Practical Airflow Considerations

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Practical Airflow Considerations

Postby CEO1 » Wed May 28, 2014 6:51 am

Discussions regarding "airflow" with AC's and heat pumps usually refer to airflow across the indoor coil...and more often than not, "low" airflow. The symptoms of low evaporator air are low(er) suction pressures/SST's and low superheat with fixed orifice systems and normal superheat with TXV systems. Very low airflow can produce SST's far enough below 32˚ to "frost" the coil.

Airflow volume across, or through, the indoor coil provides the necessary "heat of vaporization" for the the liquid refrigerant to boil (evaporate, vaporize) as it passes through the coil. When the amount of liquid entering the coil and the amount of air flowing through the coil are at the design quantities, the resulting vapor will produce the design pressure and saturated suction temperature (SST), and the liquid will completely vaporize at a location resulting in the design superheat value.

When/if the airflow (heat energy) is low, the liquid can't vaporize at the design rate...the rate of boiling (evaporating, vaporizing) slows down, so less vapor is produced, resulting in less compression and lower pressures. If the metering device is "fixed", the liquid boils away at a point farther along the evaporator circuit(s), resulting in the vapor having less time to warm up (superheat) , and lower superheat values.

Design evaporator SST's are usually in the 40˚-50˚ range, with the low end of the range more likely applicable to lower SEER equipment. The pics below show suction pressure/SST characteristics for some 10 and 13+ SEER Trane equipment.


Courtesy Trane

For a given set of indoor and outdoor conditions, the older R-22 equipment runs a few degrees lower evaporator temp than the 410A equipment. And needless to say, the suction pressure/SST value varies with indoor and outdoor conditions, with the design airflow. Low indoor temps have the same effect on evaporator conditions as reduced airflow.

But generally speaking, if you see SST's significantly less than 40˚, with low to normal superheat (depending on the metering device) at steady state conditions, there's an airflow issue.

Low airflow is most often due to dirty coils (some times dirty filters or blower wheels)...certainly if a system has been running without "low airflow issues" for a number of years, then suddenly shows symptoms of low air, you wouldn't normally blame the ductwork design.

Maybe more common than ductwork deficiencies are undersized return air filters...filters need to be sized for a maximum 300 ft/min velocity. That comes to about 200 square inches per ton (400 CFM)...In practical terms, that would dictate a 25" X 25" filter for a 3 ton system. And if the original duct design was based on the low resistance "bug catcher" type filters, and the HO has installed the far greater resistance "high efficiency" type filters, the undersized filter effects are exacerbated...

You can usually evaluate the duct system by measuring the "total external static pressure" (ESP). For a heat pump air handler, that would be the sum of the static pressures measured near the air handler inlet and outlet duct connections. If the total is greater than 0.5 inWC, the blower likely can't deliver the nominal 400 CFM per ton.

In the field, you will oftentimes find fixed orifice equipment with "normal" looking suction pressures/SST's and very low superheat values. If you measure the head pressure/subcooling, you'll likely see higher than "normal" values. What has happened in this case, is someone before you overcharged the system, raising the head pressure to "force" more liquid into the evaporator coil, raising the suction pressure to the normal SST value.

I've seen it a lot over the years...the appropriate fix would be an indoor coil cleaning, but overcharging is much simpler. On the other hand, some service people don't really understand airflow and pressures, so they just add Freon till the suction pressure is "normal"...but those same techs will try the same trick with TXV systems, where overcharge won't change the suction pressure. Then they simply conclude something else is wrong, because the "system won't take refrigerant". :roll:

Here are a couple of links, by an "accepted authority" supporting low airflow symptoms and causes:

Restricted Evaporators

Static Pressure

The point where my "philosophy" differs from many, is in going to all the trouble to measure and calculate airflow, when all the symptoms are telling me, the airflow is low...does it really matter how much low it is? You either got enough, or you don't... :| ..and if you don't, the realistic "next step" is to determine why. Confirming what I already know, is a waste of time.

I've got several "airflow measurement devices" I bought years ago to "measure" airflow, and now, they're all occupying space on a shelf somewhere. There are occasions where an airflow volume "number" is necessary, but not to make a diagnosis, on a system that once blew good air, and now doesn't. If I end up "measuring" anything, it's static pressures at different locations, attempting to determine the point of restriction.

One of the MFIC's...

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