I've followed other contractors onto several jobs this year where the system had poor cooling, high electric bills, and were running a higher-than-expected head pressure.
One thing all these systems had in common was that the “ugly” condenser unit was hidden inside an enclosure. It seems that many newer housing developments have it in their bylaws that the heat pump or air conditioning outdoor unit must be hidden from view. So the general contractor builds a wall all the way around it. Great fix, right? Sure, except for the simple fact that outdoor air can't get inside the enclosure, so the discharge air from the condensing unit re-circulates back through the condenser coils again.
There are other things that can cause condenser airflow to re-circulate. Placing a condenser under a building or deck is common practice at condominiums in my area. Dense bushes can have the same effect as a fence enclosure.
I went on a call recently when it was 95F outside. Inside the enclosure, next to the condenser, it was 115F. The system was running constantly, but it just couldn't keep up with the cooling load placed on it.
A check of the manufacturer's data shows what happened. The system was rated at 3 tons. At AHRI conditions (80F dry bulb, 67F wet bulb indoors and 95F dry bulb outdoors), the cooling capacity was 35,600 BTU. However, at 115F outdoors, the cooling capacity was only 31,200 BTU. That's a 12% capacity loss of 4,400 BTU, or more than one-third of a ton. Think of what that does to the electric bill, and how it affects comfort.
So what's the solution? The best thing would be to leave the outdoor unit out in the open. In areas that don't allow that, consider using a lattice enclosure with a high percentage of opening. Or, just block the side facing the street so that at least two sides are open. Other options are to leave a 6-in. gap at the bottom of he enclosure so air can get in all around, or make the enclosure significantly bigger than the outdoor unit.
Anything that restricts or recirculates airflow at the condenser reduces the performance of the system. Remember, air conditioning begins with air.
Kevin O'Neill, CM, is the co-owner of O'Neill-Bagwell Cooling & Heating, Myrtle Beach, S.C. He has 31 years experience in the HVAC service business, is a 24-year member of RSES, and was a finalist in the 2005 NATE Certified Technician Competition. Kevin can be reached at 843/385-2220; email firstname.lastname@example.org.