Did you know: Hot Gas Reheat provides NO benefit on a design day?

Did you know: hot gas reheat operation (for dehumidification) provides NO benefit on a summer design day?

This can be an easy mistake to make. (to clarify from the start, this is NOT a discussion about DOAS, but rather a standard return air rooftop unit).

If you have a customer that would like to maintain a specific space temperature and humidity, be careful not to assume that hot gas reheat will operate on the hottest days of the year. 

Example: 

Customer would like to maintain a 74 DegF space temperature at 50% RH. 

Ambient Design Temperature: 98/78 db/wb

Sensible load: 229,000 btuh

Latent load: 84,000 btuh 

Equipment selected at design provides 236,000 btuh sensible capacity and 72,600 btuh latent capacity. 

Discharge temperature of selection: 56.5/55.3 db/wb

Equipment selection shows that hot gas reheat performance is 105,000 btuh latent capacity

Since the base unit is slightly short on latent capacity, the selector adds hot gas reheat. 

Notice the mistake?

Hot gas reheat performance doesn’t matter on a design day, because the equipment will be running in full cooling. Therefore, hot gas reheat sequence is never energized and provides no benefit!

The first step in this case is to select a unit that will provide the correct amount of capacity AND a cold enough discharge temperature to meet the desired space temperature and humidity. 

Do you work on projects with specific space temperature and humidity requirements? 

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Brad Telker

Vice President of Sales, Applied Systems Group at cfm Distributors, Inc.
Brad joined the cfm team in 2006, and now as Vice President of Sales, Applied Systems Group he focuses on business development, as well as helping contractors and engineers find creative and unique solutions to any size HVAC project. When he’s not at work, Brad enjoys reading, running and spending time with his family.
Brad Telker
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