Because the fans in a parallel FPTU only operate in the heating and dead-band modes, the fan is usually sized for the airflow needed to meet the design heating load. The design heating loads shown in Table 4 were used to determine the design airflows in Table 5 for each of the zones.
Design Heating Loads for Each Zone and City City Design load, Btu/hr (kW) Space 1-1 Space 2-1 Space 3-1 Houston, TX 28,569 (8.37) 4723 (1.38) 28,891 (8.47) Phoenix, AZ 20,074 (5.88) 2630 (0.77) 20,387 (5.98) Chicago, IL 47,193 (13.83) 6311 (1.85) 47,879 (14.03) New York, NY 37,835 (11.09) 4931 (1.45) 38,105 (11.17) San Francisco, CA 19,413 (5.69) 2855 (0.84) 19,683 (5.77) City Design load, Btu/hr (kW) Space 4-1 Space 5-1 Houston, TX 4770 (1.40) 2557 (0.75) Phoenix, AZ 2651 (0.78) 2237 (0.66) Chicago, IL 6303 (1.85) 3732 (1.09) New York, NY 5152 (1.51) 2279 (0.67) San Francisco, CA 2906 (0.85) 2130 (0.62) Table 5.
These systems were designed to meet the design heating load using 140[degrees]F (60[degrees]C) supply water at 1.5 gpm (0.09 L/s).
For modulating, condensing boilers, design the system to meet the design heating load such that the return temperature does not exceed 13.0[degrees]F (54[degrees]C).
For centrifugal compressors, the minimum capacity may be only 40% to 50% of the design heating load
before the refrigerant flow may become unstable, otherwise known as "surge." The minimum capacity requires the return temperature of the HHW loop to be lower than 147[degrees]F (63.9[degrees]C) at the design flow of 2039 GPM (128.7 L/s) for the case-study chiller, with a leaving temperature of 155[degrees]F(68.3[degrees]C).
The ACCA Manual J design heating load
for the downstairs unit is about 21.0 kBtu/h (7.0 kW) at an OAT of 19[degrees]F (-7[degrees]C).
This is especially true for centrifugal compressors, where minimum capacity may be only 40% to 50% of the design heating load
, before the refrigerant flow may become unstable; this is known as "surge." Surge is not an issue for positive-displacement compressors; therefore, single-compressor screw heat pumps are able to unload to a minimum capacity of 25% to 35% of the design heating load
After assessing the impact of climate change on the durability and energy performance of one retrofitting strategy for the building envelope, the design heating load
was assessed based on the heat balance approach (McQuiston et al.
The fluid temperature drop across the heat pump at the design heating load
is 5[degrees]C (9[degrees]F).
The total annual design heating loads
are larger than the cooling loads by about 10%.
First, buildings in colder climates often have appreciably smaller design cooling loads than design heating loads
. Second, ASHP heating capacity and coefficient of performance (COP) decrease as the outdoor temperature, [T.sub.o], decreases because the temperature lift across the compressor increases.