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Author Topic: Charging the 13 SEER System: Different, But Nothing You Can't Handle  (Read 4870 times)

Offline Icehouse

  • Technician
  • Member Since: Oct 2008
  • Posts: 569
    • hvacbob

When the new federal minimum energy efficiency standards take effect next January, contractors and technicians will encounter a lot of change:
•   Higher energy efficiency. New federal regulations will make it illegal to manufacture air conditioners and heat pumps rated below 13 SEER (seasonal energy efficiency ratio) after January 23, 2006. You can still sell lower efficiency systems as long as supplies last, but you can't count on long-term availability.
•   Larger equipment. By its very nature, increased energy efficiency comes largely at the cost of expanded coil surface. On average, you can expect most manufacturers’ 13 SEER models to be about 30% larger than their 10 SEER models. (If you’re already selling higher efficiency systems, you may find that the new regulations push you to step even higher. The same size rules will apply as you climb up the efficiency ladder.)
•   More thermostatic expansion valves (TXV). This change will drive product lines into the 13-18 SEER range and virtually eliminate the use of fixed orifice refrigerant metering devices. Most air conditioners and heat pumps will rely on thermostatic expansion valves (TXVs).
•   Growing reliance on R410A. International Comfort Products’ research indicates that R410A refrigerant growth will accelerate in 2006 as manufacturers introduce new product lines less than four years away from the deadline for completing the shift away from R-22.
•   Increased importance of charging. As air conditioners and heat pumps move toward higher energy efficiency and alternate refrigerants, proper charging procedures will become more important than ever for three reasons:
1.   Energy efficiency. A system will achieve its rated energy efficiency only when the system carries the proper refrigerant charge.
2.   Capacity. An undercharged or overcharged system cannot achieve its rated capacity.
3.   Reliability. An improper refrigerant charge places an air conditioner or heat pump under additional stress and may shorten the system’s life.
The Laws of Thermodynamics Don’t Change
In the face of all this change, it’s important to remind yourself of one key fact: the laws of thermodynamics do not change. When you work with higher SEER systems and new refrigerants in 2006 and beyond, you will continue to work under the same rules that governed the operation of the first air conditioner over 100 years ago. So, while your charging procedures and techniques may change in the months to come, they won’t be anything new.
Let’s take a few minutes for a basic refresher course in refrigerant charging for higher efficiency systems.
When to charge. When do you need to charge a unit or check its refrigerant charge? There are many scenarios where charging may be necessary. These include new installations, pre-season start up, and replacement of a component in the refrigerant system.
How will charging change? With a thermostatic expansion valve, you can’t check a unit’s charge by using the superheat method. TXVs control refrigerant flow by maintaining a constant superheat (for instance, 8F to 10F). With constant superheat values, the condition of the system charge cannot be determined using superheat. You must look to the condenser and the liquid side to verify proper performance.
Basic charging procedures. In any case where you are required to check the system charge, the technician must consider the following:
1.   Airflow. Check the condition of coils, blower wheels, and the blower motor speed. Measure airflow by using the temperature rise method. Check pressure drop across coils using the manufacturer’s coil specification sheets. Or, use the velocity pressure to calculate airflow. Don't guess. The airflow calculation is very important because it helps you determine evaporator load, and therefore will have a significant effect on system pressures.
2.   Operating pressures. Check the system operating pressures. Connect the hoses from your manifold gauge set to the pressure taps on the liquid and suction service valves. Measure and record the liquid and suction pressures.
3.   Outdoor ambient temperature. Measure and record the outdoor ambient temperature.
4.   Wet bulb/dry bulb. Measure the wet bulb and dry bulb of the air entering the indoor unit in the return duct. This step is very important because it helps you determine the evaporator load, and therefore will have a significant effect on system pressures.
5.   Liquid line temperature. Measure the liquid-line temperature so that subcooling can be calculated. Use a good thermometer with a probe that can be strapped tightly to the line. Install the probe on the liquid line about 6-in. or so from the liquid service valve, then measure and record the liquid-line temperature.
6.   Calculate subcooling. Measure the high side pressure at the liquid-line service valve pressure tap. Using a pressure temperature chart, convert high side pressure to saturated temperature. Then simply subtract the liquid-line temperature from the saturation temperature of the refrigerant in the condenser to determine the subcooling.
7.   Compare to the manufacturer’s data. Always refer to manufacturer’s data sheets to find the proper operating pressures for the conditions of the air that you’ve measured. Do the same for required subcooling levels.
8.   If subcooling is too low, there may be an insufficient amount of refrigerant to maintain proper subcooling.
9.   If subcooling is too high, there may be too much refrigerant in the condenser. However, high subcooling can also be caused by a restriction in the refrigerant lines or a TXV that fails to open. That is why checking a system for proper pressure performance involves both the high-side and low-side pressures.
A look at suction pressure can help further your diagnosis. If both pressures are high, you have an overcharge; if both pressures are low, you most likely have a restriction.
Measure and calculate carefully since an overcharged or undercharged system will suffer from poor efficiency, capacity, and reliability.
Two Notes on R410A
If you need to charge an R410A system, it must be charged as a liquid. Turn your R410A cylinder upside down.
If you need to remove R410A from a system, you must recover the refrigerant. You can’t just vent it into the atmosphere. Even though R410A will not harm the earth’s protective ozone layer and is thus considered to be an environmentally sound refrigerant, it’s still a greenhouse gas and is governed by federal regulations. It must be recovered.
Along with an accurate load calculation and proper equipment selection using actual design conditions, the proper charging of a 13+ SEER or an R410A unit is of the utmost importance in the new world that we’ll see in 2006 and beyond.
NATE, NCCER, PHCC,HVAC Certified Instructor
Member RSES, US Army Refrigeration Specialist(Retired), Former Refrigeration Teacher NYC Board of Ed.
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