The question we get most often from contractors is not “which coil is better” but “why is there a cabinet at all”. The cabinet is the whole point. A cased coil and an uncased coil can contain comparable coil surface and still behave completely differently in the same basement, because one of them arrives with its air path, its drain pan and its structural connection already resolved.
Here is how we explain the difference to distributors, and where each one actually wins.
The physical difference
Cased coil. The coil, drain pan and metering device are assembled inside a sheet-metal cabinet with formed flanges. The cabinet is the interface: it bolts to the furnace outlet and accepts plenum or duct connections. Insulation and the air seal are part of the assembly.
Uncased coil. The same coil assembly, supplied without that cabinet. It is installed into a field-fabricated cabinet, an existing transition, or into equipment that provides its own enclosure — an air handler, or an OEM chassis where the cabinet is part of the larger unit design.
That is the entire difference in principle. In practice it drives everything that follows.
Side by side
| Consideration | Cased coil | Uncased coil |
|---|---|---|
| Cabinet | Supplied, factory-formed flanges | None — field-built or host unit provides it |
| Air seal at the coil | Determined at the factory | Determined on site |
| Drain pan | Factory-fitted, with a defined slope | Factory coil pan, but pan position set on site |
| Duct connection | Bolt to furnace, plenum to flange | Transition must be fabricated and sealed |
| Installation time | Lower in a like-for-like replacement | Higher; depends on the fabricator |
| Skill dependence | Lower | Higher — sealing and slope are on the installer |
| Fit in tight spaces | Constrained by cabinet dimensions | Can be adapted to unusual spaces |
| Service access | Designed access panels | Wherever the fabricator put them |
| Multi-position use | Casing options for retrofit and new-build | Position set by the assembly |
| Stocking | One part number per size and position | Fewer parts, more site work |
| Best fit | Residential replacement and new build | OEM programmes, air handlers, tight or unusual cavities |
Where the cased coil wins
Residential replacement, almost always. When a contractor pulls a failed coil out of a furnace in a finished basement, the goal is a predictable outcome in a short window. A cased coil arrives with the air seal, the pan slope, the flange geometry and the drain connections already decided. The remaining variables — refrigerant circuit, control wiring, condensate routing — are the ones the technician is equipped to handle on the spot.
Cased coils also protect the distributor. When two contractors install the same cased part in the same furnace model, they produce two installations that are at least geometrically identical. With uncased coils, the same part produces two different installations, and the one that leaks air or holds water comes back as a warranty claim with no clear answer.
Our A-Coil family is offered cased, with multi-position casing options for retrofit and new-build, and with custom dimensions, circuiting and casing when a standard size genuinely does not fit.
Where the uncased coil wins
Three situations, and they are all legitimate.
OEM and private-label programmes. If the coil is going inside a unit that already has a cabinet — an air handler, a packaged unit, a custom chassis — adding a second cabinet is waste. The uncased coil is the right component, and the enclosure, airflow path and service access are the OEM’s design responsibility.
Air handlers with an integral coil section. Where the host unit provides the enclosure and the drain pan arrangement, an uncased coil is the compatible part.
Tight or non-standard cavities. In older housing stock, the space above a furnace may be an odd shape with a flue running through it. A field-fabricated transition around an uncased coil can solve a problem that no standard cabinet will fit. The trade-off is that the sealing and slope quality is now a skills question.
Our engineering team quotes custom coil configurations for OEM work — dimensions, circuiting and casing — because that is where a coil stops being a catalogue item and becomes a specification.
The checklist that decides it
Run this before you specify either type:
- Measure the opening at the furnace outlet and the available height to the plenum.
- Confirm the required coil size for the system capacity, and check the airflow the system will actually deliver.
- Confirm the refrigerant the system will use — R-410A, R-32 and R-454B are not interchangeable, and a coil’s rating belongs to a refrigerant.
- Decide the position: upflow, downflow, horizontal, or a retrofit position that changes.
- Choose casing type based on who is doing the site work and how much time they have.
- Verify the drain pan access and the slope direction relative to the condensate route.
- Verify service access: can the coil be cleaned and the metering device reached without cutting the plenum?
- Confirm the matched-system documentation for the combination you are about to install.
- Confirm the Canadian certification mark on the equipment nameplate — the “c” prefix version or a combined mark — and the NRCan registration status where applicable.
Mistakes we see on both sides
- Uncased coil in a field-made box with a flat bottom. Water sits in the pan. The pan needs a slope, and the drain needs a trap with enough height to work against the fan’s static pressure.
- Cased coil used to solve a ducting problem. If the ductwork is undersized, a cabinet does not fix it. Static pressure is a system property.
- Coil changed without changing the metering device. The device has to suit the outdoor unit and the refrigerant.
- A coil specified for the wrong position. A coil installed at a different orientation than intended drains differently.
- Assumed interchangeability between refrigerants. A coil rated with one refrigerant does not become a coil rated with another because the fittings match.
What to ask us
Send the furnace model, the outdoor unit model, the capacity, the refrigerant, the position, and the space you actually have. We will tell you the coil size, whether a standard casing fits, and whether a custom casing or an uncased configuration is the honest answer.
Do not accept a coil recommendation that comes with a capacity number but no test conditions, and do not accept a claim that a combination is “the same as” a listed one. If it is not listed, treat it as unmatched and say so on the paperwork.
Start with the A-Coil family page for the sizes and configurations we build, then the air-conditioning solutions hub for how a coil is specified alongside the outdoor unit and the air handler. When you have a project with a real space constraint, send the dimensions through the RFQ form — custom coil work starts with a drawing, not a catalogue number.