Help Meet IEQ Requirements with Modulating Gas Heat

Help customers meet their Indoor Environmental Quality (IEQ) requirements through modulating gas heaters on the Fraser-Johnston® Relia™ Select Commercial Rooftop Units. 

Air conditioning within the North American market is dominated by the packaged rooftop unit space. In cold weather climates, these packaged rooftop units most commonly use gas heaters to warm the air going down the ductwork and into the space. How these gas heaters are controlled can be critical to ensure proper Indoor Environmental Quality (IEQ).

In commercial applications, it is very common for the rooftop unit gas heater to only operate during morning warmup. After that the heat load from people, computers, and lights can often maintain the load. For morning warmup applications, staged heat is typically fine.  In some applications, where the ventilation rates are high or the outside air temperatures are very cold, the mixed air within the unit may need to be tempered. This supply air tempering, otherwise known as low limit tempering, can require tighter control on the discharge air temperature than what would be achievable through a staged heater. 

Why does it really matter?

Let's take a look at a 40-ton Relia™ Select unit during a part load operating point of 10,000 CFM, a mixed air temperature of 45 degrees F, and a discharge air temperature setpoint of 55 degrees F.  With a modulating burner that goes down to 140 MBH input, the unit would heat the air by 10 degrees and supply the 55 degree leaving air temperature. In comparison, a staged unit which has a minimum of 400 MBH input, would heat the air by 30 degrees and produce a 75 degree leaving air temperature. The staged control would result in over/under shooting the discharge air temperatures and ultimately lead to comfort issues within the space. This is an example that displays the benefit of the turndown capabilities of a modulating burner over a staged burner.  There are other examples with increased ventilation or design days with reduced mixed air temperatures where an increased level of turndown is required. 

What can I do about it?

Relia™ Select rooftop units offer modulating gas heat that can help to reduce the large swings in discharge air temperature that can be seen in staged heat control. There are variations in the turndown ratios, and these turndown ratios should be considered when understanding the need for the modulating heater. For additional information on the modulation capabilities, please see the technical guides. 

Modulating gas heat is available to select, price, and order on Relia™ Select units, and is available on vertical-configured Relia™ Select units. Look for horizontal configurations with modulating gas heat to be available on Relia™ Select rooftops later this year.