A rule of thumb for air conditioning compressor motors is that there is one horsepower per ton. This rule will not work for applications other than comfort cooling air conditioning applications. The number of horsepower required to provide each ton of cooling varies with the suction pressure and head pressure. The higher the head pressure or the lower the suction pressures, the more motor horsepower required to achieve a ton of cooling. Another way of stating it is that as the pressure difference between the suction and head pressures increase, the system tonnage decreases. This is why it is so important to keep condensers clean, evaporator air filters replaced and airflow on both condensers and evaporators up to normal. Anything, which increases head pressure or decreases suction pressure will decrease system capacity and at the same time increase operating costs.
Some things, in my experience, that have caused reduced capacity in otherwise nicely installed systems are:
- Refrigerant charges more than 5% over or under spec
- Economizers with broken, frozen or missing drive system components, outside air or mixed air sensor failure, low changeover temperature setpoint, or the use of a single-stage cooling thermostat
- Low airflow (less than 300 CFM/ton, normal is 400/ton)
- failed sensors
- misadjusted or failing expansion valves
Almost every rooftop unit that I came up against had a wrong refrigerant charge and a bad or misadjusted economizer. Ensuring that all stages of cooling are energized when the refrigerant charge is checked and upgrading to solid state economizer will almost always allow a previously underperforming RTU to work more near it's rated capacity.
The actual equipment to measure the actual performance of a particular unit, in tons of cooling, is super expensive. It involves setting up a variety of sensors on the unit and connecting them all to a box that hooks to a laptop. You're in the 20 grand range here. The CheckMe! tool is used by some people, but it won't display actual tonnage performance.
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