Having the ability to modulate various components of our HVAC systems (pumps, fans, valves, dampers, boilers, chillers, etc.) is a good thing. Advancements in various technologies over the years have allowed for increased levels of modulation in these components. Modulation allows for more stable control, energy savings, and extended equipment life expectancies. When troubleshooting or fine-tuning the automation of HVAC systems, I now carry a general suspicion there will be room for better modulation that will solve our known issues or improve the operation of the systems in some other way. I carry these suspicions because I’ve seen so many different oversights that hinder the ability to modulate HVAC systems. This article outlines a few such examples, where hindered modulation of a single HVAC component generated issues for the larger system as a whole.
A controlled device is a controlled component in an HVAC system. Supply or return fans at air-handling units (AHUs) are examples. I often find the minimum speed setting the fans are allowed to operate at configured inappropriately on the variable frequency drive (VFD). I had a supply fan’s minimum speed found at 35 hz (ASHRAE now recommends as low as 6 hz1) when we started investigating noise coming from a VAV box during a mild day. Nearly all served VAVs were at minimum airflow, and the supply fan could not slow down enough due to this inappropriately high minimum speed setting, resulting in static pressure in the supply duct being two-and-a-half times the set point. This resulted in VAV dampers throttling down excessively and forcing the air through a much narrower opening, which generated the noise we were investigating. It also meant controllability at the VAV boxes was poor, fan energy was being wasted, and the fan was operating within the surge region of its fan curve.