Airflow is Important

I have heard many technicians and contractors in Florida tell customers that it is normal to find suspected mold (I can’t say mold without laboratory testing) on their air conditioning equipment and ductwork. This morning I was called to consult on another such system and tracked down the actual cause of this growth as well as several other associated problems.

This system is serviced on a yearly basis by a licensed contractor. Every year the thermostat has been replaced as the technicians note that the thermostat is not working properly and not maintaining temperature. The customer changes his own filters every three months.

System Information:

Air Handler: 36,000 BTU/Hr (3 Ton)
Heat Pump: 30,000 BTU/Hr (2.5 Ton)

(Note, in this case the mismatch in size is OK as it is per manufacture specifications for efficiency)

Outside Unit Inspection:

Found an older heat pump unit that needs to be cleaned. The fan motor is drawing 3.5 amps with a rating of 3.2. Found the capacitor was weak and only measuring out to 2.2 mfd rather than the full 5 mfd that it is rated. Fan motor felt hot to the touch. Checked refrigerant operating pressures and they were within range for a cooler day. Captured gauge readings. Checked resistance on all compressor winding connections as well as to ground. Compressor ohmed out OK, no short to ground. Compressor amperage is 8.8 and it is under rating of 13.5. Compressor capacitor checks out normal. Contactor has burnt points and insect/reptile parts stuck to it. Condenser fan protector has a deceased frog in one corner.









Outdoor Unit Recommendation:

Replace the 45/5mfd combo capacitor and based on age of unit consider replacing the outdoor fan motor. The capacitor should solve the problem but the motor has been running with bad capacitor and it felt extremely hot to the touch. Possible winding damage.

Outdoor Unit Pictures:































Air Handler and Plenum Inspection:

Upon arrival inspected air handler. Found significant possible biological growth (needs tested but strong possibility of mold) on air handler and supply plenum. Air handler is in interior hallway closet and in conditioned space.














Return plenum is under air handler with single 12 inch round to return grill in living room under thermostat.  Checked filter and found extremely dirty filter. Customer had a new one in the garage and I changed it for him.










Condensate line is incorrect. Per manufacture specifications there should be a trap as close to unit as possible. As this would cause a double trap possibility with condensate line running under floor, there will need to be a vent installed after the trap. As this air handler is inside conditioned space and the possibility of water damage exists there should be an overflow cuttoff switch on the condensate system.










Opened air handler and inspected blower motor and electrical components. Blower motor winding ohmed out properly and no short to ground. Capacitor checked out per specifications. Noticed growth on blower motor and interior of electrical compartment and blower motor housing. Blower wheel needs to be cleaned. Dust and dirt buildup on wheel. Inspected indoor coil and found normal dirt/dust buildup but airflow is still OK. Does not need to be removed to be cleaned at this time. Checked blower amperage and it is normal.


















Reassembled air handler, put covers back on and started unit to check temperatures and pressures. After ten minutes of run the return air temp was 76.1  degrees F with a 59.6% relative humidity and a wet bulb temperature of 66.1. The supply temperature was 70.8 degrees.  This is a temperature split of 6 degrees. Far too low. Checked the supply static pressure and found .01 inch water column. Checked the return static pressure right behind the filter and found .83 inch water column. Add both absolute values together for a total static pressure of .84 inches water column. Rated static pressure per the nameplate is .50 inches water column. This should be split relatively equally between supply and return.









Based on the static pressure differential and sound the unit was making decided to verify airflow at the return. Took two readings, one with filter, and the other without filter using a CPS flow hood and software. With the filter we had an average of 612CFM of air for 2 minutes of testing. Removed the filter and repeated test and achieved an average of 728CFM of air for 2 minutes of testing.
















A three ton air conditioning coil requires 1200CFM of airflow across the coil to operate properly (400 CFM per ton). In this case the lack of return air flow caused extremely high static pressures in the return duct and across the coil that was on the negative side of the system (before blower). This in turn pulled air across the coil far to fast to cool and dehumidify the air (low temp split and higher humidity). This fast moving air also pulled all of the moisture off of the coil, and because of the open pipe to the drain pan also pulled water out of the drain pan. As there was not enough pressure on supply side of the blower the warmer, moist air remained far to long in the ductboard supply plenum which absorbed moisture and became a growth point for biological growth.  The problem with this system originates with an undersized return duct. The outdoor unit problems are wear and tear.

Repairs Needed:

1) Replace the supply plenum ductwork and re-seal to supply duct and top of air handler.
2) Completely clean the air handler including blower wheel.
3) Replace the combo capacitor in outdoor unit and to be totally safe replace the outdoor fan motor.
4) Replace the contactor in the outdoor unit.
5) Clean the outdoor unit and remove dead frog and reptiles.
6) Install second return grill with a minimum of 12″ round duct on other side of wall near dining room.
7) Test and balance system.
8) Install proper trap and vent on condensate line. Put emergency shutoff switch into overflow.