Roof top heating and cooling systems are the preferred HVAC systems for designers.  Self contain and no loss of usable space inside the building.   The units are designed to last 15 years and will typically work last 20 years.  As they unit age, parts will start to fail and repair cost can get expensive.   

 

Replacing a Heat Exchanger and Compressor can be very expensive, particularly with R22 systems.  

 

Replacing the unit before it starts to need major repairs is far more cost effective and reduces and save a lot of tenant complaints before they occur. 

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Selecting the proper roof unit can result huge energy savings.  New units use half the electricity a typical 20 year old RTU and improves comfort.