Testing the Capacity of Air Conditioning Systems

For an existing system that you are going to service, to start, do not change or adjust anything before you test in! This means test in before you change the filters, clean coils, and even before hooking up the gauges. Knowing where you are starting is a powerful way to show the customer the value of the service that you provide. Testing in benchmarks the system performance and allows you to compare the test in to the test out and definitively prove the value of the service that you provide. Testing in is simple and only requires two measurements and two tools and can be done without even putting your gauges on the system.

To test in you will need an initial airflow measurement made with the Fieldpiece STA2, and the change in enthalpy across the coil using the Fieldpiece SDP2 (One of the coolest tools I have seen this year).

To calculate system capacity the standard air formula has proved reasonably accurate and easy to use. BTUh= 4.5 x CFM x ?h. (?h = change in enthalpy). There are also apps available like "PSYC IT" from the Apple® App Store or the TruTech Tools version of Psychart from Handsdown Software) that will provide slightly more accurate results. Most capacity problems (low BTUh output) are refrigerant charge and airflow related, which, on the airflow side, can also be directly related to dirt accumulation on the filter, evaporator, or condenser coils.

1. If this is a new system being commissioned, make sure the system has been properly installed and evacuated. A proper evacuation is critical to proper performance. If you need help with this process, click HERE!
2. Clean the filters and the coils. You cannot charge or check the charge on a system with dirty coils, filters, or a dirty blower. The condenser and the evaporator must be clean. The blower must be capable of moving the correct amount of air. Visual inspection of the evaporator can be aided with the use of a video scope by removing the high limit of most furnaces and fishing the shaft up through the heat exchanger to the bottom side of the coil.

Airflow is one one of the least understood and least performed measurements in the HVAC industry. However, it's most important for achieving designed capacity and creature comfort! You cannot charge a system without correct airflow. After the system is confirmed to be clean, the airflow must be set to the manufacturer's recommended settings. Typically this is 400 CFM/Ton +/- 10%. Measuring airflow with the Fieldpiece STA2 is a quick and easy process. Because it is such a cost-effective solution it is simply one tool no technician can be without. The video below demonstrates how easy it is to quickly get an accurate airflow measurement in any ducted system.

If you have measured and set the required airflow with the Fieldpiece STA2, in the big picture you are in good shape, but you should verify that the static pressures lie within the design range of the equipment to identify opportunities for duct system improvements in efficiency, operation, and air noise reduction. Additionally, some equipment will not perform correctly when the static is out of range.

While you may be able to achieve airflow requirements above design static you are doing so at the sacrifice of the efficiency of the evaporator fan motor. High static can be caused by something as simple as a restrictive filter or as extensive as an undersized duct system that would require renovation to repair the problem. Checking both return and supply static pressures independently as well as the total external static pressure (TESP) will tell you a lot about where the opportunities for better operation will lie. Achieving airflow is critical to proper operation. As a general rule, and by typical design, for straight cooling applications 400 CFM per ton is recommended, for heat pumps 450 CFM/Ton, high sensible loads up to 525 CFM/Ton, and high latent load also 400 CFM/Ton. Reducing airflow below 400 CFM/ton is not recommended because the lower airflow will cause problems with grill performance (throw and spread), lower coil temperatures, and result in lower efficiency and capacity, chances of freezing the coil also increase at typical indoor air temperatures. For some quick tips on measuring static pressures see the video below.

Digital gauges are not only faster, more accurate and more fun to use, but they also in the case of the Fieldpiece SMAN460 combined with the Fieldpiece SDP2, dramatically increase your productivity and charging accuracy when it comes to charging fixed orifice systems. Load (the heat that is removed from the home) is a dynamic thing. The amount of load and the percentage of sensible to latent load (heat to humidity) can and will dramatically impact the target superheat requirements.

The SDP2 communicates wirelessly with the SMAN460 to deliver the target superheat requirements. saving you steps, and time and providing increased accuracy. Due to the high-accuracy design (capacitive humidity sensor coupled with a thermistor for high-temperature accuracy), you can be assured of levels of accuracy and performance never before possible in the field. Previously, you would need expensive a lab-accurate or custom measurement solution.

Charge the system using the superheat and or sub-cooling method, for detailed charging procedures go HERE.

It is first important to understand that rated capacity and efficiency (SEER) is only achieved and measured under a specific set of conditions. Capacity can increase or decrease with factors like indoor load, outdoor air temperature, line set length and lift, and supply voltage. The changes are small, but they are cumulative, and almost always result in capacity losses. Very few if any systems will ever test in at rated capacity (under full load) unless the charge and the airflow are correct, the line set meets design requirements and the system is well maintained. Many times what we are looking for as much as the absolute output is the change in capacity that is achieved under similar load conditions. Typically this is considered a field capacity or field EER (energy efficiency ratio) but no less a valuable piece of information that shows the technician and the homeowner what they need to see which is the actual work that the air conditioning or heat pump system is doing. If we consider the electrical consumption (watts) including both those from the condenser and the evaporator fan, we can then show the work and the efficiency of the change in efficiency at which it is done. The reason we also want to consider the electrical energy is twofold.

A system can and will increase in amp draw if it is moving more BTUs. Moving heat energy is the work that is being done. What is important to remember is the ratio of BTUs moved to that of the watts used is what will dramatically change.

A system operating under less than full load can still operate at full capacity but at an electrical consumption disadvantage. Consider something like a dirty condenser, the evaporator may still be operating at its full capacity and the system can reject the heat at a higher than normal head pressure on an 80 deg F day, not exhibiting a loss of performance until the outdoor air temperature is closer to design. Cleaning the condenser coil on that 80 deg F day may not increase and maybe even a slight decrease in capacity but, a huge increase in EER as the ratio of power consumed due to the lower head pressure per BTU moved is significantly increased. A technician only looking at capacity and not efficiency may miss or even be perplexed at the outcome if only the capacity is considered.

Before you remove the Fieldpiece SDP2 probes from the duct, verify the TEET (Target Evaporator Exit Temperature) and or ?T (temperature drop across the coil) across the evaporator are correct. The SDP2 will not only show you what they should be, but it also calculates how far away from the target you are. Verifying that these are in line assures you that the system measurements, specifically evaporator performance and airflow are optimal. The evaporator exit temperature and the temperature drop will vary with the ratio of sensible to latent load, and the Fieldpiece SDP2 will consider this and provide you far more accurate results than looking for about a 20 deg. F drop. Evaporator temperature drop can vary from 16-24 degrees F with ease, and the only way to know when it should be is a lookup chart or manufacturer's table or to simply read it from your SDP2.