motor amp draw versus nameplate.
- Inspect and clear condensate drain and trap; test overflow switch if installed.
- Measure refrigerant circuit performance: suction and discharge pressures, superheat and subcooling, temperature split across the coil.
- Test electrical components: capacitor microfarads against rating, contactor contacts, wiring terminations, voltage and amp draw on every motor and compressor.
- Exercise the economizer: command to full open and closed, confirm actuator travel and sensor readings.
- Gas heat: inspect burners and heat exchanger, verify gas pressure, ignition sequence, flue draft, and combustion where appropriate.
- Verify safeties and controls: high/low pressure cutouts, freeze protection, limit switches, thermostat or BAS communication.
- Document with photos and readings tied to the unit ID, and note anything trending (e.g. amp draw creeping up).
That last step is the reason we built the DaVinci Portal. A reading means little in isolation; a three-year trend of the same compressor's amp draw is what tells you whether to repair or budget for replacement.
Orange County conditions that change the plan
- Coastal salt air (Newport Beach, Huntington Beach, Costa Mesa shoreline areas): Accelerates coil and cabinet corrosion. Specify coated coils on new units and shorten coil-cleaning intervals.
- Inland heat (Anaheim, Fullerton, Yorba Linda, Orange): Summer peaks of 95°F and higher push condenser head pressure. Clean condenser coils and healthy fans matter most here.
- Santa Ana wind and wildfire smoke events: Heavy particulate loads clog filters and coils fast. After an event, inspect filters sooner than the schedule says.
- Rooftop access and crane logistics: Many Orange County business parks restrict crane staging. Plan replacement dates with property management well in advance.
For the climate reasoning by area, see our Orange County commercial HVAC climate guide.
Repair or replace: the lifecycle view
Commercial RTUs commonly last 15 to 20 years when maintained, less in harsh or high-run-hour environments. The decision should weigh several factors together, not just one quote.
| Factor | Leans repair | Leans replace |
|---|---|---|
| Age vs. expected life | Under about 10 years | 15+ years, especially with deferred maintenance |
| Failed component | Motor, capacitor, contactor, board, sensor | Compressor on an old unit, cracked heat exchanger, corroded coil or cabinet |
| Refrigerant | Current refrigerant, parts and refrigerant readily available | Phased-out refrigerant (e.g. R-22) where service costs are rising |
| Repair history | First or second repair in several years | Repeated repairs across systems in the same year |
| Efficiency and code | Unit still performs close to rated capacity | Significant efficiency gap and incentives or code triggers favor new equipment |
| Budget timing | Repair buys planned time to a funded replacement | Cheaper to replace now than pay for emergency replacement later |
A common rule of thumb says to replace when a repair exceeds about half the cost of a new unit; we cover how to pressure-test that rule in our RTU repair vs. replace framework. For the replacement process itself (crane, curb adapters, permits, sequencing), see our commercial RTU replacement page.
Specifying a replacement RTU
- Match load, not just the old nameplate. Buildings change: added equipment, changed occupancy, and envelope upgrades all alter the real load. Undersized units never catch up; oversized units short-cycle and handle humidity poorly.
- Check the curb. Dimension mismatches are common between generations. Verify before ordering, and budget for a curb adapter when needed.
- Decide on heat type early. Gas, electric, or heat pump affects utility service, venting, and (in South Coast AQMD territory) emissions compliance for gas equipment.
- Specify corrosion protection for coastal sites and grease-exposed locations.
- Plan controls and monitoring. Connect to the existing BAS if there is one, and decide whether a standalone thermostat with schedules is enough.
- Confirm lead times. Larger units can run weeks to months from order; temporary cooling plans may be needed for critical spaces.
Frequently asked questions
How long should a commercial rooftop unit last?
Commonly 15 to 20 years with consistent maintenance. Units in coastal air, near commercial kitchens, or running many hours per day can wear out sooner.
Why does my RTU keep tripping the high-pressure switch?
It is a safety doing its job. Typical causes are a dirty condenser coil, a failed condenser fan or capacitor, an overcharge, or non-condensables in the refrigerant circuit. The fix is finding out which. Resetting the switch only hides the symptom.
Do I need a crane to replace a rooftop unit?
Almost always for units over a few tons, and for anything on a multi-story roof or where hoisting by hand is unsafe. Crane scheduling, street permits, and rooftop structural limits should be part of the replacement plan.
Is an economizer required?
California's energy code requires economizers on many new or replaced commercial cooling systems above certain capacities, with exceptions. Whether it applies to your project depends on the unit size, climate zone, and the scope of the alteration. We confirm during planning.
Can you service rooftop units from any manufacturer?
Yes. We work on commercial equipment from all major manufacturers, and the service approach is the same: diagnose the cause before replacing parts.
Have a site this applies to?
Davinci Mechanical is the commercial and union division of Scottish Tom's Heating & Air. Send us the equipment list or the problem and we'll tell you what we'd check first.