The Bane
What a waste. Air is cooled, then heated. Or, it’s heated, then cooled. In air handlers, that’s simultaneous heating and cooling. What’s the point of this dance? It’s like a bad way to barbeque–bringing burgers from the fridge, icing them in the freezer, and then tossing them frozen on the grill. You use energy to cool the meat, then you use energy to heat them up. What’s the point? Better see some inedible examples.
The Scenarios
A building in Someville, Somewhere, notorious for its energy waste. It’s the middle of January; the building needs more heat. I trudge to the mechanical room for adjustment, just to realize our air handling unit is… calling for cooling. With the discharge setpoint at 55°F, which was needed to cool the building during last summer’s hot spell, the mixed air is cooled to that setpoint using excess outside air. I hear the 55°F supply air rumble down the ducts toward the terminal reheats. I sense the boiler funneling hot water to the boxes; must be reheating to 90°F. If the discharge temperature was set appropriately, the unit would be drawing minimum outside air for a mixed air temperature of 60°F. What a waste. Now we have to heat an additional 5°F. From my calculations, this additional heating uses 1,200 therms a year at 80% thermal efficiency and 15,000 CFM. That’s spent gas worth ~$1,300/year.
Six months later in the soggy summer season, I trek to another building in Someville, Somewhereelse. It’s a typical heat pump system with an outdoor air makeup unit. A contractor claims effective dehumidification, but my gut told me there was waste. As he departs the grounds, a paper slips from his pocket into my view (see below).
Scribbled beneath this graphic: “to dehumidify, you must decrease the dew point to remove water vapor.” Fancy language for a controls engineer. I expose the code of the energy management system and spot the supply setpoint of 70°F from winter has yet to be changed. Spaces beg for average 60°F air. Mixed air comes from the cooling coil at 54°F for moisture removal, and is heated to 70°F. Note here that there is 10°F of overheating on average, and then, 10°F cooling must be extracted again by the heat pumps to provide comfort conditions. What a waste. The heating uses an additional 2,100 therms a year at 80% thermal efficiency and 15,000 CFM. The cooling uses 12,000 kWh a year at 13 EER. That’s spent energy worth ~$3,000/year.
The Solutions
“Don’t fix it unless it’s broken.” Mother’s maxims are usually right, but this time, I’m not listening. Air handling units may cool great in one season, but gas bills reveal inappropriate setpoints as the season changes. Spot such setpoints beforehand and make the fix.
“Learn by example.” I know, Mother, I’ll give examples. In the summer, reset discharge setpoints to 55°F only when needed to dehumidify. Leave the air handler heating coil off–let the terminal reheats do the work. In the winter, reset discharge setpoints to 65°F or 70°F to prevent any mechanical or natural cooling–remember, dehumidification does not apply in the winter.
“That’s smart money.” That’s right, Mother. Solving simultaneous heating and cooling can garner estimated savings as shown in the table.
The Endeavor
Distressing. So many buildings fall into this style of disrepair. Waste persists and we’re oblivious to its persistence… until the bills arrive. I hope mankind does its part to rally against this energy foe.
I’ll write again when new stratagems for waste come about. For now, hunger calls. I’m off to grill burgers. The right way.
