Intelligent Controls Improve System Charging

"Charge View" by Johnson Controls

Units with intelligent boards that assist in system charging are available. Many VRF systems can assist technicians in charging the unit. They are so complex that some type of automated assistance is really necessary. With multiple heads and variable capacity compressors there is really no way to use system pressures to determine the correct charge. Computer assistance is available through installation and charging applications that run on laptop computers, to evacuation and charging modes built into the system controls.

Trane introduced split system units with “Charge-Assist” back in 2008 in their Xli line. These systems have pressure transducers and temperature thermistors which are used to operate the electronic expansion valves in the unit. The board can also use the input from these sensors to determine if the system charge is correct. An external  “Charge Assist” solenoid can be controlled by the board to allow the unit to charge itself. On these units, the technician only sees a blinking LED on the unit control board.

Johnson Controls (York, Coleman, Luxaire) are now offering units with built in pressure and temperature monitors and a screen to display system pressures, liquid line temperature, suction line temperature, superheat, and subcooling. The system will also tell you if it is correctly charged. It is like having a digital gauge set built into the unit. The main point is that you can check the unit charge without attaching any gauges or temperature probes. That means you will not lose any refrigerant while checking the charge.

These examples represent only the very high end systems from a few different manufacturers, but I believe it shows the direction the industry is headed. Systems will have sensors and intelligent controls monitoring system operation. I am sure that as the technology matures, its cost will come down, making this technology attractive to other manufacturers and in more main line units. Another driving force will be the desire to insure actual equipment performance and efficiency match the design. The most efficient system available installed incorrectly may perform worse than the lowest builder grade equipment available. Designing intelligent controls into a system is a way to improve system installation and service by taking guesswork out of charging. With systems employing these intelligent controls you really have no excuse for leaving the unit improperly charged.

How Much Refrigerant Does it Take to Make the Fan Blow Harder?

My brother Richard sometimes finds himself helping younger technicians who are stumped and need the help of an experienced professional technician. However, they have to really need help because they know Richard is going to twist their tail a bit in the process.

One day a frustrated tech called and told Richard that he was working on a system with a low suction pressure that was frosting up. He further explained that his new digital gauges were telling him that the superheat was 0, so he added refrigerant. However, no matter how much refrigerant he added, the superheat would not increase.

Richard asked him, “how much refrigerant do you have to add to make the fan blow harder?” There was no response, so Richard asked again, “Tell me, I really don’t know. How much refrigerant does it take to make the fan blow harder?” Finally, the tech responds: “Your question makes no sense! There is no relationship between the amount of refrigerant in the system and how hard the fan blows.” Richard then replies “So why are you trying to fix an airflow problem by adding refrigerant?”

While it is true that an undercharge can cause an air conditioning system to frost, the most common cause of a frosting air conditioning coil is actually low airflow. Always look at airflow issues first when trying to remedy a frosting air conditioning system. Common airflow issues include a dirty air filter, a dirty evaporator coil (caused by dirty air filters), closed registers, and poor ductwork.

One tip-off is superheat.  A system with airflow issues will operate with a low superheat while an undercharged system will operate with a high superheat. Note that if the coil is frozen over it will need to be defrosted before any pressures or temperatures are checked. The ice covering the coil makes its own airflow restriction.

Cold Weather Heat Pump Charging

Charging an air source heat pump during cold weather has always been a bit of a problem. The problem is that the amount of refrigerant circulated decreases as the outdoor temperature drops. Why is this? Well, as the outdoor temperature drops, the evaporator temperature has to drop in order to be able to absorb heat from the outdoor air. The lower evaporator temperature produces a lower evaporator pressure. The lower evaporator pressure increases the compression ratio because there is now a greater difference between the suction pressure and the discharge pressure. The higher compression ratio means that the compressor does not circulate as much refrigerant.

At a 45°F outdoor temperature,  a typical air source heat pump produces a heating capacity roughly equal to its nominal cooling capacity. At 17°F outdoor ambient, it produces about half as much heat as it does at 45°F. This difference in capacity is directly related to the amount of refrigerant being circulated. The rest of the refrigerant is just sitting somewhere – normally in either the accumulator or the charge compensator. So a system operating at 17°F outside could have perfect pressures even if it only had half of its factory charge. That is why you can be way off checking a heat pump by pressures in the heating mode.

Some manufacturers provide heating performance pressure charts, but refer to them as “check” charts. They are intended to check the system operation at specific conditions, but are NOT intended as guides for adding refrigerant. The problem is that you don’t have a good way to judge how much refrigerant is stored out somewhere in the system. I can hear a bunch of you saying that measuring superheat and/or subcooling solves that problem. While I AM a fan of checking both, they still just measure the refrigerant that is circulating.

There have been some interesting methods used, such as measuring discharge superheat. For discharge superheat, you measure the temperature and pressure of the discharge line right as it leaves the compressor. It should be somewhere around 60°F warmer than the discharge saturation temperature. So if you have a 410A system running at a discharge pressure of 318 psig (saturation temperature 100°F), the discharge line should measure 160°F. A lower temperature reading indicates an overcharge and a higher temperature reading indicates an undercharge. The surest way to charge a heat pump in the winter is to recover the refrigerant, evacuate the system, and weigh in the correct charge. If you have performed a repair on the refrigerant system, then this will save you time and insure a correct charge.

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.