Indoor Coil Leaks

With A/C season officially here, I have been asked to address an issue that has been around our industry for a long time now and that is indoor coil leaks also known as formicary corrosion.

Indoor coil corrosion failures are an issue in the HVAC industry today. Although the occurrence rate of these failures is low nationwide, some geographic areas have experienced higher incidence rates. For instance, some homes experience multiple failures while those around them have none. Failures are typically characterized by leaks that form in the fin pack area of the coil after one to four years of installation and use. This issue exists industry-wide. A competitive study has shown identical corrosion failure leaks in all coil brands investigated. (I have attached an industry study paper that Carrier put out that goes into great detail on this industry wide problem. There is a  lot of great information in this paper and some excellent pictures showing microscopic views of the various types of corrosion).

There are two main forms of pitting corrosion found in indoor coils: (1) general pitting; and (2) formicary corrosion, sometimes called “ant’s nest” corrosion.

(1) General pitting corrosion is caused by aggressive anion attack on the copper tube. An anion is a negatively charged chemical species. Due to this negative charge, anions aggressively search for positively charged species called cations. Copper is an abundant source of cations.Large pits resembling bite marks characterize the footprint of general pitting. These pits can often be observed with the human eye. Chlorides are the most common source of the aggressive anions known to cause general pitting corrosion.

Common household substances that may contain chlorides include:
• Aerosol sprays                                             • Carpeting
• Degreasing and detergent cleaners         • Dishwasher detergents
• Laundry bleach                                            • Fabric softeners
• Paint removers                                            • Tub and tile cleaners
• Vinyl fabrics                                                 • Vinyl flooring

(2Formicary corrosion, on the other hand, appears as multiple tiny pinhole leaks at the surface of the copper tube that are not visible to the human eye. Upon microscopic examination, the formicary corrosion pits show networks of interconnecting tunnels through the copper wall, hence the association with ants’ nests. The agents of attack involved in this corrosion mechanism are organic acids.

There are three conditions required for formicary corrosion to occur:
• The presence of oxygen
• The presence of a chemically corrosive agent (organic acid)
• The presence of moisture.

There is increasing evidence linking the primary cause of indoor coil leak failures to agents present in the household environment. Significant levels of corrosive agents known to cause these failures have been quantified in indoor condensate sampling during studies. (see list in attached paper) The trend toward decreased home ventilation rates likely contributes to the elevated levels of indoor contaminants. In addition, increased environmental awareness to identify and fix refrigerant leaks will continue to focus attention on these indoor coil failures as an industry issue.

So why didn’t it occur long ago and what is being done to address this issue? Both are very good questions.

So why didn’t this occur long ago? In an effort to increase efficiency of air conditioning systems, the heat transfer capabilities of the coils needed to be increased. To accomplish this, thinner walled copper tubing is now used and that tubing has rifling (grooves) inside the tubing which also decreases the wall thickness of the copper tubing. In much older systems, the copper tubing had thicker walls but the corrosion was still taking place — it just took a lot longer for it to occur and cause leaks like we now see  occurring in  the short time with present day coils.

When the coil leaks in the fin pack, the only solution is to replace the coil. But putting in another “copper tube” coil will eventually fail again depending on the “indoor environment”. To address this, manufacturers are using both “tinned” copper tubing in the coils or have gone to or will be going to all aluminum coils. Since aluminum and tinned coils do not have the anions available for the cations to attack they should resist the general pitting/formicary corrosion.The other issue that will help is providing more air changes to the occupied space.

Feel free to use this post to help explain it to your customers when you encounter the indoor coil leak.

Thank you to Carrier Corp for the information in the attached paper.



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Air Conditioning and Humidity Control — Comments from Readers

This was fast – already have had a few suggestions and responses. Thank you to the responders.

One thing I did not go into was “controls” other than the economizer and this is what the responders all made suggestions about.

What could happen if a single stage thermostat was used on on 2-stage unit? Chances are, even though the unit WAS properly sized but now, because it only runs at full capacity with the single stage stat, there are times it is oversized causing it to short cycle and not properly dehumidify.

Or, being a bank, (they do keep them pretty cold), is it short cycling due to low loads at times. They have it set for a certain people load and now there is only 1 or 2 or no customers in the bank.  Maybe they need hot gas bypass or a RAWAL Valve (see post on HGBP).

So many things can cause a problem only because someone didn’t properly install a system or look at the complete application.



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Air Conditioning and Humidity Control

I recently received this question from one of this site’s readers:

Is it a good or bad idea to leave the ahu fan on in a commercial system?This facility is a bank and is experiencing a higher than normal moisture content. The system is a 15 ton Trane and it is a matched system.”

As we all were taught, one of the primary purposes of any air conditioner is dehumidification. So, why isn’t this system controlling the humidity in the space?

The writer says it is a “matched system” which always helps but the question that may need to be addressed is possibly the sizing of the system.  An oversized air conditioner will short cycle and will not run long enough to do proper dehumidification. The longer a cycle lasts, the more humidity the system will remove.

He asked about running a continuous fan? Most commercial applications and codes require a certain amount of air changes and, for the most part, run continuous fans during occupied periods. If the fan is moving too much air, the air does not stay on the coil long enough to dehumidify. Being a commercial system, the TXV will try  to regulate the superheat to prevent other problems which could eventually lead to compressor failures (see post on Discharge Temp as a Diagnostic).

What could be a possible factor here is whether or not the AHU is equipped with an economizer or ERV. If the unit is equipped with either of these, they might not be properly set up or not functioning properly and allowing humid air from outside into the space. The continuous fan would only compound this situation if that is the case.

Lastly, it could be an IAQ (indoor air quality) issue. If there is not proper amount of outside air allowed into the space? They could be experiencing “sick building syndrome” where the CO2 level and humidity in the space is not controlled.  This goes back to proper air changes in the space.

In my response, these were the suggestions I made for the tech to check at the site.  As we all learned — a properly sized air conditioner will be a dehumidifier. When it isn’t, then other things need to be looked at.

We all need to remember that an air conditioner is a SYSTEM and we need to look at all possible causes for a problem. I also appreciate the fact the the reader was looking for suggestions. That is the sign of a good tech — ask when you are not sure!

Please let me know if you have other possible ideas and I will pass them on to the tech who asked the question.

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Maintaining Equipment Efficiency

I participate in a blog called HVAC SERVICE SELLING  and this was posted on the site by Robert Baker who does commercial heating and cooling.  I felt this tied in nicely with my “Spring Cleaning” series of posts and thought I would pass it on to everyone.

“Do you take your car to a mechanic for preventive maintenance? Sure you do. Why? Well so it’s safe to use, runs to its full potential, saves fuel, prevents costly repairs, and extends the life of the vehicle.”

“The air conditioning and heating in your home or building also needs preventive maintenance; just like your car, for the same benefits I listed. Replacing your equipment and major repairs are costly and improper maintenance may increase your utility bills. Electricity fuels your equipment, so this spring call a licensed professional to make sure your equipment is running at its full efficiency.”

The day of the “do it yourselfers”  is gone.  “Beer can cold and sweating”, outdoor ambient +30, and all other ‘rules of thumb’ for charging can no longer be tolerated with the high efficiency equipment of today. Proper diagnostics need to be performed to keep equipment functioning properly.  Using “matching and AHRI rated” components is critical to proper efficiency.

So  — Robert said this very well and I really feel this is something that you, as professionals, should be passing on to ‘end users’. I alway preach about you being properly trained and now is the time to properly train your customers.  Feel free to pass this on or use it if you feel it will help your business.

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Compressor Electrical Diagnostics — Revisited

In reviewing posts from time to time that have the most views i feel it is a good thing to bring back a post that was previously written.  The following was originally written back in 2011 but there have been a lot of views so here it is again.  Hopefully, this will remind you of some basic checks before we really get into the air conditioning season. Keep in mind that this is true of all electric motors and compressors.

A compressor is an electric motor.  Since it is a motor, it has windings just like any other motor.  These windings can be checked for shorts, grounds, or opens.

To check compressor windings, first shut off the power to the unit.  Next, go to the compressor electrical connection box on the compressor and open the cover. At this point you will see the terminations of the windings where they enter the compressor.  Whether the unit is single phase or three phase, there should be three terminals from inside the compressor. These terminals will be marked (C) common, (S) start, and (R) run on a single-phase unit or T1, T2, and T3 on a three-phase unit.  On the single-phase unit it is important to note which wire went to which terminal so they can be properly replaced.  On a three phase reciprocating compressor, it does not matter which wire goes to which terminal.

CAUTION: On a three-phase SCROLL compressor, the rotation must be checked to verify proper rotation when finished with electrical checks.

To properly check the windings, the wires must be removed from these terminals.


With the wires removed, take an ohmmeter and set it on the R X 1000 scale. Take one probe from the meter and find a good ground. (You may need to scratch the paint on the compressor or scrape the copper tubing at the compressor to assure a good connection).  Take the other probe and touch it to EACH terminal in the compressor.  ANY reading to ground from any terminal indicates a short to ground and the compressor must be replaced.


Next we want to check for an open winding in the compressor.  Again, with your ohmmeter set on R X 1000, take the probes and go between pairs of terminals. If an “infinite” reading is obtained between pairs of terminals, there is an open winding in the compressor.  This does not necessarily mean the compressor is bad.  Compressors have internal overloads that open due to temperature or high amperage.  Feel the compressor.  If it is hot, chances are an internal overload may be open.  Note:  on a single-phase compressor, if the open is read between the start and run winding, the compressor is bad and needs to be replaced.  If the compressor is hot and an open winding was found, it may take an hour or two for the compressor to cool down and the overload to reset.

While waiting for the overload to reset, check the contactor for pitted or worn points.  Check the capacitor on a single-phase unit and make sure it is good. Check all wiring connections for loose terminations.  Check the condenser coil to make sure it is clean.  Verify that the unit has proper voltage present at the disconnect.  All of these things can cause the internal overloads to trip.


With an ohmmeter set on the R X 1 scale, we can check the windings for internal shorts. On a single-phase compressor, the windings should always “add-up” by pairs.  What this means is when reading the resistance between windings on a single phase compressor, Common (C) to Start (S) plus Common (C) to Run (R) should always equal Start (S) to Run (R).  (C-S) + (C-R) = (S-R).  Common to run should be the lowest reading.  Common to start should be the mid-range reading. Start to run should be the highest reading.  Example: (C-S)= 3 ohms and (C-R)= 1 ohm then (S-R) should = 4 ohms.  If these readings cannot be obtained, chances are there is an internal short in the windings of the compressor

On three-phase compressors, all the windings should read the same. Again, if there is a variance in the readings, there could be an internal short in the windings.

Since these checks are electrical checks of the MOTOR of the compressor, it does not matter if it is a reciprocating, scroll, hermetic or semi-hermetic compressor.  All the windings checks, for opens, shorts, and grounds are performed the same.

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SUPER-HEAT as a Diagnostic

Whether you are working on a 1½ ton unit or a 50-ton unit, the principles of the refrigeration cycle is the same.  The most important thing to remember about compressors is that they are VAPOR PUMPS. Compressors do not like liquid in them.  Liquid will damage valves, wash out oil leading to bearing failure, and break down the windings of a compressor.  The suction line should contain vapor and the discharge line puts out vapor. This is accomplished through super-heat.

Let’s look at a normally operating refrigeration cycle.  By understanding what is normal, it is easier to detect when there is a problem.

When a compressor is energized, refrigerant in the form of vapor is brought to the intake valves of the compressor through the suction line.  The compressor actually compresses this vapor in the cylinders and releases it out the discharge valves of the compressor into the discharge line.  This is hot, high-pressure vapor.  This vapor then enters the condenser coil where heat is removed causing the vapor to condense to a liquid.  Near the end of the condenser coil, there are always extra passes of tubing through the coil.  This serves a very important function.  When you have your gauges attached to the unit and are reading the liquid pressure, you are reading the saturated liquid pressure of the condenser coil.  This point occurs before the liquid leaves the coil.  The extra passes are in the coil to take the saturated liquid and sub-cool that liquid.  This is essential to the proper operation of the metering device since it relies on a continuous column of liquid at the metering device to function properly.  Normal sub-cooling on today’s equipment is between 10 and 15 degrees of required sub cooling.

The sub-cooled liquid leaves the condensing unit and is pumped through the liquid line as high-pressure liquid until it reaches the metering device.  The metering device can be an expansion valve, capillary tube, or an orifice.  If there is not sufficient sub-cooling of the liquid, the liquid can “flash” off in the liquid line en-route to the metering device.  When this happens, the metering device cannot properly control the flow of refrigerant to the coil causing improper cooling or reduced capacity.  When there is sufficient sub-cooled liquid present at the metering device, the metering device takes the high pressure liquid, forces it through the device, and releases it as a low pressure liquid.  In this state, the refrigerant is now capable of “absorbing” heat in the evaporator coil thereby, cooling the air.

As this low-pressure liquid passes through the evaporator coil, it again changes state.  The evaporator coil does just what its name says – it evaporates the liquid and turns it back into a vapor by absorbing heat from the air passing over the coil.  Just as there were extra passes in the condenser coil, there are extra passes in the evaporator coil.  Again, with your gauges attached to the unit you read the suction pressure.  This is the saturated vapor pressure in the evaporator coil.  The extra passes in the coil super-heat the vapor assuring that the compressor receives only vapor at the suction valves.  This super-heated vapor then returns to the compressor and the cycle starts over again.

So, understanding how the basic refrigeration cycle works should give you some clues as to how super-heat can be used as a diagnostic. Let’s look at it.

If your system has an expansion valve for the metering device, depending on the system, it can have either a fixed super-heat or an adjustable super-heat setting. Most residential expansion valves are of the fixed super-heat variety and are usually set between 10 & 15 degrees of super-heat. Knowing the valve type and setting will help you diagnose possible problems with an expansion valves. If you know the valve should be set for 12 degrees of super-heat and you are out of that range,  you should look at the valve. Start with the position of the sensing bulb on the suction line, is it insulated? Is it making good contact? Is it in the correct position in regards to the size of the suction line?.  Take the bulb off and put it in a cup of hot water — does the valve open and you see the pressures changing?  If the valve was possibly over-feeding, put the bulb in a cup of ice water.  The valve should close and the pressures should respond accordingly.

As you can see, super-heat will help diagnose a possible problem with an expansion valve but what about capillary tubes or orifice metering devices?  Super-heat is the preferred method of charging units with orifices and cap tubes. To properly do this, though, you need to not only know the indoor dry bulb temperature but you need to know the indoor wet bulb temperature to properly charge by super-heat. The wet bulb temperature really determines the amount of super-heat the system needs to make sure no liquid gets back to the compressor.  Most manufacturer’s have charts for this purpose. There is no guessing when it comes to proper super-heat.

If your system shows signs of Low system super-heat ––you need to remember that compressors are vapor pumps and do not like liquid.  Not enough super-heat could allow the vapor to condense back to a liquid in the suction line and now you could cause a problem in the compressor.  As stated at the start – liquid is the enemy of the compressor. An over-feeding metering device could cause low super-heat. Not enough air flow across the coil with an orifice metering device will keep the evaporator coil from “evaporating” all the refrigerant and allow liquid back to the compressor. This cold be caused by a dirty coil, dirty blower, improper blower speed, etc.. In any case, the heat is not being added to the refrigerant properly allowing the possibility of liquid back to the compressor.

If your system has High system super-heat — this is typically a result of an evaporator being starved of refrigerant.  Starving is caused by a TXV or orifice under-feeding, plugged liquid line filter-drier, kinked or restricted liquid line or any other form of refrigerant restriction. Since we are “starving” the coil, the refrigerant picks up more “heat” in the evaporator.  The compressor discharge temperature reflects the latent heat absorbed in the evaporator, evaporator super-heat, suction line super-heat, heat of compression, and compressor motor-generated heat.  All of this heat is accumulated at the discharge of the compressor and must be removed. Also keep in mind that too much air flow  across the evaporator will cause high saturated suction temperature. The increase load on the coil transfers too much heat to the refrigerant.  More refrigerant is vaporized, elevating the temperature & pressure.

In both scenarios, the hotter refrigerant entering the  compressor needs to be removed. The condenser requires more of the condenser surface to reject the heat.  More condenser is needed, less room for sub-cooling. This equates to a lower sub-cooled refrigerant and it becomes a vicious cycle until the compressor fails.

Super-heat is a very important function in the refrigeration cycle. Most importantly, it protects the compressor from liquid  but also from the problem of “burning up” due to too much super-heat.  Hopefully, this post on SUPER-HEAT as a diagnostic will become part of your “arsenal” on proper diagnostics of air conditioners.

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Quality is a Two-Way Street

The other day I  came across this description of MANUFACTURING QUALITY  and I thought about it since I had been the service rep for a manufacturer and decided this would be an interesting topic for a post.

You can call something quality by testing for quality.
You can test for quality by knowing how to measure quality.
You can measure quality by knowing the specs.
You know the specs by knowing the customer.                                                                         Now read it backwards!                                                                                                 (author unknown)

In our industry. “QUALITY” is a buzz word that manufacturers like to use as part of their marketing. The best manufacturers use “focus groups”, employee input, distribution input, and their own expertise in the design of a product. They work the quality statement from the bottom back to the top.  But does that guarantee a quality product?

Today, most manufacturers use vended components and, other than the physical design, basically “assemble” products. They may make the basic cabinet, the coils, or heat exchanger, but the operating controls are a purchased product. The manufacturer also chooses how “robust” the cabinetry is by either metal gauge or design.. So, other than the “appearance” of the product and the “brand name” on the cabinet, it all comes down to  what I always said was it is all just “BTU’S ON A BOX”.

There is not a manufacturer out there that intentionally designs a BAD PRODUCT. Sooner or later, every manufacturer will have a problem, and it is usually with a vended component. A responsible manufacturer will address these issues and provide some form of “extended warranty”, labor, or both to correct the issue. The vendor of the problem component is also asked to participate in this as this directly reflects on their quality.

Now we get to  what the title of this post is saying. Above, we talk about the relationship between the manufacturer, engineering, and their vendors. Now we need to look at our side of the business and our relationship with the manufacturer.

A manufacturer could have the best product out there, but if the installers don’t install it properly, it will never work properly.  Also, if there are problems, do the service techs know how to properly address the problem and correct it?

Too often, a home owner doesn’t know who the original installing contractor was or he couldn’t get there soon enough to their liking  and “randomly” calls a contractor who advertises they work on  ALL BRANDS because they have a problem and want it fixed now! Of Course the problem here is they never ask if they are familiar with the product they have in their home. This is especially true with a manufacturer’s HIGH END products. All manufacturers have them and they can have very sophisticated controls or sequences of operation. The manufacturer have made some major investments in design  and efficiency. They also have made major investments in being able to train the contractor with manuals, PowerPoint programs, on-line training etc. and  usually require training on for the contractor to be able to purchase the product. They want to assure the “quality” of their product is not compromised because of “lack of knowledge” on the technology used in the product.

Now, the contractor who was randomly called gets to the home, starts working on something he is not familiar with, tries to figure it out but, if he can’t, he tells the home owner, “Yeah, we have nothing but problems with this brand!” and may even try to sell them a new one all because he does not understand the specific product operation. — It happens all too often! What does this do to the manufacturer’s reputation? Too often the manufacturer gets the “bum rap” for quality when it really is the contractor had never been to training on that product.

Our industry needs to stop the practice of saying “We sell XYZ Brand but we work on  anything”. That was possible in the days of thermocouples, standing pilot furnaces, fan and limit controls, belt drive motors, 6 SEER A/C’s etc, but it just is not possible today for an individual contractor to be familiar, let alone have the parts on their truck, to fix every brand unit out there, and not to mention the training expertise to do the same.  Contractors need to ask new customers what the equipment is that they have and be honest enough to tell them that they need to call a ‘dealer who is familiar with that brand and who would probably have the necessary parts to get them going right away. If they do get to a customer’s home or site, and find something they are not familiar with, to, again, be honest and tell the home owner that they don’t have the parts or expertise to handle their needs.  Lastly, and most importantly, contractors need to keep up with the newest technology by attending manufacturer’s training, community college training, union training, etc.

So, the two-way street is manufacturers need to partner with vendors who provide quality components, use those components to produce quality designs, and have quality training programs in place for the contractor.  The contractor, in return, needs to attend the training, have the necessary parts for that brand available, and be honest enough with the customer to tell them they are not “set up” to handle their specific product. The day of “we work on anything” are gone.

Quality starts with the manufacturer but, if you read the opening description above in reverse as it says to, it also starts with the contractor/customer.

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