ACHRNEWS

For performance and comfort: Don’t ignore the indoor coils

April 19, 1999
New products boasting efficiencies in the 90% range and SEER ratings in the low- to mid-teens can make a family and its energy budget more comfortable.

But often ignored in the comfort equation is the role of the indoor coil. When properly maintained and matched to the outdoor system, an indoor coil will help enhance comfort and improve system performance.

While indoor (evaporator) coils don’t usually require annual maintenance, they do require periodic cleaning and inspection. Well-maintained coils help ensure that the system operates efficiently for maximum comfort. They also prolong the life of the system.

The frequency of servicing a unit can range from every two to five years, depending on where the unit is located. The overall upkeep of the unit will also determine the frequency. Chances are, if the homeowner doesn’t keep the rest of the system in good repair, the coils have been neglected as well.

It can be difficult to convince homeowners of the importance to service the coil simply because it’s not something they can see; nor are they generally aware of its existence. They may even be skeptical that maintenance is necessary at all.

Unfortunately, service technicians often don’t press the point, since coil cleaning is time-consuming, tedious work within the small confines of the coil housing.

Rather than backing off, however, service technicians should take the time to explain how routine maintenance today can help prevent costly problems in the future.

One hint: If the filter is dirty, use that as an icebreaker to talk to the homeowner about preventive maintenance.

Also, check the pressure at the outdoor condenser. Suction pressure will drop when the coil begins to ice, so a low pressure reading could be another indication of a dirty coil. This, again, can be used to demonstrate to the homeowner the importance of checking the coil.

On some coils, opening the front cover provides access to see the upstream side of the coil. Technicians might also need to remove a delta plate to access the inside of the coil, which is where dirt starts to build up. A look at one leg of the coil will quickly determine whether cleaning is necessary.

Dirty Coils

When a coil is dirty, it reduces the amount of airflow into the home. While some variable-speed furnaces automatically adjust to speed up the blower when an obstruction, such as dirt, is detected in the coil, most furnaces installed in homes today do not have this capability.

When they’re dirty, they end up operating with a lower-cfm airflow. This is called air starvation. The result is that some rooms are too warm in the summer and too cold in the winter.

A dirty coil can also damage other parts of the hvac system. A worst-case scenario is severe damage to the compressor.

Frost can develop when dirt restricts airflow. As frost develops, the refrigerant flowing through the coil gets increasingly colder. This propels a downward spiral in efficiency and reliability. Ice will form on the coil and further reduce the amount of airflow.

With the loss of airflow across the coil, the suction and head pressure begin to drop, and the refrigerant returns to the compressor in a liquid state. Typically, the valves will fail in units with a reciprocating compressor. In scroll compressors, oil is washed out of the bearings or the scroll plates flood, resulting in a failure.

How To Clean Them

Use a vacuum cleaner to clean a lightly soiled coil right in the casing. It’s fast and doesn’t require special equipment.

The best tool to reach back into the coil is a crevice or soft- bristle brush attachment. This provides concentrated suction and makes it easy to reach into corners. Care must be taken to not damage the coil fins with the vacuum attachments.

Another method is to use a chemical cleaner, which also doesn’t require removing the coil from its casing. Many chemical coil cleaners come with a wand to use in hard-to-reach areas. A nylon or other soft brush can remove the buildup in most situations. A soft brush won’t bend fins.

Sometimes a coil is so dirty, it’s necessary to remove it from the casing to ensure proper cleaning. There are spray-on cleaning chemicals for this specific use available through hvac parts distributors.

Some contractors use a garden hose to remove the chemicals and dirt. Caution must be used with this technique regarding refrigerant recovery and preventing water from getting into the coil.

Whenever chemicals are used, read the manufacturer’s instructions and abide by any cautions regarding use.

The Big Picture

While a clean coil is key, efficient operation may depend on other factors. Critical to system efficiency is proper coil selection. Relatedly, using an older-model coil can affect efficiency.

Coil design and technology have dramatically improved operating efficiency and capacity in the past 20 years. Homeowners using an older coil are not reaping these cost savings and comfort benefits.

One of the major improvements in today’s coils is the use of a piston or thermostatic expansion valve (TXV), which meters the rate of refrigerant entering the coil. Older models have a less-precise (and therefore less-efficient) method of adjusting refrigerant flow.

Newer indoor coils have a larger surface area, enhanced fin design, and grooved tubing. These features provide a larger area for heat transfer, improving efficiency and expanding capacity. Typical older coils may only have one-third to one-half the face area of these redesigned coils.

Some homeowners opt not to replace their indoor coil when they replace the outdoor unit. Serious complications can result if there’s a mismatch between the components.

A few years ago, engineers at Bryant Heating & Cooling Systems studied the effects of an improperly matched system. They took a high-efficiency coil dating back to the 1970s, cleaned it thoroughly, and ran it through a U.S. Department of Energy test procedure to measure coil efficiency. Then they compared it to a new, standard coil with the enhanced features. The results were dramatic.

The average drop in efficiency was 9.5%. That means a 10-SEER system would be operating at only 9 SEER. The average drop in capacity was 5.5%, or about 2,000 Btu on a nominal 3-ton system. This measurable loss in efficiency wastes precious energy dollars every day. And, with such a substantial capacity loss, the customer no doubt also feels less comfortable.

Properly pairing a coil with the outdoor unit is the best way to ensure that customers reap all the efficiency and capacity benefits your equipment can harness.

Not just any new coil will enhance a unit’s efficiency and provide greater comfort. A manufacturer may rate its condenser with its own indoor product, and also rate it with another manufacturer’s indoor coil.

But it may not be an apples-to-apples comparison when the ratings are the same, especially when both indoor coils are rated with the same outdoor product.

For example, take a 3-ton coil with 4 sq ft of area and match it with a 10-SEER outdoor air conditioner. It is rated 10.0 SEER as tested by the manufacturer, who makes both the indoor and outdoor coil.

Another manufacturer rates its coil with the same condenser as 10 SEER, but its coil has 22% less surface area. The 10-SEER rating may have been achieved by adjusting the airflow in a simulation. This same rating would not be achievable in actual use.

With replacements for R-22 now available, contractors must also make sure the coil is compatible with the refrigerant the unit was designed to use. Not all new coils work with the new refrigerants. If in doubt, check with the manufacturer.

The piston retainer must be made of specific materials that will handle all the refrigerants and oils. Having a retainer disintegrate because of use of the wrong refrigerant can result in leaks around the piston. Even worse, pieces of the material could float around the system and destroy the compressor.

Nelson McGuire is senior product manager for Bryant Heating & Cooling Systems.