Beyond the Build

Commodities Pricing

Hot Gas Reheat: Worth the Extra Cost?

Last week, I was having a conversation about pricing on an HVAC system with Heath Cather in our office. Why had the mechanical engineer chosen hot gas reheat? It’s more expensive and sometimes unnecessary, I offered.

He mentioned a trend he’s noticed. With all the mold law suits lately, we are seeing recommendations for a sure bet against moisture: hot gas reheat.

What is hot gas re-heat?

With this system, you have two coils in the air handling unit. The air first travels across a cooling coil which de- humidifies, then it crosses the hot gas coil that raises the air temp back up 15-20 degrees to further dehumidify and render the air neutral, eliminating over-cooling.  Once the thermostat calls for cooling, the hot gas drops out in order to lower the space temp.

Why are we seeing more use of hot gas reheat?

I talked with a mechanical contractor relationship of ours for details. This is what he told me:

Recent requirements to introduce large amounts of outside air into the workplace can result in the rise of the  indoor humidity level in the space. People want to combat this by purchasing a unit with excessive cooling tonnage, a “bigger is better” mentality. The over-powered unit causes the system to short cycle, not running long enough to de-humidify. We’re left with excess moisture, which can lead to mold.

Thoughts on combating the moisture with hot gas reheat:

  • Energy code limits the use of electric re-heat to 42,000 btu and below.
  • With hot gas reheat, the average 5-ton unit sees a price increase between $2,800 and 3,000, rising significantly as the tonnage increases.
  • If the space is cooling, it is de-humidifying, whether humidistat is calling or not.
  • Humidity control from a properly sized unit can lead to energy savings. Lower humidity in summer will make you feel more comfortable at 75-77 degrees, while if the humidity is high , 70-71 degrees may not seem comfortable. The opposite is true for winter operation.

Takeaways:

Hot gas is more expensive. A standard  and properly sized system has been proven to be just as effective in humidity removal and comfort control.
Know what you’re building and communicate it. What will the building be used for, and are there special climate issues involved? This is another reason design build works for many clients.
Application is key. HVAC is not a one size fits all discipline.From a contractors stand point, we see the addition of hot gas reheat on the increase and the majority of the time justified.  However one should stop and evaluate: do we really need this, or is it just insurance for a 3rd party?
Merrill Stewart Jr.

Merrill Stewart is Founder and President of the Stewart Perry Company, a commercial building contractor based in Birmingham.