NTU vs FTU: Which HVAC Metric Matters More for Energy Efficiency?
NTU (Number of Transfer Units) counts how many theoretical “perfect” heat-exchange stages your coil achieves; FTU (Fan Terminal Unit) measures how much conditioned air a fan-powered box can deliver to a zone.
Contractors swap the acronyms because both live on the same submittal sheets and both affect efficiency, yet one governs heat transfer area while the other dictates fan energy—causing confusion on the jobsite.
Key Differences
NTU is dimensionless, driven by coil geometry and flow rates; higher NTU equals more heat exchange with less pumping power. FTU carries units of airflow (CFM or L/s) and adds fan watts to push air through ductwork, raising operating cost.
Which One Should You Choose?
If your goal is minimizing chiller or boiler energy, chase a higher NTU design. If occupant comfort in variable-volume systems is the priority, pick a rightsized FTU and accept the fan penalty. The smartest buildings balance both.
Examples and Daily Life
A retrofit hospital swapped old 6-row coils for 10-row units—doubling NTU and cutting chiller kWh by 18%. Meanwhile, a school upgraded to low-pressure, ECM fan-powered FTUs, trimming fan energy 25% while maintaining classroom airflow.
Can a high NTU coil save money if the FTU fan is oversized?
No. Excess fan watts can erase the chiller savings; size the FTU to actual zone load.
Is NTU only relevant for chilled-water systems?
It applies to any heat exchanger—hot water coils, heat recovery wheels, even condensing boilers.
How do I measure NTU on an existing unit?
Use temperature and flow data to calculate effectiveness, then divide by the log-mean temperature difference.