Why Super-Heat or Sub-Cooling?

When I started in the HVAC business in the 70’s, a tech only needed a manifold gauge set, a crescent wrench and a pocket thermostat, to look cool. After the A/C unit was installed, the service valves were opened, you turned on the A/C unit and checked for cold air coming out of the registers and hot air coming out of the top of the condenser. To charge the A/C unit, we made sure, sweat on the suction line was coming back to the service valve (Beer Can Cold) and the liquid line was so hot, you could not touch it.

Weigh-in Method Doesn’t Get You to the Finish Line With today’s equipment you sell more than just a 2-ton condenser, you sell a 2-ton system. You sell a condenser and AHRI Rated evaporator coil to make up this 2 ton system. With today’s condenser we can match the condenser with different AHRI rated evaporator coils to get different SEER ratings. The difference in the coils will be the size; the larger the coil, the more rows you may have and smaller the coil, less rows you may have. This will make a difference in your refrigerant charge. The rating plate on the condenser will give the total ounces of refrigerant in the condenser for line-set up 15 feet, but it does not reflect which evaporator you are matching up to the condenser. So, the weigh in method is just a starting point on split systems, but it doesn’t get you to the finish line.

Always follow the manufacturer’s installation manual that comes with the equipment. After you have brazed the line set and pulled a vacuum to make sure you don’t have moisture or a leak in the line set, you can start charging the A/C system. If your line set is over 15 feet you need to add .6 ounce per foot of line set. Before you open the service valves, use the negative pressure in the line set to pull in your additional refrigerant into the system. When opening the service valves, always open the suction service valve first.

Today’s High-Efficiency Equipment Demands Super-Heat or Sub-Cooling Calculation

With today’s high efficiency equipment, the only way to know the equipment is performing at peak performance, is to do a Super-Heat or Sub-Cooling.

Super-Heat is used on an evaporator that is utilizing a piston metering device. After the system has run for 10 minutes, then you can check the Super-Heat and adjust the charge accordingly. Always use the Super-Heat chart that is included in the Installation manual that come with the equipment.

Super-Heat Formula: Suction line Pressure converted to temperature minus Suction line temperature = Super-heat

Sub-Cooling is used on an Evaporator that is using a TXV metering device. After the system has run for 20 minutes, then you can check the Sub-Cooling and adjust charge accordingly. Always check the installation manual that comes with the equipment for the Sub-Cool value. With Sub-Cooling, make small charge adjustment, and allow 20 minutes in between each adjustment for the TXV to react to the charge adjustment.

Sub-Cooling Formula: Liquid line Pressure converted to temperature minus Liquid line temperature = Sub-Cooling Have a great cooling Season.

Article: Don Mauck, Service & Technical Manger, Stevens Equipment Supply