The Mystery of the Frozen Coil

It was on a hot day in July that I received a call from Potomac, Maryland resident, Miss Magnificent (as I will call her).  She already had an operational A/C system on her lower level but had been informed by prior HVAC technicians that she would need to replace her system on the upper level of her home. After discussing her options with her, we selected a 3 ton system from one of the most reputable HVAC manufacturers on the market, complete with furnace, condensing unit and coil.

All went well on the day of installation and the system seemed to be working beautifully when we left that day but 2 days later, we received a call from Miss Magnificent, reporting that the system was not working properly.  We came right over to determine what had gone wrong and found the coil frozen.

After examining the entire system carefully and eliminating many of the possible reasons for the frozen coil such as:  inadequate return air ducts, blocked air ducts, a dirty air filter, a malfunctioning fan, blocked drain pipe, and low outdoor temperature, we arrived at the conclusion that there was a restriction in the flow of the refrigerant and that our best course of action would be to replace the TXV in the coil.  Fortunately, the initial coil had been brand new so it was still under warranty.

We immediately replaced the coil and, to our delight, the system began working beautifully.  A full two weeks later, Miss Magnificent called us again and informed us that the coil had frozen up again.  I came right over and re-evaluated the situation.  I suspected yet another restriction but considered other possibilities as well:  could it be that there was a leak in the refrigerant?

  1. I checked the refrigerant lines, testing them carefully to rule out the possibility of a leak.  I found no leak but was prepared to replace the refrigerant lines if it were warranted.
  2. I asked Miss Magnificent to help us rule out the possibility that the frozen coil was being caused by an excessively low nighttime thermostat setting by setting the nighttime thermostat no lower than 70 degrees.

Days later, I received a call from another HVAC company, who had been called to the home in Potomac.  He reported that Miss Magnificent was very upset about the system’s failure and had called him in to get a second opinion.

I felt terrible.  After all, Miss Magnificent had been so patient, so gracious about every step of the troubleshooting and solution process. Who could blame her for her frustration? I was frustrated too.  I had been back and forth from her house numerous times by this time, had diligently checked and researched the issues and tried various solutions.  The system had even worked perfectly for a full two weeks!

But now, my troubleshooting prowess, which has been a point of pride for me for decades of HVAC work, seemed to be failing me. Worse yet, my customer, whom I hold in high esteem to this very day, was losing faith in me and had paid good money for a system that was not working. This brings me to a previously classified AMS secret:

AMS contractors is most motivated to reach an Effective solution to HVAC issues for their most patient, gracious customers.  We would rather drink freon than let you down.

After having informed Miss Magnificent that we would not abandon her and that we would do whatever was necessary to get the system working at no additional cost to her, I summoned one of my most respected and experienced HVAC colleagues to the scene and called the HVAC manufacturer’s technical support.  My colleague confirmed every one of my conclusions.  We checked the refrigerant lines and confirmed that they were the proper size, length and diameter.

Technical support reached the same conclusion that I had reached a few weeks before and suggested that we replace the TXV AGAIN.  I followed their instructions to the T but a nagging feeling inside me told me that this coil, too, was destined to freeze.  The pressure values and other relevant numbers were not where I had hoped they would be.

A friend unrelated to the HVAC world suggested that, perhaps, there was some manufacturer’s defect.  My colleague, who had visited the home and confirmed my diagnosis, was suddenly faced with the same issue on a project he had completed elsewhere.  After each of us called technical support at two different highly reputable HVAC manufacturing companies, we were informed that there was an industry-wide defect in many 3 ton coils,specifically the TXV, causing 10% of expansion valves to seize up and 5% of them to seize up even after a second coil replacement.

Much to these HVAC manufacturers credit, they were willing to admit the issue and even had a solution to the problem.  The source of the problem is a rust inhibitor that runs in the system, which causes 10% of expansion valves to seize up.  In fact, they have just issued a bulletin publicizing the problem. Their recommendation was to replace the expansion valve and lubricate the system with a special kind of oil called Supco.

That very same morning, I bought the Supco and a 4th coilwith new TXV, reinstalled it and voila!!! The system began working beautifully and all the numbers, the pressure, etc.were perfectly in line with industry recommendations. I was relieved and so happy that Miss Magnificent, who deserved nothing less than a seamlessly working system,had finally received the service she so richly deserved.

Lessons Learned:

  • When in doubt, contact technical support.

  • Take excellent care of your clients.

  • If you have tried everything, done your due diligence and are stumped, consider the possibility of a manufacturing defect in the system or one of its’ components – it truly can happen.