WHAT IS A ROOFTOP UNIT (RTU)?

A packaged rooftop unit, or RTU, is a type of HVAC (Heating, Ventilation, Air Conditioning) system that contains all the components needed to provide conditioned air in one concise unit.

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Packaged rooftop units typically connect directly to a ductwork system that distributes the conditioned air through the interior space and returns it to the packaged rooftop unit.  A basic thermostat typically controls packaged rooftop units, however, they can be a part of a more complex control system depending on the application.  Depending on the size of the building, some facilities will have more than one unit to provide HVAC for different zones.

Many commercial or industrial facilities use rooftop units in some capacity.  Everything from a standard office building to a factory is likely to have HVAC units mounted on the roof to provide for proper environmental controls year round.  Single-story buildings are best suited, since the conditioned air doesn’t have to travel very far to the living or working space.  However, RTUs can be used in multi-story buildings provided they don’t exceed 10 stories and there is accessible space on the roof and ductwork in place.

Another fairly common application for RTU HVAC systems is restaurant kitchens.  In these settings, the RTUs installed are known as ‘make-up air units’ or ‘100% dedicated outdoor air units’.  This means that all of the air running through the unit is coming entirely from outside and the unit uses no air returning from the inside space.  These specialized applications require increased insulation, additional parts, and potentially more energy to accomplish their design criteria.  Their function is to provide additional ventilation and conditioned air to replace air that’s vented out through high-powered stove hoods.

In all cases, rooftop units need to be sturdily-built and weather-proof to deal with exposure to harsh sun, snow and wind.

WHY THE ROOF?

There are a number of reasons why vital heat pump units are almost always housed on the roof of a building, but it starts back in the early days of commercial and industrial HVAC.  Before the development of the suburbs, most commercial and industrial facilities were in crowded downtown areas where there was hardly any space around the building to place the HVAC equipment venting heat and drawing heat in.  Putting units on the roof was the logical solution.

They’ve remained there because of other advantages:  prevention of damage and vandalism, easy access for maintenance and repairs without disrupting operations, reduction of indoor noise, easily modular additions, and saving space around the building.

HOW DOES A ROOFTOP UNIT WORK?

A rooftop unit is also sometimes referred to as a packaged unit.  This separates it from the ­split system, which you’re familiar with from homes.  A split system is a heat pump or air conditioner that has indoor and outdoor components.  One set of coils is indoors and the other outdoors; the air handler is indoors and the compressor outdoors.

With a rooftop unit, all these components are stored in one cabinet attached directly into the ventilation system of the building.  The fan of the unit draws air out of the building through a duct as well as fresh air, and then depending on the mode the unit is in, it either heats or cools the air by passing it over the two sets of coils.  The blower fan then pushes the conditioned air down another duct and into the main ventilation system of the building.

HOW IS AN RTU HVAC DIFFERENT FROM OTHER HVAC SYSTEMS?

Besides RTU HVAC units, other types of HVAC equipment that are situated on a roof include split-system air conditioning condensers, or water-cooled chiller systems.  In an air-cooled split system, the condenser is typically located outside, and the evaporator and fan are inside.  Also, you’ll need a separate boiler or furnace for heat with a split system.

The difference with a RTU HVAC system (or ‘packaged unit’), is that it includes all the components of an air conditioning and heating system in one box:

  • an AC evaporator coil, which absorbs heat
  • an AC condenser coil, which releases heat
  • an AC compressor, the heart of the air conditioning system
  • a heat source
  • a fan to blow heated and cooled air through the ducts
  • an intake for outside air (to improve ventilation)

The RTU HVAC unit provides all the heating and air conditioning needed for your space.  The heated and cooled air reaches different areas in the building via ductwork and registers.  You’ll also have return registers to pull air back into the system.

Read on for the major pros and cons of choosing a RTU HVAC:

RTU HVAC pros:

  1. They use less energy.  That’s because all the components have been assembled and configured in a factory under optimal conditions, so the system works at maximum efficiency.  That translates to lower utility bills for you.
  2. They don’t take up indoor space.  Everything is on the roof, except for the ducts.  You don’t need to waste expensive indoor space for air handling equipment.
  3. Installation is simpler (therefore less expensive).  Most of the job has been done at the factory.  That’s why the overall RTU HVAC cost tends to be less than for other types of systems.
  4. Distribution may already be in place.  If you are replacing an existing system, chances are you already have the existing ductwork.
  5. Faster diagnostics and repairs.  Since all the components are in one place, it’s faster for HVAC technicians to diagnose problems and fix them. Again, lowering your costs.

RTU HVAC cons:

  1. They require roof space and access.  To install an RTU HVAC system, you’ll need relatively easy access to the roof, as well as space of the roof to install the unit.  Remember that you’ll need to maintain the unit for years to come.  If it’s difficult to get up there, that adds to the maintenance cost.
  2. It’s exposed to the elements.  The entire system is exposed to high heat, extreme cold, wind, rain, snow, ice and pollution which can take its toll over time.
  3. Possible animal infestations.  Since everything is outdoors, and in a location that’s away from traffic and people, animals can make their way in and cause damage.
  4. They are not suitable for tall buildings.  There’s only so far that conditioned air can move through ductwork and still heat and cool the building.  RTU HVAC systems are best for buildings under 10 stories.
  5. Out of sight & out of mind. RTU HVAC equipment tends to suffer from neglected maintenance more than other types of systems.  This can lead to breakdowns that interrupt business and shorten the life span of the unit.

CHOOSING THE RIGHT MODEL AND MAINTENANCE

Just as you’d need the right-sized furnace and air conditioner for your home, the same is true for a rooftop unit.  Your contractor can help assess your building size to determine the right size of unit.  RTUs range in size, typically with three to 40 tons of cooling capacity, or more for custom units.

The price of a rooftop HVAC unit will vary depending on the size of the building.  The smallest unit, suitable for a one-story building, would start at around $6,000 installed.  Costs would increase to $12,000 installed for a large building such as a plaza or industrial facility.

Rooftop units typically have a lifespan of around 15 to 20 years.  However, older models and units that aren’t properly maintained could be using more energy than necessary.  In such a case, a proactive replacement, even before a unit’s lifespan is up, may be a good idea.

Historically, many builders over the years have turned to rooftop units because they’re more cost effective, but they weren’t always the most energy-efficient choice.  Over the past several years, though, rooftop units have become much more efficient, so having an RTU-based system doesn’t mean it can’t also be energy efficient.  By working with a qualified contractor, rooftop units can help curb energy use while keeping your space more comfortable.

Being mounted on the roof, RTUs run the risk of being ‘out of sight, out of mind’.  Regular maintenance is particularly important for these units as their remote location makes it harder to see and hear the signals that it’s time for a service call.  For commercial applications, it is recommended to have your equipment inspected 3 or 4 times per year by a knowledgeable HVAC technician.

Carrier and Lennox are recognized as leading brands in this market, both of which and have been in operation over 100 years and offer a complete line of options.