What is an RTU HVAC System? How Does It Work?

What is an RTU in HVAC?

A rooftop unit (RTU) is a type of packaged HVAC system which is installed on the roof of commercial and sometimes residential buildings. All the HVAC components are contained in 1 big metallic box which is then connected to the house’s ductwork to provide cooling, heating and ventilation.

There are 2 types of HVAC systems: split systems and packaged systems. Split systems have an outside unit and an inside unit connected through refrigerant lines while packaged units have all the HVAC components in one metal cabinet and always located outside the house.


Packaged units can be installed on concrete slab outside the house (on the ground) or on the roof of the house. They are then connected to the return and supply air ducts either from the back or from the bottom.

The main advantage of RTU packaged units is that they save valuable outdoor spaces since the entire unit is installed on the rooftop. They are also a good choice for houses that do not have enough indoor spaces like crawlspaces or basements.

An RTU works by drawing hot air from the house, mixing it with outside (ventilation) air and then cooling it and supplying it back to the house via the ductwork. For heating, an RTU passes cold air through an electric coil, natural gas chamber or a heat pump where it’s heated and supplied back to the house.

So, what is the difference between an RTU and ab AHU (air handling unit)? Do they do the same thing?

An RTU is a complete HVAC unit with all the components housed together in a single cabinet and installed on the roof to cool and heat the house. An AHU unit however just moves air. It pulls outdoor and indoor air, filters it, adjusts its temperature and humidity and then supplies it to the house.

Another question I am often asked is “Hoe does and RTU heat”?

An RTU heats the house using natural gas, electricity or a heat pump. Some RTUs will only be designed to cool the house while some will be designed to only heat the house. There are also some which will cool and heat the house.

How Does an RTU Unit Work?


An RTU packaged HVAC unit is made up of the following components:

  • Air intake
  • Modulating dampers (optional)
  • Air filter
  • Condenser coil
  • Heating coil (optional)
  • Gas burner (optional)
  • Blower/fan
  • Compressor
  • Expansion valve
  • Ductwork connections

As I mentioned earlier, the function of an RTU is to heat the building when the indoor air is cold, cool the indoor air when it’s too hot as well as bring in fresh air and remove stale indoor air (ventilation).

Let us now look at how an RTU does all of that in more details.

How an RTU Cools

The temperature of the indoor air is controlled by a thermostat which is centrally located inside the house. When the temperature of the indoor temperature increases more than what’s set in the thermostat, the thermostat sends a signal to the RTU unit to kick in.

When the RTU unit starts, the fan pulls the hot indoor air through the return air ducts. Depending on what type of RTU unit you have, 2 things will happen:

  • All the stale indoor (return) air will be removed through and exhaust fan
  • Some of the return air will be removed while the rest will be mixed with fresh outdoor air.

While removing all the return air and replacing it with fresh outdoor air sounds ideal, it is inefficient.

The reason for that is because the fresh outdoor air will be hot (that would be most likely during summer) and therefore cooling it would be expensive. The return air would only be warm and mixing it with the outdoor air would reduce your heating cost.

The RTU has an air intake that allows air into the unit. When the unit starts, the fan pulls air from inside the house and from outdoors (ventilation air).

Modern RTU have modulating dampers which are basically devices which open and close to allow or block air into the unit. They make sure that only the right amount of air goes inside the unit to mix with the return air.

The mixture of return air and ventilation air then passes through a filter which removes all the suspended dirt in the air. Filters need to be replaced regularly otherwise the unit will struggle to work.

At that point, we will have fresh but hot air. The next steps will be to cool it and supply it to the house. That happens inside the evaporator coil.


1. Evaporator Coil

The evaporator coil, condenser coil and compressor are connected together using copper pipes to form a closed loop. A chemical known as a refrigerant (like Freon or Puron) is then circulated between these components where it changes from gas to liquid and back to gas as it transfers heat from one point to another.

The refrigerant enters the evaporator coil as a very cold liquid. At that point, the fan will push the air (return and ventilation air) across the coil.

The refrigerant will absorb heat from the air and that is how cooling happens. The same fan pulls the cooled air and sends it back to the house through the supply air ducts.

After absorbing enough heat from the air the refrigerant (refrigerants have a low boiling point) evaporates and flows out towards the compressor.

2. Inside the Compressor

In order for the refrigerant to go round the cycle and come back for another round of cooling, it needs to remove the heat it has absorbed.  The problem is that the outside air would be hot as well, and for heat transfer to happen there needs to be a temperature differential.

And that is where the compressor comes in. Primarily, the compressor compresses the refrigerant gas to increases its pressure. But that is not the only thing that happens.

When the refrigerant gas is compressed, its temperature also increases way more than the outside air. At that point, heat transfer from the refrigerant to the outside becomes possible.

3. The Condenser Coil

The condenser coil works just like the evaporator coil (heat exchange) but in reverse. That is where the refrigerant loses the heat it absorbed from the inside air to the outside air.

As the refrigerant flows through the condenser coil, the fan blows cooler air across it. The cooler air absorbs heat from the refrigerant and takes it away with it.

By the time the refrigerant is being ejected out of the condenser coil, it will have lost so much heat that it will have condensed back to its liquid state.

4. Expansion Valve

Just before the refrigerant goes back to the evaporator coil, it is passed through a throttling device known as an expansion valve.

For a start, the expansion valve ensures that only the right amount of refrigerant is allowed to the evaporator coil. When too much of it flows to the coil, not all of it will manage to vaporize meaning that some liquid refrigerant will enter the compressor.

The compressor is only designed to compress gases. Liquids inside the compressor will therefore damage it.

Secondly, as the refrigerant goes through the expansion valve, its pressure falls drastically, and so is its temperature. That results in the creation of a super cold refrigerant.

At that point the refrigerant is ready to go back inside the evaporator coil. The cycle goes on and on until the building is sufficiently cooled.

How Does an RTU Heat?

Whether an RTU is being used to heat or cool the house, ventilation has to take place. Therefore, the system will continued drawing fresh air from the outdoors as I explained above, mixing it with return air and supplying it back to the house after heating.

As I mentioned earlier, RTU units can be designed with only 1 coil, to heat or cool the house. Some will also have 2 coils, one for cooling and the other one for heating the house. There are also those which have a single coil which heats the house and also cools it, and are known as heat pumps.

Unless you have a heat pump, heating in RTU units is by natural gas or electricity. Even when you have a heat pump. You need an auxiliary (back up) heating source which is usually in the form of electrical resistance heating.

If an RTU unit is only for heating purposes, it will have an electrical heating coil or a gas burner. Both however work the same way.

After the ventilation and return air has been filtered, it will be pushed across the heating coil/heat exchanger where it will be heated and then supplied back to the house via the supply air ducts.

Some RTU units will have a cooling coil (condenser coil) and a heating coil or gas burner. That type of unit will cool the house during summer and cool it during winter.

Another type of RTU is the one connected to a heat pump. A heat pump looks and works exactly like an air conditioner (as described above) only in reverse.

When used to heat the house, a heat pump’s evaporator coil works like a condenser coil while the condenser coil works like an evaporator coil. Let me explain.

A heat pump has a device known as a reversing valve. This valve reverses the direction of flow of the refrigerant.

As such, a liquid refrigerant whose temperature is lower than that of the outdoor air is forced through the condenser coil. Due to the difference in temperature, the refrigerant extracts heat from the outside air even when it is evidently cold outside.

After absorbing heat from the outside, the refrigerant evaporates and enters the compressor. Inside the compressor, the refrigerant gas is compressed to increases its pressure and temperature.

The refrigerant therefore enters the evaporator coil as a superheated gas. When the fan pulls the indoor and outdoor air and passes it across the coil, the air absorbs heat from the refrigerant.

The hot air is then supplied to the house via the supply air ducts. And that is how an RTU units heats the house.

Pros and Cons of RTU HVAC Units

Are RTU units right for you? Should you install an RTU HVAC system or should you go with the more popular split systems? What should you consider?

To answer those questions, we will now look at the pros and cons of RTU units starting with the pros.

The Pros

The following are the advantages of RTU HVAC units:

1. Easy/Fast Installation                                     

Compared to split systems, RTU units are way easier and quicker to install since they are compact units. As a matter of fact, you could say that most of the installation work of RTU units is already done in the factory.

With split systems, you need to install and outdoor unit and an indoor unit and then connect them together using refrigerant lines and electricity lines. All that requires human labor and time, which are both quite expensive.

2. Saves Spaces

With RTU units, you do not need to worry about having enough space inside or even outside your house. These units are securely installed on the rooftop, space which you almost always never need.

Unlike the large condenser units outside your houses, these units also do not mess up with the aesthetics outside your house.


If you therefore don’t have enough space indoors and outdoors for other types of HVAC systems, you can consider installing an RTU.

3. High Efficiency

The compact nature of RTU units makes them very energy efficient since they are connected directly to the ductwork. These units are also factory-assembled and tested and are therefore not likely to result in heat loss.

Other types of HVAC systems that are assembled together on-site are prone to mistakes which results in heat losses, refrigerant likes and other problems which reduces their overall efficiency.

4. Easy Repairs

Since all the components are in one place, it is very easy, fast and therefore cheap for HVAC technicians to find and repair these units.

With split systems, the technician has to troubleshoot both the outdoor and indoor units as well as refrigerant lines before they can find the problem which takes time and is therefore expensive.

The Cons

The following are the disadvantages of RTU HVAC units

1. Space Constrains

Not all roofs can accommodate RTU units. To start with, the roof needs to be easily accessible (for installation and maintenance work) so if it isn’t that will be a challenge.

Another thing to remember is that the space underneath the roof and above the ceiling needs to be free in such a way that it allows the connection of ductwork to the unit. If there are pipes, sprinklers and other installations then an RTU may not work for you.

2. Exposure to Elements

Although RTUs are covered using a thick metal cover, they are still exposed to extreme weather conditions like heat, cold, wind and rain. That is unlike split systems where some of the units are indoors and properly protected.

The exposure to this elements have the potential to shorten the lifespan of RTU units.

3. Animal Infestation

RTU HVAC units are always out-of-sight and out-of-mind. As such, it is easy to forget about them (as long as they are working) which can result in the infestation by rodents, birds ad bugs.

These animals can damage some of the small but vital components of the unit. Some will even block air ways when building their homes inside the unit which will reduce the efficiency of the system.

What is the Difference between AHU and RTU Units?

An air handling unit (AHU) and a rooftop unit (RTU) almost looks the same and even do the same thing and hence the reason most people have a hard time telling them apart. So, what is the difference between AHU and RTU?

To start with, RTUs as their name implies are specifically installed on rooftops. On the other hand, AHUs can be installed indoors or outdoors (on rooftops).

Secondly, RTU are packaged units meaning that they are self-contained. These units do everything from heating, cooling and ventilation of the house.

AHUs do not have an in-built cooling system (like the one in RTUs) where a refrigerant cools indoor air and releases the heat to the outside. Instead, they are connected to a central water plant or a cooled chiller.

Check out more information on those units in this post.


And basically that is everything about packaged HVAC rooftop units (RTUs). I hope that this guide was as helpful as you wanted.