Capacity is the rate at which a system moves heat, measured in BTU/hr. For a cooling system, capacity is how many BTU per hour the system removes from the indoor air. For a heating system, it is how many BTU per hour the system adds.
A 3-ton air conditioner has a rated capacity of 36,000 BTU/hr (12,000 BTU/hr per ton). That number comes from the manufacturer's testing at standardized conditions: 95F outdoor, 80F indoor dry bulb, 67F indoor wet bulb, and rated airflow. In the field, conditions are rarely that clean.
Measured capacity tells you what the system is actually delivering right now, in the conditions it is operating in.
measureQuick uses the enthalpy method to determine delivered capacity. The calculation works like this:
The formula, simplified:
Delivered Capacity (BTU/hr) = 4.5 x CFM x (Return Enthalpy - Supply Enthalpy)
This is more accurate than a simple temperature-split calculation because it accounts for latent heat (moisture removal), not just sensible cooling. In humid climates, latent capacity can represent 30% or more of total capacity.
The nameplate on the outdoor unit says 3 tons. That is the rated capacity at standard test conditions. Measured capacity in the field will differ for several reasons:
Measured capacity below rated capacity is normal. The question is how far below.
The capacity ratio is the percentage of rated capacity the system is actually delivering:
Capacity Ratio = (Measured Capacity / Rated Capacity) x 100
What the numbers mean:
| Capacity Ratio | Interpretation |
|---|---|
| 90-100%+ | Excellent. System is performing at or near rated capacity. Conditions may allow above-rating performance on mild days. |
| 80-90% | Typical for a well-functioning system in real-world conditions. Minor losses from non-ideal conditions are expected. |
| 70-80% | Below expectations. Investigate airflow, charge, and coil condition. |
| Below 70% | Significant performance loss. One or more problems are reducing output substantially. |
A system delivering 75% of its rated capacity on a 95F day is underperforming. The same system delivering 75% on a 105F day may be operating within its limits given the extreme conditions.
Airflow is the single largest controllable factor in delivered capacity. The standard target is 400 CFM per ton. Reducing airflow to 350 CFM per ton can drop capacity by 5-10%. Reducing it to 300 CFM per ton (common with restrictive ductwork or dirty filters) can reduce capacity by 15-20% or more.
Low airflow also reduces the system's ability to remove moisture, making the space feel clammy even if the thermostat setpoint is reached.
Undercharged systems produce low suction pressure, high superheat, and reduced evaporator capacity. Overcharged systems produce high head pressure, high subcooling, and wasted compressor energy. Both conditions reduce the effective capacity delivered to the space.
A dirty evaporator coil restricts airflow and insulates the coil surface from the air stream. A dirty condenser coil raises head pressure and reduces the system's ability to reject heat outdoors. Both reduce capacity.
As outdoor temperature rises, the temperature difference between the refrigerant and outdoor air shrinks, making it harder for the condenser to reject heat. Capacity drops. Manufacturer expanded performance data (AHRI ratings or OEM tables) shows the expected capacity at various outdoor temperatures.
An oversized system cools the space quickly but short-cycles: it reaches thermostat setpoint before running long enough to remove moisture. Symptoms include:
An undersized system runs continuously on hot days without meeting thermostat setpoint. Symptoms include:
Both conditions are diagnosable by comparing delivered capacity to the building's cooling load. measureQuick provides the measured capacity; a Manual J load calculation (or equivalent) provides the required capacity.
When a customer says "my system runs all day," there are three common explanations:
The capacity measurement in measureQuick helps you distinguish between these. If measured capacity is close to rated, the system is working correctly and the issue is sizing or load. If measured capacity is well below rated, the system has a performance problem you can diagnose and fix.
On a 75F day, the condenser rejects heat easily and the system may deliver more than its rated capacity. This is normal. Evaluate capacity relative to rated conditions, not just as a raw number.
The capacity calculation depends on accurate airflow data. If airflow is estimated from fan tables rather than measured with TrueFlow or equivalent, the capacity result inherits that uncertainty. Use measured airflow when capacity accuracy matters.
A system can deliver full rated capacity while consuming excessive energy. Capacity tells you what the system is delivering. Efficiency (EER, SEER) tells you how much energy it uses to deliver it. Both matter, but they answer different questions.
In humid climates, latent capacity (moisture removal) can account for 25-35% of total delivered capacity. A simple temperature split does not capture this. The enthalpy method in measureQuick accounts for both sensible and latent capacity, which is why it is more accurate than supply-minus-return temperature alone.
Prerequisites (complete these first):
Follow-up articles (next steps after this one):
Related in the same domain:
If you have questions about capacity analysis or interpreting capacity results in measureQuick: