Best Practices in Fan Coil Unit Design
Doing things because “That is the way we always did it” does not make it the right way. Using new technologies is how humankind progresses. Doing it the old way would bring us back to carrying people in litters rather than using wheels. Certainly it would be simpler and more economical to build litters than automobiles and jets.
The technology behind the controls sequence is not new. It is used on large equipment every day. Every day, we design buildings with air handlers that are designed to be used in VAV applications. This is for the purpose of saving energy and investment money by using smaller equipment to satisfy only the simultaneous building load rather than the total building peak load. This allows us to take advantage of building diversity and save money both in first cost and in operating costs. What is new is that the ECM motor gives us the opportunity to improve the old way.
The old-style fan coil units using PSC motors cause the hotel to be conditioned with equipment sized for the total load; however, the room very rarely sees total load conditions. Having on/off control with 3-speed motors and 2-position valves does not allow the fan coils to address part load conditions. This new design allows for the room equipment (the fan coils) to address not only full load conditions, but also part load conditions where the room lives at least 85% of the year. The new design will meet the building design requirements under all operating conditions.
Once the ECM motor was available, equipment could be designed that would take advantage of the calculated load in the space. And because the motor efficiency and blower efficiency both increase as the motor rpm and airflow are decreased, the owner or user pays lower utility bills while the equipment provides a superior environment and lower sound levels.
Forward looking companies should require that new technologies be used when available and when they can improve the building so that their clientele, the occupants, are able to take advantage of lower energy costs, improved IAQ and lower sound levels. ECM motors and dynamic fan and valve control designed to regulate discharge air temperature certainly meet that criteria.