What does RTU stand for, and how is it different from a split system?

RTU stands for rooftop unit — a packaged HVAC system where the compressor, condenser, evaporator, blower, heating section, and controls are all built into one weatherproof cabinet on the roof, with conditioned air ducted down into the building. A split system, by contrast, separates the equipment: an outdoor condensing unit connected by refrigerant lines to an indoor air handler or furnace. The practical difference is that an RTU keeps all the equipment and service on the roof and off the leasable floor, which is why packaged rooftop units are the default for single-story and low-rise commercial buildings. If you're weighing the two, see our RTU vs. split system and packaged vs. split commercial HVAC comparison guides.

What are the main parts of a rooftop unit?

Inside the cabinet you'll find the compressor and refrigeration circuit, an evaporator (indoor-side) coil, a condenser (outdoor-side) coil with fans, a supply blower and motor, a heating section (a gas furnace on gas/electric units or a reversing refrigeration circuit on heat-pump units), an economizer with outside-air dampers for free cooling, air filters, and the control board or building-automation interface that sequences and stages everything. The unit sits on an insulated roof curb that seals it to the building and routes the ductwork.

What size rooftop unit does my building need?

It depends on a load calculation, not a guess. Capacity is measured in tons (one ton equals 12,000 BTU/hour of cooling), and most commercial RTUs run from about 3 to 25 tons, with larger buildings using multiple units. Rough benchmarks put many commercial spaces around 250 to 400 square feet per ton, and the U.S. DOE reports the RTU stock averages roughly 488 square feet per ton — but those are starting points. Actual sizing accounts for occupancy, internal heat gains, glazing, insulation, and ventilation. A restaurant or dense office needs far more cooling per square foot than a warehouse. Oversizing causes short-cycling, poor dehumidification, and premature wear, so right-sizing from a real load matters. See our commercial HVAC system sizing guide for the full method.

How long does a commercial rooftop unit last?

A typical commercial rooftop unit lasts about 15 to 20 years, and ASHRAE lists a median service life of roughly 15 years. Units that get regular maintenance tend to reach the upper end of that range or beyond, while neglected units often fail early — sometimes at 10 to 12 years — because rooftop equipment is exposed to weather and runs thousands of hours a year. Climate, run hours, maintenance, and build quality are the biggest factors. For guidance on the replace decision, see our commercial HVAC lifespan and when-to-replace guide.

What's the difference between a single-zone and a multi-zone rooftop unit?

A single-zone RTU conditions one area to one thermostat — ideal for an open retail floor, a warehouse, or a simple office where the whole space behaves the same. A multi-zone rooftop system serves several areas with different needs from one unit, typically using VAV (variable air volume) boxes and controls so each zone gets its own temperature. Multi-zone setups suit buildings with mixed occupancy, varied loads (a server room versus a lobby), or many small tenants. The choice follows how your building is partitioned and used, and it affects both first cost and control complexity.

Should I choose a gas/electric or a heat-pump rooftop unit?

Gas/electric units cool with electricity and heat with a natural-gas furnace section — long the standard in much of the country and often lower-cost to operate where gas is cheap. Heat-pump rooftop units have no combustion; they heat and cool with the same reversible refrigeration circuit, usually with electric backup heat for the coldest days. Heat pumps matter increasingly because they eliminate on-site emissions, which is directly relevant to NYC buildings facing Local Law 97 thresholds. The right choice weighs fuel costs, your climate's heating demand, electrical capacity, and electrification goals. See our gas vs. electric commercial heating guide to think it through.

What maintenance does a rooftop unit need?

The basics carry most of the value: change filters on schedule, keep both coils clean, inspect and replace blower belts, lubricate bearings, verify economizer and damper operation, check refrigerant charge and electrical connections, clear condensate drains, and — on gas units — inspect the burners, ignition, and heat exchanger for safe combustion. Most of these are inexpensive, and skipping them is what turns a cheap tune-up into an emergency compressor or heat-exchanger failure. A planned maintenance program, like the rooftop coverage in Com+ Mechanical's commercial maintenance and RTU preventive maintenance services, is the most reliable way to reach the high end of an RTU's lifespan.

When should a rooftop unit be repaired versus replaced?

Repair usually makes sense when the unit is under roughly 12 to 15 years old, the fault is isolated, parts are available, and the cabinet, coils, and heat exchanger are sound. Replacement deserves pricing when the compressor or heat exchanger has failed on an aging unit, repair costs approach a large share of replacement value, the unit uses an obsolete refrigerant like R-22, or you want higher efficiency to support Local Law 97 targets. The honest call depends on equipment age, run data, and your hold period for the building — which is exactly the analysis Com+ Mechanical documents before recommending either path.

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The Commercial Rooftop Unit (RTU) Guide: How Packaged HVAC Actually Works

The rooftop unit is the workhorse of commercial HVAC — the single packaged cabinet that heats, cools, and ventilates most retail stores, offices, restaurants, and low-rise buildings in the country. If you own or manage a building, you live with RTUs whether you understand them or not: they drive your comfort complaints, a large share of your energy bill, and some of your biggest capital decisions. This guide explains what an RTU is, the components inside the box, the gas/electric and single- vs multi-zone variations, how tonnage is sized, how long they typically last, what goes wrong, and the maintenance that keeps them alive. It is written to make you a smarter owner — not to sell you a unit. When you do need rooftop work in New York City, Com+ Mechanical repairs, maintains, and replaces packaged units across the five boroughs of New York City and surrounding counties.

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How a Rooftop Unit Works: The Key Components

Compressor and refrigeration circuit

The compressor is the heart of the cooling side — it pressurizes refrigerant and drives the refrigeration cycle. Paired with the evaporator coil (which absorbs heat from the return air) and the condenser coil and fan (which reject that heat to the outdoors), it is what actually makes cold air. Scroll compressors are the most common type in commercial RTUs; larger or higher-efficiency units may use multiple compressors or variable-capacity designs to match part-load demand. The compressor is also the single most expensive component to replace, which is why its condition often decides repair-versus-replace on an aging unit.

Evaporator coil, condenser coil, and condenser fans

Refrigerant moves heat between two coils. The evaporator (indoor-side) coil sits in the return airstream and absorbs heat, cooling and dehumidifying the supply air. The condenser (outdoor-side) coil, with its fans, dumps that heat to the atmosphere. Dirty or bent coils are one of the most common efficiency killers on a rooftop unit because they sit exposed to weather, pollen, and rooftop grime. Keeping both coils clean is one of the highest-value maintenance tasks there is.

Supply blower and motor

The blower (supply fan) pushes conditioned air through the ductwork into the space and pulls return air back. Belt-driven blowers need belt and bearing service; many newer units use direct-drive or variable-speed (ECM) motors that modulate airflow to demand and save fan energy. Airflow problems — a slipping belt, a failing motor, or a clogged filter starving the blower — show up as weak airflow, uneven temperatures, and coils that freeze or overheat.

Heating section: gas furnace or heat pump

On a gas/electric unit, the heating section is a gas-fired furnace with burners, an ignition control, a gas valve, and a heat exchanger that transfers combustion heat to the airstream while keeping flue gases separate. A cracked heat exchanger is a safety condition that takes a unit out of service. On a heat-pump RTU, there is no combustion — the refrigeration circuit reverses to pull heat from outdoor air, often with electric-resistance backup for the coldest days. Which one you have shapes both your fuel costs and your electrification/Local Law 97 options.

Economizer and outside-air dampers

An economizer is a set of motorized dampers and sensors that lets the unit pull in cool outdoor air for 'free cooling' when conditions allow, instead of running the compressor. Standard on most modern commercial RTUs, a working economizer saves real energy — but seized actuators, failed sensors, or bad linkages are extremely common and quietly waste energy or cause comfort complaints when dampers stick in the wrong position. Economizer faults are among the most under-diagnosed rooftop issues.

Controls, filters, and the curb

The control board, thermostat or building automation system (BAS) interface, and safety switches sequence everything and stage capacity to demand. Filters protect the coils and the airstream and are the cheapest, most-skipped maintenance item on any rooftop unit. The roof curb is the insulated, sealed frame the unit sits on; a failing curb or flashing causes leaks and roof staining below the unit. None of these are glamorous, but neglected filters, controls, and curbs cause a large share of avoidable rooftop service calls.

What a Rooftop Unit Is — and Why Buildings Use Them

A rooftop unit, or RTU, is a packaged, self-contained HVAC system: a single factory-assembled cabinet that contains everything needed to cool, usually heat, and ventilate a commercial space, mounted on the roof over a curb and ducted down into the building. The word that matters is 'packaged' — unlike a split system, where the compressor and condenser sit outside and the air handler sits inside connected by refrigerant lines, an RTU puts the compressor, condenser, evaporator, blower, heating section, controls, and filters all in one weatherproofed box on the roof. Buildings favor rooftop units for a simple reason: they keep all the mechanical equipment, and all the service that goes with it, off the leasable floor and out of the occupied space. A technician works on the roof; the tenants below are undisturbed. That makes RTUs the default choice for single-story and low-rise commercial buildings — strip retail, big-box stores, restaurants, warehouses, schools, and many offices — where roof access is straightforward and interior mechanical space is valuable. Capacities commonly run from about 3 tons up to 25 tons or more per unit, and a larger building simply uses several units, each conditioning its own area or 'zone.' Most commercial RTUs are gas/electric — they cool with an electric refrigeration circuit and heat with a natural-gas furnace section — though all-electric heat-pump rooftop units are increasingly important as buildings electrify and face emissions rules like NYC's Local Law 97. Understanding the box, its parts, and how it is sized is the foundation for every repair-or-replace, efficiency, and budgeting decision you will make about it. The sections below break it down, and where you want hands on the equipment, Com+ Mechanical handles rooftop service across New York City.

What This Guide Covers

What a packaged rooftop unit is and how it differs from a split system

The major components inside the cabinet — compressor, coils, blower, heat section, economizer, and controls

Gas/electric vs. heat-pump rooftop units, and where each fits

Single-zone vs. multi-zone (and variable air volume (VAV)) rooftop configurations

How tonnage is sized to a building, and why rule-of-thumb sizing is risky

Typical RTU lifespan and the factors that lengthen or shorten it

The most common rooftop unit problems and what causes them

Maintenance basics that protect efficiency, capacity, and equipment life

Where Com+ Mechanical fits — rooftop repair, maintenance, and replacement across New York City

Why Buildings Choose Packaged Rooftop Units

All mechanical equipment and service stay on the roof, off the leasable floor and out of tenant space

A single packaged cabinet is faster to install and replace than field-assembled split equipment

Modular by design — a larger building scales by adding units, each serving its own zone

Well suited to single-story and low-rise retail, restaurants, warehouses, schools, and offices

Available in gas/electric and all-electric heat-pump versions to fit fuel and electrification goals

High-efficiency tiers with economizers, multi-stage or variable capacity, and ECM blowers are widely available

Our Simple Process

From call to comfort in 4 easy steps

Start With a Load Calculation, Not a Rule of Thumb

Proper sizing begins with a cooling and heating load calculation that accounts for the building's square footage, occupancy, internal heat gains, insulation, glazing, and ventilation needs. Rough rules of thumb exist — commercial spaces often fall somewhere around 250 to 400 square feet per ton of cooling, and the U.S. DOE's Better Buildings program notes the RTU stock averages roughly 488 square feet per ton — but those are starting points and benchmarks, not a substitute for a real load. Use is everything: a office or restaurant needs far more cooling per square foot than a warehouse.

Why Oversizing Is the Common Mistake

Rooftop units are frequently oversized 'to be safe,' and it backfires. An oversized unit satisfies the thermostat too quickly, short-cycles on and off, never runs long enough to dehumidify properly, wears out its compressor faster, and costs more to buy and run. Right-sizing — matching capacity to the actual calculated load — delivers better comfort, lower energy use, and longer equipment life. This is why a measured load beats a generous guess every time.

Match the Configuration to the Building

Sizing isn't only about tons. It includes choosing gas/electric vs. heat pump based on fuel cost and electrification goals, single-zone vs. multi-zone based on how the space is partitioned and occupied, the efficiency tier based on run hours and energy cost, and the heating capacity for the climate. In the heating-dominant Northeast, the gas-heat or heat-pump capacity and cold-weather performance matter as much as the cooling tonnage.

Confirm the Roof, the Curb, and the Power

Finally, selection has to respect the building: roof structural capacity to carry the unit, a compatible or adaptable curb, adequate electrical service and gas supply, rigging and crane access for the set, and any code or permit requirements. A perfectly sized unit still fails the project if the roof can't carry it or there's no crane path. Com+ Mechanical confirms these realities before specifying a replacement on any New York City building.

Types of Systems We Install

Gas/Electric Packaged RTUs

The most common commercial rooftop configuration: electric refrigeration for cooling paired with a natural-gas furnace section for heat. A strong fit where gas is available and affordable and the building has meaningful winter heating demand, as in the Northeast.

•Electric cooling, gas-fired heating in one cabinet
•Often lower heating operating cost where gas is cheap
•Heat exchanger requires periodic safety inspection
•Widely available across tonnages and efficiency tiers

Heat-Pump Rooftop Units

All-electric packaged units that heat and cool with a single reversible refrigeration circuit, usually with electric-resistance backup heat. Central to electrification strategies and Local Law 97 planning because they eliminate on-site combustion emissions.

•No on-site combustion — supports emissions reduction
•Reversing valve provides heating and cooling
•Electric backup heat for the coldest design days
•Pairs with high-efficiency and cold-climate options

Multi-Zone & VAV Rooftop Systems

Larger rooftop units that serve several zones from one cabinet using variable-air-volume (VAV) boxes and controls, so each area gets independent temperature control. The right answer for buildings with mixed occupancy, varied loads, or multiple tenants.

•Independent comfort for multiple zones from one unit
•VAV boxes modulate airflow per zone
•Suited to mixed-use, multi-tenant, varied-load buildings
•Integrates with building-automation (BAS) controls

Why Property Teams Use Com+ Mechanical for Rooftop Work

Commercial Rooftop Is What We Do

Com+ Mechanical services packaged rooftop equipment across the five boroughs of New York City — single-zone and multi-zone, gas/electric and heat-pump units. The diagnosis and the recommendation start from the rooftop unit's nameplate, load and condition, and we document what we find.

Root-Cause Diagnosis, Documented

When a unit fails, we test the refrigeration, electrical, airflow, and control circuits under load to find the true cause rather than resetting the symptom, and we document findings with photos so building staff have a clear record and an honest repair-versus-replace recommendation.

Built for Occupied, Multi-Site Buildings

We coordinate roof access, certificates of insurance, tenant notice, and crane windows, and we support property-management companies running one rooftop or a portfolio across multiple buildings and boroughs — one vendor, consistent documentation, coordinated scheduling.

Efficiency- and Compliance-Minded

We weigh efficiency tier, economizer performance, and electrification against operating cost and Local Law 97 emissions exposure, so a rooftop replacement is one you can afford to run and that helps your building's compliance position — not just a like-for-like swap.

Transparent Pricing

No fees. No surprises. Just honest service.

Preventive Maintenance

Custom Quote

Planned rooftop maintenance scoped to your units and run hours to protect efficiency, capacity, and equipment life across the year.

Coil cleaning, filter changes, belt & bearing service
Economizer, damper, and control verification
Heat-exchanger and combustion safety inspection (gas units)
Condition reporting and 24/7 emergency dispatch

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Replacement / Changeout

Custom Quote

Turnkey rooftop replacement — equipment selection, rigging, curb adaptation, tie-ins, controls, and commissioning.

Load-based sizing and efficiency-tier selection
Rigging, crane set, and disposal of the old unit
Curb adaptation, electrical, gas, and condensate tie-ins
Controls/BAS integration, startup, and commissioning

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Pricing shown is a structure, not a quote. Rooftop repair, maintenance, and replacement are confirmed in writing after assessment, because tonnage, fault, access, efficiency tier, and tie-in scope drive the real cost and vary by building. Repair diagnostics are a $269 flat business-hours fee, credited toward the repair.

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Equipment & Brands We Service

Factory-trained technicians for all major HVAC manufacturers

TraneThe Apple of HVACFactory Authorized

CarrierThe OG of Air ConditioningFactory Authorized

LennoxPremium High-EfficiencyFactory Authorized

American StandardTrane's Smarter TwinPreferred Partner

RheemReliable & Drama-FreePreferred Partner

BryantCarrier's Quieter SiblingCertified

GoodmanHonest ValueCertified

RuudRheem's Reliable TwinCertified

Mitsubishi ElectricGold Standard for DuctlessFactory Authorized

DaikinWorld's Largest HVAC ManufacturerFactory Authorized

BoschGerman Engineering ExcellencePreferred Partner

LGSurprisingly LegitPreferred Partner

Don't see your brand? We service all major manufacturers! Call us to confirm.

Serving the New York City Metro Area

Commercial HVAC service for buildings across the five boroughs and nearby counties

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Frequently Asked Questions

Get answers to common questions about our services

What is a rooftop HVAC unit (RTU)?

A rooftop unit (RTU) is a packaged, self-contained HVAC system — compressor, coils, blower, heating section, filters, and controls in one factory-assembled, weatherproofed cabinet — mounted on the roof and ducted down into the building. It cools, usually heats, and ventilates the space without occupying interior floor area, which is why it is the default HVAC choice for low-rise commercial buildings.

How long do commercial rooftop HVAC units last?

Typically 15-20 years when properly maintained — and meaningfully less when maintenance is skipped. Salt-air exposure near the coast, economizer neglect, dirty coils, and oversized short-cycling units all shorten life. A well-maintained unit at year 18 is a replacement candidate; a neglected unit at year 12 usually already is.

What tonnage RTU does my building need?

There is no reliable rule of thumb — roughly 1 ton per 400-600 sq ft is a starting guess that is wrong often enough to be dangerous. Real requirements depend on envelope, occupancy, ventilation, equipment loads, and hours of use. Com+ Mechanical sizes every unit from an actual cooling and heating load calculation, which regularly un-sizes legacy units that were oversized (wasting money) or undersized (never keeping up) from day one.

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Have a Rooftop Unit Question — or a Rooftop Unit Problem?

Understanding your rooftop equipment is the first step; having a commercial team that can actually service it is the next. Whether you want a second opinion on a failing unit, a load-based sizing for a replacement, a preventive-maintenance program to reach the high end of your RTUs' lifespan, or an efficiency and Local Law 97 strategy for your portfolio, Com+ Mechanical services packaged rooftop equipment across the five boroughs of New York City and surrounding counties. Call (332) 600-4640 or request service to talk it through.

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