HVAC Maintenance Recovery & Preventive Maintenance Lesson #30
Clean what’s dirty: condensing coils

By: Tom Olson - Tuesday, August 21, 2007
Source: Climate Makers, Inc.

It has come to our attention that air-cooled condenser coil cleaning was missed on our initial outline and so it was also missed in our lessons.  Given the timeliness of that discovery, we're going to interrupt your regularly scheduled lesson for this special lesson.  This information may help save your air-conditioning compressor this season.

In our lessons, "Cleaning what's dirty" takes a pretty high priority.  We put a great deal of effort into detailing how to pressure wash unit vent and air-handling unit heating and cooling coils.  But, the fact of the matter is, all of these coils have, or should have, some level of protection against getting dirty, with an air filtering system ahead of them.

All that happens with air-conditioning is that the indoor heat is transferred outside.  In 1973, the Educational Facilities Laboratories, in their book entitled The Economy of Energy Conservation in Educational Facilities, stated that a small layer of debris can affect the heat transfer efficiency by 25 percent.  Without any air filtering systems, a small layer of cottonwood lint, grass clippings, dandelion seeds, and other debris, can easily collect on air-cooled condenser coils in one cooling season.  If you're not currently cleaning your air-cooled condenser coils annually, we respectfully suggest that you're wasting an awful lot of energy. probably not providing the desired comfort on really hot days, and prematurely wearing out your compressors, well before their time.

There are two schools of thought when it comes to cleaning air cooled condensers:
1. Use compressed air or CO2
2. Use high pressure spray washing

Generally speaking, only the smallest of air-cooled condensers should ever be considered for cleaning with compressed CO2.  And that only applies to those units that are too remote to be easily cleaned by other methods.  To do a thorough job, it simply takes too many canisters of CO2.  As with all air-cooled condenser coil cleaning tasks, the debris is best eliminated by blowing, or washing, in the opposite direction (counterflow) from which it entered the coil.  Don't even think about doing it any other way.

Whether by air, CO2 or water, care must be taken to blow (wash) perpendicular to the coil.  If you don't, coils can easily be flattened by any of these methods, decreasing the heat transfer efficiency of the coil, rather than increasing it.

For many of the condenser coils we are responsible for keeping clean, compressed air is the preferred method.  It's quick and pretty thorough.  Setup is the biggest deterrent and you either need a few really big units to make it worth while, or lots and lots of smaller units.  First thing in the spring, and/or at the end of the cottonwood lint season, which is right now, we frequently rent a 100 h.p. air compressor for this task and just move it from one building to the next.

There are three specific instances when cleaning with compressed air is not the preferred method.  The first is if your condenser coils are so plugged that even lots of high velocity air can't penetrate the debris.  You'll sometimes get some debris out with compressed air, and that will definitely speed the process, but these coils often take some sort of chemicals to loosen and emulsify the debris.

The second condenser coil type for which compressed air isn't very effective is what's called split condensing coils.  This is where one coil is applied over another coil, spooning the first coil, if you will, creating a pocket between the two coils.  You may even find that the outside coil doesn't look at all dirty, but the layer of debris between the coils affects the air flow, thus the heat transfer efficiency, through both coils.

To gain access to these coils, the entire top of the condensing coil, which includes the condenser fan, has to be removed.  Then, the two condensing coils need to be carefully separated, first pressure washing the outside coil--again, always counterflow to the normal direction of the airflow--then the inside coil.  You don't want debris from the second coil spraying on the coil that has already been cleaned.  You will most likely need to use a coil cleaning agent with this process.

Another type of coil that can be tricky to clean is the lanced fin coil.  This is, generally, a single row coil, but it has little serrated cuts in the aluminum fin.  While this increases the coil heat transfer efficiency, it creates little edges that like to grab and hold cottonwood lint and other debris.  It is best, if possible, to always use compressed air first, and then use a chemical agent, if required, to get the remaining debris.

If the surface of the coils looks clean, inspect more than just the surface.  The outside does not always tell the entire story.  With a small pocket screw driver, CAREFULLY open a small area of fin to get a better view between the fins.  If you see any debris inside, it should be cleaned.  Then, carefully bend the fin(s) back in place.

A good indicator of dirty coils is high suction on the unit service door, if there is one.  If you cannot readily remove the compressor access door, with the condenser fan operating, there is a good chance the condenser coil is plugged.

Another good indicator of a dirty coil, on a vertical discharge condenser fan, is to put your hand directly over the top of the fan.  If the discharge seems minimal, or even downward into the top of the fan, try bringing your hand up the side of the condenser.  As your hand clears the top of the condenser, if there seems to be an almost sideways discharge of air off the top of the condenser, the coil is likely plugged.

If all other inspection methods fail, just know that in Minnesota and Wisconsin, every air-cooled condenser coil requires ANNUAL cleaning!  Keeping systems clean, on a regular basis, requires much less work than recovering from years of neglect.

Finally, you just can't be in a hurry cleaning air-cooled condenser coils.  I can't tell you the number of times a new mechanic has called in, stating that he had just washed the condenser coil, but that the head pressure on the unit was still too high.  Told to wash it again, he calls a second time to state that the system head pressure is still too high.  Finally, after about the fourth washing and rinsing, with a reduced system head pressure, the now amazed new mechanic calls in to tell the service manager that he wouldn't believe how dirty that coil was.  Oh yes he would!  Remember, he was the guy that said to keep washing the condenser coil until the head pressure finally comes down.

Dirty condensing coils equals high head pressure.  High head pressure equals more work being done.  More work being done equals higher cost of operation and longer equipment run times.  Longer equipment run times equals shorter equipment life.  Shorter equipment life equals more frequent replacement costs. It's a vicious cycle.

Remember, 75 percent of all HVAC service is simply keeping systems clean, dry and oil-free.  No one can perform those services more cost-effectively than in-house personnel.  But occasionally, even the best in-house personnel need help on some of these, as well as more technical tasks.  When that happens, don’t hesitate to call in a professional.  This is not the place to be pinching pennies.

This HVAC Maintenance Recovery Lesson is provided to you, compliments of
Climate Makers, Inc.  Climate Makers is a HVAC, temperature control and building automation system contractor, specializing in energy optimization, comfort improvement and indoor air quality.  Since 1978, the staff at Climate Makers has been sharing their 'back-to-basic' ideas for recovery from years of under-funded maintenance budgets, throughout Minnesota and Northwest Wisconsin.


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