Expensive Top Offs

For years, many technicians and customers have been content to “top off” leaky air conditioning units, feeling the time and expense required to find and repair small leaks exceeded the inexpensive price of adding a few pounds. After all, adding refrigerant is pretty easy, does not take long, and most customers were perfectly happy to have their systems gassed up every so often. With the cost of adding refrigerant steadily rising, the era of cheap “top offs” is coming to an end. Newer units also hold considerably more refrigerant than older models, and the refrigerant is quite costly. A bill of $500 for “topping off” a system is quite possible. At $500, I don’t want to call you back next month for a repeat performance. I want the problem fixed. Customers are going to start demanding that we fix the leaks – not just gas up the units.

The temptation to just add refrigerant is strong. Locating and repairing leaks takes time and there is really not a sure-fire leak test that can prove a charged system has no leaks. We detect the refrigerant coming out to prove a leak. However, not finding any leaks does not necessarily mean there are none – it just proves that you did not find them. Like many things, better tools often yield better results. Everyone should have a good electronic leak detector and good quality soap bubbles that will form micro-bubbles. An ultrasonic detector is also a good idea for detecting leaks using nitrogen. You also need to use the tools enough to feel confident and comfortable using them. A fancy electronic leak detector is not much help if you don't trust it or use it .

Some leaks are really not repairable. Take for an example an evaporator coil with multiple pin-hole leaks. Plug a little hole today and a new one springs up tomorrow. The repair is to replace the coil, which is expensive. However, not replacing the coil is also expensive. Not only is the cost of adding refrigerant quite high, so is the cost of electricity. A system with a leak loses capacity and efficiency from the day it is charged. The unit has to run longer and longer to accomplish less and less, until it simply can’t meet the demand. The combined costs of added refrigerant and increased operational cost can add up to the cost of replacing the coil. Topping the charge off is no longer an easy and inexpensive solution. It is now just the lazy and expensive cop out.

Cold Weather Heat Pump Charging

Checking the refrigerant charge of a heat pump in cold weather can be confusing for many technicians, at least partly due to the many different methods recommended by different manufacturers. Although there are many methods, most are described as heating performance charts or heating cycle check charts. A check chart is just that, a chart for CHECKING the charge during the heating cycle. It is not intended to be used for actually ADDING refrigerant. Why? Lower outdoor temperatures cause a lower evaporator temperature and pressure. Lowering the evaporator pressure increases the compression ratio, so the compressor capacity is reduced. Less refrigerant is being circulated in the refrigerant system because of the diminished compressor capacity at low outside ambient temperatures. The refrigerant that is not being circulated must sit somewhere, usually in the accumulator. For this reason, a charge which is adequate to maintain correct pressures and temperatures at 25°F may not be adequate at 35°F. How? Suppose that the unit was undercharged according to the check chart. If you add just enough refrigerant to bring it up to the required pressures, you have only added enough refrigerant for correct operation at that one condition. When the outside temperature rises, the unit will be capable of pumping more refrigerant. However, since you didn't add any extra refrigerant, there won't be any more to pump. The system will be undercharged. Just adding a little extra is not a great ida either because you really have no way to determine how much extra to add. This is why manufacturers say that the check chart cannot be used to CHARGE the system, only to check its operation. If the system is undercharged, the recommendation from many manufacturers is to recover the refrigerant in the unit and weigh in a total system charge according to the manufacturer's instructions. 

Charing - Read the Fine Print!

There is an old saying that “the devil is in the details.” That is certainly true of refrigerant charging. There are so many details to keep track of. Outdoor ambient temperature, indoor wet bulb temperature, evaporator airflow, condenser pressure, evaporator pressure, suction line temperature, liquid line temperature, superheat, subcooling and approach. I am sure I left a few off, but that list is long enough to make my point: there is simply a lot to keep track of. The confusion caused by trying to make sense of so many variables can cause some technicians to look at only one thing, often they look at only the suction pressure. This is like covering your ears so you won’t hear the thunder. It does not keep you from being struck by lightning. It is true that no manufacturer specifies all those things in their charging information. However, many of these details are assumed so that system performance can be judged by one or two key indicators. The problem is that even a manufacturer’s charging chart will not be accurate if the conditions you have do not meet the assumptions used with the chart. You must read the fine print. Most manufacturers disclose their assumptions somewhere on the chart or in their service manual. A commonly specified condition is that the evaporator airflow is 400 CFM per ton and the return air temperature is 80°F 50% relative humidity. In our shop we have a packaged unit with a very simple charging chart. It gives the suction and discharge pressure for five specific outdoor temperatures. However, it is assuming standard airflow across the evaporator and indoor conditions that meet AHRI rating conditions – 80°F dry bulb 50% relative humidity. It is seldom that warm in our shop, so the pressures are usually lower than those specified on the chart. In the field, it is common for the indoor temperature to be above the AHRI rating condition, that is why you are there in the first place. You can expect higher pressures as a result. Superheat charts that list only an outdoor temperature have a similar problem. They assume an indoor temperature and humidity level. Even charts that specify an indoor wet bulb and an outdoor temperature are still assuming standard airflow across the evaporator coil. For that matter, all charging charts are assuming both coils are clean, that you have good airflow across both coils, and that all the refrigeration components are performing correctly. If any of these are not right, no charging chart will work. You should make a habit of checking airflow, air filter, and condenser cleanliness on every call. Then read the fine print to check any specified conditions used with that chart. 

How much refrigerant does it take to make the fan blow harder?

Have you ever noticed that right after you buy a car you seem to see that particular model everywhere?  Similarly, once you solve a problem, every problem you see seems to be another iteration of that same problem. My theory is that we are programmed to repeat our successes. The brain says “that worked before, lets do that again!” We are all occasionally tempted by the same quick fix demon that whispers in our ear “this worked the last time!” The temptation is just to repeat our latest success without really looking carefully at the actual problem before us. We convince ourselves that this problem is just like the previous one. I think it is more difficult for younger technicians to resist the quick fix temptation. They have not seen as many problems, so their brain is less confused – it is certain the solution is whatever they have seen recently! Often the quick solution involves adding refrigerant. My brother Richard is a senior technician at his company. Having seen and experienced many things, other technicians often call him for help. Richard is also a joker, so they know they are going to have their tail twisted a bit when they call. Recently a young tech called Richard and said he could not get the suction pressure on an R22 unit above 50 psig even though he had added refrigerant three times and every time the pressure settled down to 50 psig. He didn’t understand why adding refrigerant did not raise his suction pressure. Richard asked him “how many pounds of refrigerant do you have to add to make the fan blow harder?” No reply. So he asks again “Say, I really don’t know, how many pounds of refrigerant DOES it take to make the fan blow harder?” Finally, the tech replies: “adding refrigerant won’t make the fan blow any harder no matter how much I add.” Richard then asks “so why are you trying to fix an airflow problem by adding refrigerant?” Of course there are several possibilities that could cause low suction pressure, but airflow is one of the most common.  He could also have some type of refrigerant restriction or a metering device issue. However, more information is needed to differentiate these from each other. A high superheat and low subcooling could indicate an undercharge. A low superheat could indicate an airflow problem. A high subcooling and high superheat would point to a refrigerant restriction or metering device issue. The problem is that the tech was only looking at the suction pressure and nothing else. He had already decided what the problem was and how to fix it before actually finding the problem. To his credit, he did recognize that he was pursuing the wrong “fix,” that is why he called for help. The next time you can’t understand why your fix is not working, ask yourself if you are trying to make the fan blow harder by adding refrigerant. Then call Richard.  

Dry R22 Units

I got my first look at a “dry” unit the other day. A wholesale house had one on its showroom floor with a big sign “we have dry units!” I looked at the nameplate to see what it said about refrigerant. A refrigerant amount was actually listed. Instead of the familiar “factory charge” preceding the refrigerant quantity, it had “Field Charge” and then the amount. I asked about coil matching for older systems. They recommend 1/2 ton smaller condenser than the original for heat pumps and the same size condenser for air conditioning. Then I asked about warranty and they said the units came with a 5 year compressor warranty regardless of the coil connected to it. They pointed out that this was less than the warranty on new matched equipment. Many of the units are being sold to replace condensing units that have been stolen or chopped up in place, often in rental property. Another wholesaler requires you to take coil measurements and then uses software to try and figure out the coil SEER. If the calculated SEER is less than 13, they recommend against buying the dry unit. Companies that just a few years ago were cautioning against the perils of mismatches units are now selling units that will almost certainly be mismatched. They cannot provide any meaningful charging chart without knowing what coil is on the other end of the system.  

If you do decide to install a “dry’ unit, make sure and follow good refrigeration system installation practices. The fact that you are creating your own special blend of equipment makes it all the more important to be careful, not careless. It would be wise to treat the job like a compressor burnout. Flush the lines and coil, install a filter drier, pull a good vacuum, and weigh in the charge using an accurate digital scale. You can see more about line flushing at Mainstream Engineering. They sell a flush designed for this type of use. You will need to adjust the data plate charge for line length and possibly coil mismatch, but the nameplate charge should be in the ballpark unless you have very long lines. I think it might be wise to start a little under, so I would not adjust for line length until after starting the unit.  I found a good set of instructions for installing dry systems online provided by Century, a supplier that is offering dry units. No, these are NOT the folks who suggested the 1/2 under ton heat pump coil adjustment. The link to the guide is:

http://www.centuryac.com/DryChargeInstructionsandchart.pdf

The New 70

Many of the old timers I talk to who are still leery of R-410A want to know what the “new 70” is. They want a target number for the suction pressure of an R-410A air conditioning system. Many service technicians are in the habit of charging to a target number on the suction side. This is frequently 70 psig on R-22 systems. The saturation temperature of R-22 at 68.5 psig is 40°F and for many years the standard evaporator temperature at design conditions has been 40°F. But 70 is a nice round, easy to remember number and close enough for quickie rules of thumb. The problem is that systems seldom operate at design conditions. Another big problem is that conditions other than charge can cause low suction pressure. If a technician is only checking the suction pressure, they are not collecting enough information to recognize other system factors that can contribute to low suction pressure. A common error of inexperienced techs using the “70” method is overcharging systems that have low evaporator airflow. I have gone behind someone using the “70” method who had overcharged the system so severely the compressor stalled at startup and pegged my high side gauge past 500 psig. The customer was told that the compressor was bad. To be fair, most experienced technicians that use the “70” method understand how airflow and system operating conditions affect system pressures, they just don’t want to bother with manufacturer’s charts. They just modify the target up or down as they judge conditions affecting system pressures. These folks can usually get a system cooling; that is why they still have a job. However, the system will often not be performing optimally when they leave. Today’s customers are paying a premium for systems that are more efficient and have less environmental impact than older systems. Beginning January 2010, even the least expensive unit a customer can purchase will be non-ozone depleting and more efficient than the least expensive unit they could purchase just a few years ago. This means everybody is paying for efficiency and reduced environmental impact. However, if technicians don’t charge units properly, customers are not getting what they are paying for. Your students can be the vanguard of a new era that values professionalism. Make sure your students know how to handle new refrigerants like R-410A and know how to read and follow manufacturer’s charging charts. That will put them in a position of leadership early in their career.

Fundamentals of HVAC/R has a detailed discussion of system charging in Unit 27 Refrigerant System Evacuation and Charging. Variables affecting system pressures, common methods of determining the correct charge, and common methods of adding refrigerant are all covered. Charging is also discussed in other units including

• Unit 35 Residential Split system Air conditioning installations
• Unit 36 Troubleshooting Split System Air Conditioning
• Unit 52 Heat Pump Installation
• Unit 53 Troubleshooting Heat Pump Systems
• Unit 83 Troubleshooting Refrigeration Systems
• Unit 84 Installation Techniques
• Unit 85 Planned Maintenance
• Unit 86 Troubleshooting