CFM and Enthalpy

CFM and Enthalpy

CFM

stands for Cubic Feet per Minute, and is a measurement of how much air the blower moves through the duct system.

How to measure CFM

• Turn the system to heating or cooling

• Measure the total static pressure of the duct system

• Determine the fan speed setting

• Consult the cfm table for the blower using the fan speed and the total static pressure

Measuring Static Pressure

When measuring total static pressure, it is important to measure immediately before and after the blower.

To do this, first use a manometer with a pitot tube to measure the duct pressure between the filter and the furnace cabinet for the total return static pressure. This will be a negative pressure, but ignore the negative for now.

Secondly, measure the duct pressure through the furnace high limit for the total supply static pressure.

Add the two values together for the total static pressure.

Enthalpy

is a measurement of heat in a system. It includes internal energy, and include pressure and volume.

In HVAC terms, it is the total of both the latent and sensible heat of the system.

Wet Bulb to Enthalpy Conversion Chart

How to Measure Enthalpy

• Measure and record the wet bulb temperature of the supply and the return

• Consult the wet bulb to enthalpy conversion table and record the enthalpy values for the supply and return.

• Subtract the return enthalpy from the supply enthalpy. We will use ΔH to represent the difference in enthalpy.

Time to put it all together...

 $BTUH\ \left(total\right)=CFM\times4.5\times\Delta H$  

To calculate the total heat being removed by the AC, multiply the CFM by 4.5 and the difference in enthalpy between the supply and the return.

Compare the result to the nominal capacity of the unit to determine the 

Open Ended

We have a low cool call on a 3 ton system that was recently installed. The customer states that the system doesn't seem to be keeping up during 80 degree weather. Upon arrival, we determine that the filter is clean and the system seems to be running properly. The furnace is a Goodman model GMES80804B. With the system set for cooling, we find the following:

The cooling dip switches are set to off-on-off.

The return static measures -0.32 inWC and the supply static measures 0.51 inWC.

The supply wet bulb is 68.3 and the return wet bulb is 60.1.

How many BTU's is the system removing?

What can we determine using this information?

How did we do?

What is the diagnosis? Why?

Open Ended

We have a low cool call on a 3 1/2 ton system that was recently installed. The customer states that the system doesn't seem to be keeping up during 80 degree weather. Upon arrival, we determine that the filter is clean and the system seems to be running properly. The furnace is a Lennox model ML180UH110E60C. With the system set for cooling, we find the following:

The cooling speed is set to the yellow wire.

The return static measures -0.36 inWC and the supply static measures 0.74 inWC.

The supply wet bulb is 69.1 and the return wet bulb is 63.2.

How many BTU's is the system removing?

What can we determine using this information?