Commercial dehumidification and indoor air quality

The dew point problem in Middle Tennessee

Temperature is the number people feel, but dew point is the number that tells you how much water is in the air. At Nashville's airport, the July dew point averages about 68°F, and during the stickiest tenth of July hours it averages about 73°F (NOAA 1991–2020 hourly normals, BNA). Davidson County sits in ASHRAE climate zone 4A, a mixed-humid climate (OpenEI). Every cubic foot of that air brought into a building has to give up its water on a cold coil before it feels comfortable.

That is why a building can sit at 72°F and still feel clammy. A cooling coil removes moisture only while it is cold and air is moving slowly enough across it. An oversized rooftop unit reaches its temperature setpoint quickly, shuts off, and leaves most of the water in the air. A fan left in the "on" position blows the water still sitting on the coil straight back into the space. The symptoms are sweating supply grilles, damp ceiling tiles, musty smells, condensation on cold water piping and doors that swell in summer.

Dehumidification equipment we service

Some buildings handle moisture with the rooftop units they already have, using a hot gas reheat coil that lets the unit keep dehumidifying after the space is cool enough. Others use dedicated dehumidifiers: compressor-based units that pull air across a cold coil to wring out water, then reheat it with the heat they just removed. Warehouses, archives, food plants and pool rooms each end up with a different mix. On all of them we check the compressor and refrigerant circuit, the reheat coil and its valve, airflow and filters, condensate drains and traps, and the humidity sensor that decides when the whole thing runs.

The opposite problem shows up in winter. Heated outdoor air is dry, and some rooms need moisture added: IT rooms, labs, printing and storage spaces for paper and wood. We service steam canister, electrode and electric resistance humidifiers, where the usual enemies are mineral scale, a failed fill or drain valve, and a sensor that has drifted.

Humidity and IAQ faults we see

What you see	What is usually behind it
Cool but clammy space	An oversized unit short cycling, fan set to run continuously with the compressor off, reheat that has failed, or too much untreated outdoor air.
Sweating grilles, diffusers or cold pipes	Space dew point above the surface temperature: humidity control is not keeping up, or insulation on cold piping is damaged.
Pool room windows fogging, metal rusting	A pool dehumidifier down or undersized for the evaporation, an exhaust fan that stopped, or poor air movement across the glass.
Humidifier alarm or no steam	A scaled canister or element, a failed fill valve, a drain that will not open, or a lost signal from the humidity sensor.
Makeup air unit blowing cold air	A burner that will not light: flame sensor, igniter, gas valve, airflow switch or a tripped high limit.
Doors hard to open, smoke or odors hanging	Exhaust and makeup air out of balance: a broken belt, a failed motor or bearings, or a makeup air unit that is off while exhaust keeps running.

Indoor pool (natatorium) HVAC

An indoor pool is the hardest humidity job most buildings have. The water evaporates all day and all night, the air carries chlorine compounds that attack steel and aluminum, and the room is warm enough that the glass and the roof deck are the coldest surfaces in the building. Pool dehumidifiers are built for that: they pull the moisture out with a refrigeration circuit and put the recovered heat back into the room air, and often into the pool water too. Design guidance generally keeps the room air a couple of degrees above the water temperature and relative humidity somewhere around 50 to 60 percent, which limits evaporation and keeps the room from feeling cold to a wet swimmer.

When that equipment struggles, the building shows it quickly: fogged windows, dripping ceilings, rusty door hardware, corroded hangers and a chemical smell that never clears. We service the compressor and refrigerant side, the coils (which corrode faster in pool air than anywhere else), the fans, the controls and the exhaust that keeps the pool room slightly negative to the rest of the building.

Makeup air units and exhaust fans

Every cubic foot a building exhausts has to come back in somewhere. Kitchens, paint and welding areas, restrooms and process exhaust pull air out, and makeup air units replace it, tempered with gas heat in winter and sometimes cooled in summer. When the makeup air unit quits and the exhaust keeps running, doors whistle and stick, flues can backdraft and the building pulls hot, wet air in through every crack. We service both halves together: exhaust fan belts, bearings, motors and drives, and makeup air burners, flame safeguards, gas trains, filters and airflow switches. Makeup air units and exhaust fans belong on a maintenance agreement alongside the rest of the building, and when one stops mid-season it is a standard commercial HVAC repair call. We cover these systems throughout Nashville and the other towns on our service area list.

Humidity and IAQ questions