Applied rooftop heating systems offer many performance and cost benefits, including configuration flexibility, energy efficiency, and wide operating ranges, for owners and design engineers. The availability of fully modulating gas heating systems with high turndown ratios helps maintain comfort conditions. Two types of applied rooftop heating systems can benefit from a heating source with a wide range of modulation: variable-air-volume (VAV) systems at light-load, low-ambient conditions and 100-percent-outdoor-air systems. Useful for schools, health-care centers, churches, offices, manufacturing facilities, and shopping malls, rooftop heating systems can address widely varying heating loads in colder climates.
GAS FURNACES WITH HIGH-TURNDOWN BURNERS
It can be difficult to maintain constant discharge-air temperature (DAT) when only a small amount of heat is required. Steam and hot-water coils commonly are used to provide fully modulating heat, but have limitations in rooftop applications. Steam and hot-water heating systems can be expensive to provide, and coil freezeup can be a problem in cold climates.
Gas furnaces with high turndown ratios and continuous-capacity modulation at normal inlet gas pressures are an effective solution. This application results in less on-off furnace cycling, reduced thermal stresses, and better comfort caused by enhanced leaving-air-temperature control.
Gas furnaces with high turndown ratios provide a constant supply-air temperature. In comparison, low-turndown systems provide DATs that fluctuate (Figure 1). Non-modulating heat systems supply more heat than is needed and turn off for short periods. This results in discharge air that is too hot, then too cold.
With high-turndown gas furnaces, engineers are able to reduce design, installation, and operating costs while delivering precise temperature control and efficient mixed-air temperatures (MATs). High-turndown gas burners are designed for full modulation and provide turndown ratios of up to 20-to-1.
Burners with limited turndown capability cannot address light-heating-load conditions efficiently. For example, a furnace with 3-1-turndown-ratio capability can decrease modulation to only 33 percent of full capacity. For lower heating requirements, the furnace will cycle on and off between 33 percent and pilot light. In contrast, high-turndown furnaces can decrease modulation to 5 percent of full-fire capacity, eliminating almost all cycling caused by capacity control.
VAV systems often keep discharge air at a constant temperature (usually about 55°F) and vary airflow rates to meet cooling loads. Most VAV systems provide air conditioning year-round because of internal heat loads. Proper DAT can be difficult to maintain in cold weather with VAV systems. As the supply-fan air volume is reduced at light load, the percentage of outdoor air increases. MAT then could decrease significantly, leading to a need for heating to maintain 55°F DAT. However, a high-turndown modulating furnace can provide proper DAT under all ambient conditions.
Properly matching design-furnace capacity to application requirements can be as important as modulation capability. For example, let's assume that a building's specifications include using a rooftop unit's gas furnace for morning-warmup heat and MAT tempering under the following conditions:
20,000-cfm design airflow.
8,000-cfm minimum supply-fan airflow.
3,000-cfm minimum ventilation air.
-10°F winter ambient temperature.
75°F return-air temperature.
60°F DAT at minimum load, based on reset schedule.
325-mbh design capacity requirements.
Most commercial rooftop systems only offer one or two furnace sizes per unit, usually 400 mbh. Applied rooftop systems offer five to 10 sizes per unit, making it easier to select a properly sized furnace. Table 1 compares a 400-mbh furnace with a 3-1-turndown-ratio burner and a 320-mbh furnace with a 20-1-turndown-ratio burner at various outdoor-air temperatures.
In Table 1, a 20-1-turndown-ratio burner provides close temperature control, allowing a VAV unit to take advantage of a single unit-mounted furnace for main heating and MAT tempering. In contrast, a 3-1-turndown-ratio burner cannot provide the required ambient DAT between 0 and 30°F.
Cycling a burner between 33 percent and pilot light introduces large variations in the temperature supplied to VAV boxes, causing them to over-correct above or below the set point and leaving them unable to find equilibrium. This creates a system that is unacceptable unless heat sources are added to handle low outside-air temperatures and resulting MAT tempering. The system's performance is compromised, costs increase, and tenant satisfaction declines.
One-hundred-percent-outdoor-air systems require close heating control. Rooftop heating systems must provide heat that is indirectly proportional to the ambient temperature and may operate for hours with loads ranging from 0 to 100 percent. Proper DAT control demands a high-turndown, fully modulating heat source.
One-hundred-percent-outdoor-air systems generally temper outdoor air to a moderate temperature of about 60 to 75°F. If the design winter ambient temperature is -10°F, the design furnace-temperature rise is about 80°F. However, a 50 to 60°F ambient temperature causes many hours of light-load operation, and furnace-temperature rise must be tempered accordingly.
Table 2 includes gas-fired-heat specifications for a 100-percent-outdoor-air rooftop system supplying 15,000 cfm with a design DAT of 75°F and a design minimum outside-air temperature of -5°F. As Table 2 illustrates, the minimum controlled temperature rise for a 3-1-turndown-ratio burner is 28.9°F. However, if the outside-air temperature is 55°F, the DAT will cycle between 83.9°F (55°F + 28.9°F) when the burner is running and 59°F when the burner cycles to pilot light. From a control standpoint, this would be unacceptable. With a minimum controlled temperature rise of 4.3°F and continuous modulation for higher temperatures, the 20-1-turndown-ratio burner could maintain a design DAT of 75°F over the required modulating range without cycling.
Rooftop systems for VAV or 100-percent-outdoor-air applications may require heating systems with high turndown percentages to meet minimum ventilation requirements and design conditions. Properly matching furnace capacity to heat load also is critical. Oversized furnaces further restrict capacity modulation. This type of furnace is available on built-to-order applied rooftop systems. As a result, engineers can specify a customized system while controlling installation and operating costs.
A marketing manager for McQuay International, Skip (Henry) Ernst has a bachelor's degree in mechanical engineering from the University of Minnesota. A member of the American Society of Heating, Refrigerating and Air-Conditioning Engineers and an Air-Conditioning, Heating and Refrigerating Institute representative, he has 32 years of HVAC marketing experience, including 20 years specializing in applied packaged systems and air-handling units.