It’s without question that technology has improved the mechanical world around us. From my smartphone, I can instantly see the health of hundreds of our boiler plants across the country. A building management system (BMS) can sense occupancy to control everything from lighting to the snowmelt systems. Water heaters can read the voltage through their powered anode rods to calculate the life left on the tank. While technological advances like these have allowed us to bend the rules of mechanical designs, we can’t fall into the trap of using technology to skate around fundamental engineering practices. 

This leads me to the discussion of turndown. Turndown is simply the operational range of a device, i.e. maximum capacity divided by minimum capacity. A box fan that spins at 50% on its lowest speed has a 2:1 turndown (1 ÷ 0.5 = 2). For decades, a two-stage heating system able to turn down to 50% was considered “high-tech” because most were simply on/off. In fact, last year I upgraded my home’s HVAC and installed one of my company’s top-of-the-line 98% AFUE forced air natural gas furnaces with modulating 2.5:1 turndown, and it’s been amazing. On low fire and low fan speed, it’s whisper quiet. On shoulder season days, it cycles two or three times per hour, well within the six-cycle per-hour rule of thumb. It’s important to note my home’s R-value did not change, and the furnace is sized properly. Figure 1 compares the turndown ratio for five separate boilers. You can see the difference only affects the bottom end, and while a 10:1 to 25:1 sounds like a dramatic increase, when you do the math, it’s only a difference of 10% minimum fire vs. 4% minimum fire.