Cleaning the condenser coil. This has always been the number one task that Service Technicians avoid at all costs when maintaining residential equipment. Maybe it is the nature of the fast paced business when the weather gets hot? Certainly it could not be the lack of water to rinse the coil as there is almost always is a water spigot on either side of the home. I cannot figure out why, and this may be one of the largest aspects of seasonal maintenance that impacts the efficiency of the unit. Sure, the system will still operate if not cleaned, for a while. Lets explore just how the capacity and EER are impacted...

A/C Expanded Cooling Data

For an air-conditioner operating at design load, lets estimate 85F for simplicity, the condenser can deliver close to the nominal capacity. Remember, nominal capacity is in tons, from 1.5 to 5 tons, in half-ton increments for residential equipment. There are 12k btu/hr per ton. In my example equipment, the unit is 36K btu/hr or a nominal 3 tons.

• At design operating conditions, my unit will deliver 35,400 Btu/hr - which is very close to the nominal rating.

When a condenser becomes dirty or fouled, the unit cannot easily reject heat from the refrigerant. This in turn drives up your liquid line pressure and the system artificially operates at what can be determined a higher outdoor ambient than actual.

• For a R-410A system with a dirty condenser, the liquid line pressure could be as much as 60# higher than normal. This equates to approximately 20F. So, instead of rejecting heat and operating at a capacity for 85F, the condenser think it is more like 105F. This then reduces the capacity of the system by 3,700 btu/hr.

Guess, what? The condenser never gets cleaner without someone doing so! Therefor, the capacity losses only get greater throughout the season.

• To put this in perspective regarding efficiency, the system was operating at an EER of 13.8. With the dirty coil, the system is operating at 10.1 EER!

• When I said you were losing 3,700 btu/hr you probably thought, So What! But at 105F, the condenser has to work much harder to deliver less capacity!

Heat Pump Heating Data

For a heat pump, the penalty on your electric bill might be more severe - particularly in a cold climate like New England. As the condenser coil gets dirty, a heat pump operating in heating mode will begin to decrease in capacity just like an A/C.

• In my example data for a comparable heat pump in capacity to the A/C, the unit will deliver close to the nominal rating at 47F (35,400 Btu/hr).
• If the coil is dirty, it is not out of the realm of possibility to have a system operating 17F less than the actual outdoor ambient.
• In this example, that drops our capacity down to 27,500 Btu/hr, a loss of 7,900 Btu/hr.
• More important, the decrease in EER from 13.6 down to 11.3.
• What this does not take into account is the increase in defrost cycles and use of supplemental heat since the heat pump will not be able to keep up in just the mildest of weather!

One phenomenon I have yet to fully understand is the Heat Pump Mini-split. Don't get me wrong, I have installed tons of these systems over the past 15 years. But, these heat pumps tends to keep themselves clean in New England! I think it has to do with the micro channel coils and long heating seasons; they almost wash themselves off every Winter, and stay clean enough through the Summer.