Air handler — upflow draw-through

For homeowners

A draw-through air handler has the blower mounted above the cooling coil rather than below it. This is the common configuration in humid climates like Florida — air enters at the bottom through the filter, passes UP through the evaporator coil (which removes humidity), is then drawn through the blower, passes through the heat strips, and exits the top as supply air.

The advantage of this layout is that the coil pulls moisture out of the airstream before the air reaches the blower wheel. The blower stays cleaner and the motor sits in dry air rather than damp air. In a climate where return air sits at 75% relative humidity for half the year, that matters.

The tradeoff is that condensate drainage requires careful planning — the drain pan sits low in the cabinet and the drain line needs proper slope to function. Algae growth in the drain trap is the most common cause of summer service calls in humid climates.

The heat strips being above the blower (downstream in airflow order) is intentional — moving air carries the heat away from the strips immediately and the thermal cutout in the strips acts as a safety against airflow failure.

For technicians

Configuration trade-offs compared to blow-through:

Condensate drainage gets harder, not easier. The pan sits low in the cabinet, which sounds convenient, but the primary drain line still needs a trap and a slope to gravity-drain. In a slab-on-grade Florida house with the air handler in a garage closet, that often means routing the drain through the wall to outside, to a laundry drain, or to a condensate pump. The trap is the part that fails — algae grows in it, the trap clogs, the pan backs up, the float kills the system. Every Florida HVAC tech has a wet/dry vac and a brush.

The heat strips at the top of the cabinet are downstream of the blower in airflow order. That’s not an accident. The strips have a thermal cutout that opens if they get too hot, and they will get too hot the moment airflow drops. Putting them downstream means a blower failure or a blocked filter immediately starves the strips of moving air, the thermal cutout trips, and the strips de-energize. Upstream placement would heat the blower wheel and motor — bad. Downstream placement uses the airflow as the safety.

The blower wheel pulling rather than pushing changes what you find when you open the unit on a service call. In blow-through configurations the wheel collects whatever the filter misses — dust cakes on the leading edge of every vane and chokes airflow over time. In draw-through, the wet coil acts as a second filter for anything that gets past the air filter, and the wheel stays significantly cleaner. The trade-off is the coil itself fouls faster, which is why coil cleanings are a routine Florida maintenance line item.

Secondary drain pan, not shown because it isn’t always present: beneath the entire air handler, with its own float switch wired in series with the primary. Attic installs require this by Florida code — the secondary catches anything that escapes the primary and shuts the system off before the bedroom ceiling becomes a fountain. Closet and garage installs sometimes have it, sometimes don’t, depending on era and inspector.

Drain pan maintenance is the single highest-frequency Florida service item. Twice-yearly: pull the cap, run a stiff brush through the trap, suck the line with a wet/dry vac from the outdoor termination, treat with algaecide tabs in the pan. Some installs have a cleanout tee on the drain line specifically for this — locate it during initial commissioning and label it.

The coil pulls condensate the entire cooling season. A 3-ton system in coastal Florida can pull 20+ gallons of water per day during peak humidity. That water has to go somewhere reliably, and “outside the wall” or “to a properly-sized condensate pump” are the only acceptable destinations. Drains routed to laundry waste lines run into venting issues; drains run to attic floors invade ceilings.