TROUBLESHOOTING HEAT PUMP SYSTEMS / RECOGNIZING LOW AIRFLOW

One of the more common problems found in service work is low airflow across the indoor coil. This situation can be due to coil restriction, inadequate/damaged ductwork or dirty filters to name a few. Much has been written in regards to airflow and it is a critical element of heat pump operation and performance. The design value for indoor airflow with A/C’s and heat pumps has, to the best of my knowledge, always been 350-450 CFM per ton. Values less than 350 generally create operational problems like coil frosting in the cool cycle or high head pressure in the heat cycle. Values greater than 450, though the lesser of two evils, would produce situations better described as performance problems…poor moisture removal in the cool cycle, or low discharge temperatures in the heat cycle. But, with typical residential systems, excessive airflow is rarely a problem, simply because residential duct systems are rarely oversized. So, the usual situation becomes one of recognizing and correcting low system airflow.

All residential equipment, be it heat pump air handlers, gas furnaces or packaged units, is limited in its capacity to move air and the limiting factor is system external static pressure. External static pressure is for all practical purposes, a measurement of the resistance encountered by the air as it moves through the duct/air distribution system. The standard for maximum ESP is about 0.5 inWC, and residential duct systems have to be designed and sized, to meet this capability of the blower. Otherwise, the ESP may exceed 0.5 and the airflow per ton will be less than 350 CFM…

With no practical understanding of system operation, determining airflow volume would require taking some kind of measurements and doing some calculations that eventually provide a CFM value. Or, if a specific value of airflow is required, so will be the measurements and calculations. But for me, most of the time, the issue comes down to either having enough airflow, or not. And by “enough” I simply mean, the system will run 24 hours without frosting the indoor coil in the cool cycle, or producing excessive head pressures in the heat cycle. Now, if you’re attempting to evaluate overall system performance and efficiency, which depends to a great extent on airflow, “enough” probably isn’t adequate. But keep in mind, service calls are usually situations where the homeowner was content with the system performance yesterday, but not today. So, I always start with the assumption the airflow has at some point in time, been satisfactory, adequate or “good enough”…and most of the systems I find fall into this category. All I need to do is return the system airflow volume back to or near, whatever it was on Day 1, regardless what that value actually is. And that usually amounts to changing a filter, or cleaning the coil.

You don’t have to calculate airflow to determine there isn’t enough. If, in the cool cycle, the suction pressure is low and the superheat is low for fixed orifice systems, or normal for TXV systems, the airflow is low. If, in the heat cycle, the head pressure is high, the cause is either overcharge or low airflow, or both…quite often, techs will overcharge a fixed orifice system in the cool cycle to correct low suction pressure resulting from a dirty indoor coil. But you can ask a few questions to determine if that’s the case. Common sense dictates indoor coils will restrict over time, due to a variety of possible reasons. So when you see symptoms of low airflow, that’s the first thing to suspect. Occasionally you get lucky and find a dirty filter, but more often than not, the coil is the culprit.

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