Most residential air conditioning technicians don't understand the impact airflow and ductwork have on the performance of an HVAC system. It is often overlooked, when troubleshooting a system with a poor cooling capacity and especially when installing the newer high efficiency variable speed heating and air conditioning systems. Poor cooling is often blamed on the equipment and its size alone when in fact, more often it is restrictive or poorly designed ductwork that's the problem. A basic understanding of ductwork design will go a long way to broaden your HVAC skills.
The first thing you must know is that airflow directly impacts the BTU output of an air conditioner. Below is the formula for calculating sensible heat removal:
Formula
BTU = 1.08 x CFM x Delta 'T'
Let's take a look at how CFM (airflow) impacts system capacity on a 4 ton AC system:
Duct Pressure Units
The air pressure in an residential HVAC system is quite small, usually less than .025 psi. So, duct system pressure is measured in inches of water column (ICW). For a reference 27.7 IWC = 1 psi.
There a two pieces to the ductwork puzzle, the blower and the duct and its components.
The Blower
In this chart, the top row represents the ductwork resistance in IWC and the side columns represent the CFM output of the blower. Looking at model ARUF60D14, medium speed, with .4 IWC, the output is 1800 CFM. The ONLY way to change the CFM in an HVAC system is change the blower speed or change the ductwork design to lower the the resistance or static pressure.
Increasing the blower speed does not overcome poor ductwork because static pressure increases as airflow increases. For example in the chart above, you could increase the blower speed to high but, if the static pressure increases to .6 IWC, the CFM output is actually reduced.
This is part 1 of a multi-part series on residential duct systems. If you would like to learn more about residential duct systems becoming an HVAC technician or are interested in National Association of Technical Excellence (NATE) CEUs, please contact us.
by Ron Walker
The first thing you must know is that airflow directly impacts the BTU output of an air conditioner. Below is the formula for calculating sensible heat removal:
Formula
BTU = 1.08 x CFM x Delta 'T'
Let's take a look at how CFM (airflow) impacts system capacity on a 4 ton AC system:
- CFM - 1600 CFM (400 CFM per ton)
- Delta 'T' - 19.5F (an average temperature split)
- 1.08 x 1600 x 19.5 = 38,880 BTU (Sensible)
- CFM - 1400 CFM (400 CFM per ton)
- Delta 'T' - 19.5F (an average temperature split)
- 1.08 x 1400 x 19.5 = 29,484 BTU (Sensible)
Duct Pressure Units
The air pressure in an residential HVAC system is quite small, usually less than .025 psi. So, duct system pressure is measured in inches of water column (ICW). For a reference 27.7 IWC = 1 psi.
There a two pieces to the ductwork puzzle, the blower and the duct and its components.
The Blower
- Output is measured in CFM
- How much CFM depends on external resistance (ductwork)
- Blower data tables tell us how much CFM a blower can put out against a certain amont of resistance
- Blower curve (not often used) also shows CFM
In this chart, the top row represents the ductwork resistance in IWC and the side columns represent the CFM output of the blower. Looking at model ARUF60D14, medium speed, with .4 IWC, the output is 1800 CFM. The ONLY way to change the CFM in an HVAC system is change the blower speed or change the ductwork design to lower the the resistance or static pressure.
Increasing the blower speed does not overcome poor ductwork because static pressure increases as airflow increases. For example in the chart above, you could increase the blower speed to high but, if the static pressure increases to .6 IWC, the CFM output is actually reduced.
This is part 1 of a multi-part series on residential duct systems. If you would like to learn more about residential duct systems becoming an HVAC technician or are interested in National Association of Technical Excellence (NATE) CEUs, please contact us.
by Ron Walker