Warm air in a tall building does not stay where you put it. Twenty minutes after a unit heater kicks on, most of what it produced is sitting in a layer up at the roof, and the crew on the floor is still cold. That gap between floor temperature and roof temperature is heat stratification, and through a Northeast Ohio winter it adds cost every hour the building is occupied.
The spread is wider than most people guess
In a building with 30 foot ceilings and forced air heat, the difference between floor level and the roof deck commonly runs 15 to 30 degrees. Ceiling height drives most of it. Heater type, roof insulation and the outside temperature do the rest. A radiant system stratifies less than a gas fired unit heater blowing warm air up into open space, though any building tall enough to need a lift has some version of this going on.
The thermostat is standing in the wrong air
Here is the part that costs money. A thermostat gets mounted at four or five feet because that is where people can reach it, which means it reads the coldest air in the building. It calls for heat, the burner runs, the new warm air climbs to the roof, and the sensor still reads cold. The warmest air in the building ends up thirty feet above the device that is supposed to be managing it.
A hot roof deck pushes more heat outside
Heat moves through a roof in proportion to the temperature difference across it. On a 25 degree January day, a roof with 88 degree air pressed against it is working against a 63 degree difference. Mix that air back down so the deck sees 70 degrees and the difference falls to 45. Same roof, same insulation, same weather outside, and the building gives up heat more slowly.
Cleveland buildings make it worse
Cleveland runs roughly 6,000 heating degree days a year and the heating season here stretches from October into April. That is a long stretch to spend warming air nobody is standing in.
The older brick and timber stock on the near west side and through the Flats usually pairs a high bay with thin roof insulation, so the loss through the deck is severe even where ceilings are moderate. Newer tilt up distribution space in Solon, Twinsburg and out along I-480 has a much better envelope but ceilings at 32 to 40 feet, which builds a deeper column of trapped heat above the racking. Both end up in the same place for different reasons.
Checking your own building first
Before anyone quotes you a fan, get a number out of the building. An infrared gun will not give it to you, because it reads surface temperature and what matters here is the temperature of the air. Use a probe thermometer or a small logger on the end of a pole, or tape one to the basket of the lift the next time somebody goes up for lamps or sprinkler work.
Take a reading around four feet, one at fifteen, and one as close to the deck as you can safely get, all on the same trip, with the heat running and the doors shut. Mid morning on a cold day tells you more than first thing in the morning, because the heating system has had time to work. Keep the numbers. Repeat them after a fan is in and you have a before and after to put in front of whoever signs for it.
Running the fan through the winter
An HVLS fan handles this by turning the whole volume of air over slowly. At the low speeds used for winter mixing the column coming down is a gentle exchange rather than a draft, and nobody working underneath feels wind. Air moves down through the middle of the coverage area, out across the floor, up the walls and back to the fan, and it keeps doing that for as long as the heat is on.
Power draw stays low. A Humongous Fan pulls 1.5 kW at full speed, against 8 to 10 kW for the conventional units it offsets, and winter destratification runs well below full speed. Coverage reaches 22,000 square feet from one fan.
What changes
Once the air is mixed, the floor to ceiling spread drops to 2 to 5 degrees. The thermostat finally reads the same air everyone else is standing in, the burner satisfies sooner and it cycles less often. We put the heating reduction at 20 to 30 percent, with typical payback on the installed cost landing between 12 and 24 months.
Where people get it wrong
Running it too fast in winter is the usual one. Winter mixing wants a slow turnover of the whole room, and full speed in February earns complaints about drafts before it earns anything else.
Undersizing is the other. Two small fans chasing the footprint that one large fan would have covered move less air for more money and more mounting points. Diameter is what buys coverage.
Mounting height gets overlooked as well. Hung too close to the deck, a fan never develops the column it is supposed to be pushing down. Hung too low, it stops clearing racking and lighting. There is a window for it and the building decides where that window sits.
Sizing one for your building
Ceiling height sets the diameter. Bay layout, racking and whatever hangs from the structure set the mounting points and how many fans a building needs. We build HVLS fans from 8 to 24 feet across here in Cleveland, and the sizing questions do not change much between a machine shop in Bedford and a distribution center in Streetsboro.
There is more on the physics and on measuring your own building in air destratification and temperature control. To put numbers against your own square footage, ceiling height and utility rate, run them through the energy savings calculator. If you would rather have us look at it, tell us about the building and we will size a fan for it.
