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The Truth About Horizontal Coil Installs:

Why Coil Orientation Makes or Breaks Airflow
(Mike Tells You What Actually Matters Up There)

If you’ve ever crawled through a 140°F attic with a flashlight in your mouth, balancing on joists while trying not to fall through drywall, you already know:

Horizontal coil installs are a different beast.

They’re harder.
They’re tighter.
They’re hotter.
They’re more sensitive to mistakes.

And they WILL make or break the performance of your Goodman 3-ton R-32 system (GLXS4BA3610 + CHPTA4230C3 coil).

The truth is simple:

You can have the best equipment in the world and still get terrible cooling if the horizontal coil is installed wrong.

So today, we’re diving into:

airflow
drainage
coil pitch
static pressure
line set routing
TXV performance
attic heat load
and everything a contractor won’t tell you…

…because most people don’t even know horizontal installs have their own set of rules.

Let’s get into it — the Mike way.

1. Horizontal Coils Move Air Differently — And Most Installers Ignore That

Horizontal airflow is NOTHING like vertical airflow.

In a vertical coil, air moves down through the coil evenly.

In a horizontal coil, air moves across the coil — and if that coil isn’t positioned perfectly:

half the coil doesn’t get air
humidity removal drops
coil saturation becomes uneven
static pressure spikes
the TXV hunts like crazy
efficiency tanks

This is why airflow must follow design principles similar to those in the [Directional Air Distribution Engineering Note], which clearly states horizontal coils need consistent lateral airflow to maintain proper heat transfer.

Air wants to take the easiest path.
In a horizontal setup, that means it will bypass entire sections of the coil if the installation is sloppy.

2. Coil Pitch Is NOT Optional — It’s Mandatory for Drainage

Horizontal coils must be pitched ¼” to ½” toward the drain.

Why?

Because:

condensate MUST flow downhill
if it doesn’t, it pools in the pan
pooled water hits the coil
coil re-evaporates water into the airflow
humidity skyrockets
mold forms
pan overflows
ceiling gets soaked

The [EPA Approved HVAC Condensate Removal Guideline] is crystal-clear: improper coil pitch is one of the top causes of water damage in horizontal systems.

A perfectly level coil = a disaster waiting to happen.

A properly pitched coil = reliable drainage and longer coil life.

3. Horizontal Coils Need Wider Plenums — Or You Lose Half Your Airflow

You cannot cram a horizontal coil into a tiny plenum and expect perfect airflow.

You need:

wide transitions
smooth radius turns
no sudden restrictions
no directional turbulence
no bottlenecks

According to the [Residential Supply Plenum Transition Standard], horizontal coils require:

at least 1:3 transition ratio

for laminar airflow across the coil face.

If you violate that?

the center of the coil does all the work
the outer edges barely cool
coil temperature becomes uneven
TXV performance becomes unstable
efficiency falls off a cliff

This is one of the most common causes of “my 3-ton feels like a 2-ton.”

4. Horizontal Coils Create More Static Pressure Than Vertical Installs — Unless You Fix the Ducts

Horizontal coils have a wider face area but they also introduce more directional resistance.

Add in:

long flex runs
attic duct heat
poor return size
single return grilles
crushed insulation
tight furnace closets

…and static pressure skyrockets.

Static pressure is the #1 killer of:

airflow
SEER2 efficiency
coil performance
humidity removal
compressor life

This is why the [Air Handler External Static Pressure Performance Table] warns that horizontal applications need larger return ducts than vertical.

Horizontal coil = more resistance
More resistance = more blower work
More blower work = lower efficiency

Unless the ducts are upgraded, that 15.2 SEER2 system becomes a 10–12 SEER2 system in real use.

5. Horizontal Coils Must Be Installed With Correct Airflow Direction (Yes, It Matters)

There’s a reason every Goodman coil has an airflow arrow.

Horizontal airflow direction is essential because:

the coil structure is asymmetrical
the distributor tubes are arranged for specific airflow
the pan is sloped for one direction only
the TXV is positioned for optimal refrigerant distribution

Install it backward and you get:

water blowing into ducts
TXV hunting
20–30% capacity loss
high head pressure
uneven coil saturation
temperature swings
higher humidity

This is why the [Goodman Horizontal Coil Airflow Orientation Bulletin] specifically warns installers against flipping coils just to “make the duct fit.”

Orientation is not optional.

6. Attic Temperatures Destroy Horizontal Coils Faster Than Vertical Units

Horizontal coils are usually installed:

in attics
above garages
in crawlspaces
in tight mechanical closets

These areas get HOT.

When the coil cabinet heats up:

suction pressure changes
liquid line temp rises
saturation point shifts
TXV reacts aggressively
compressor amps rise

This is documented clearly in the [Attic Ambient Temperature Impact Study], which shows coil performance drops 12–18% when cabinet temps exceed 120°F.

Horizontal coils in hot attics MUST have:

insulated cabinets
insulated plenums
sealed seams
proper airflow
proper attic ventilation

Otherwise, they underperform — every single time.

7. Horizontal Coils Are More Sensitive to Line Set Routing

R-32 refrigerant is HIGHLY sensitive to:

line set slope
vertical lift
horizontal run length
trap placement
heat exposure
brazing technique

And horizontal coils usually have more complicated routing because they’re in tight attic corners or crawlspaces.

Per the [Refrigerant Piping Best Practices for A2L Systems], improper routing can cause:

refrigerant migration
oil pooling
TXV starvation
compressor overheating
throttling noise

A badly routed line set can drop capacity by 10–20%.

Installers rarely admit this — but I’ve fixed too many bad installs to ignore it.

8. Horizontal Coils Need PERFECT Air Sealing — Or You Lose 10–30% Cooling

When air leaks around the coil cabinet:

hot attic air enters the system
return air pulls in dusty air
humidity spikes
static pressure rises
temperature drop across the coil shrinks
energy efficiency dies

The [Residential Air Leakage & Return Infiltration Assessment] shows horizontal coils lose more efficiency from leaks than vertical coils because: