Being four months removed from college, my daily routine and work environment have significantly changed. I am no longer running to class, so I don’t miss the beginning of a lecture, scratching notes down as fast as they come out of the professors’ mouths and pulling late nights cramming material in hopes of passing the big exam with flying colors.

Now a days, I am assisting in design ideas, attending design meetings and putting designs on paper to be built. The keyword is design. Although college didn’t always require developing a specific, real world design from start to finish, it did require a lot of formulas and crunching numbers.  These tedious tasks are the backbone behind HVAC design.

Like most industries, HVAC design relies on technology. 

Software can spit out numbers for a given system’s size.  However, without an understanding of the inputs and methods used by the software, it is difficult to determine how precise the system sizing may be.  Each building is different and requires special attention to several key areas. Some of these key areas are building construction, building usage, and building location.

Building Construction

The materials used in the building process determine how easily heat can enter or leave the building.  All building materials have specific R and U values that represent this. The R value represents a material’s resistance to heat flow.  The larger the R value, the better insulating value of the material. R values are also additive. Therefore, if you are interested in the heat gain through a wall, the R values of the air films, exterior siding, insulation, vapor barrier, drywall, etc. can be added up.

The reciprocal of the total R value is the U factor.  The U factor represents the heat flow through the material.  The smaller the U factor, the better insulating value of the material. A sample wall construction detail is shown below.  Its R or U factor could easily be calculated to determine its overall insulating value.


Building Usage

The number of occupants and type of activity in a building can alter the type and/or sizing of an HVAC system. The number of occupants affects the amount of ventilation air brought into the building and the sensible and latent heat gains.  The ventilation (outside air) requirements are often specified by the occupancy category. For a facility requiring more outside air (such as a sports area), more conditioning will need to be done to the air on an extremely hot or cold day.  Therefore, a larger HVAC system will be needed.

The type of activity occurring in a building can predict the sensible and latent heat gain per person.  Sensible heat is the heat that causes a temperature change.  Latent heat is the heat that accompanies a change in state.  A filled gymnasium, such as the WVU Coliseum shown below, is a good example of sensible and latent heat.  The lights, videoboards, food warmers, etc. produce sensible heat. The fans attending the game and the players produce latent heat. HVAC systems have sensible and latent capacities that dictate the size of a unit based on the type of activity in a building.


Building Location

The heat transfer through building walls, roof, glass, etc., takes the temperature difference between the inside and outside air into account.  The design outside air temperature depends on the location of the building.   A building in Morgantown would have a larger temperature gradient than a building in Miami, FL, during the winter months.  Assuming the buildings are identical in construction and size, the Morgantown building would have a higher heat loss and require a larger HVAC unit with more heating capacity.

Allegheny Design Services

At Allegheny Design Services, we pay special attention to all the factors involved in HVAC design.  We understand that each building is different and are committed to providing practical solutions in each instance.  Click here to see the designs we have implemented in our recent MEP projects.

Thanks for reading,


Written by Alex Clarkson, EIT, Junior Mechanical Engineer

Alex-ClarksonAlex Clarkson joined the Allegheny Design Services team in March of 2015 as an engineering Intern. He graduated magna cum laude from West Virginia University with his bachelor’s degree in mechanical engineering in May 2015. While at WVU, Alex served as the treasurer of the student branch of ASHRAE during the fall 2014 semester and president during the spring 2015 semester. Prior to joining ADS, Alex worked as an MEP engineering Intern at Miller Engineering. He is now a junior mechanical engineer at ADS, primarily focusing on the design of mechanical and plumbing building systems.