GPOAS 12x12 Heat Exchanger Water To Air, Forced Air Heating with 1" Copper Ports Hot Water Coil for Outdoor Wood Furnaces, Residential Heating

Why I chose this coil

I needed a compact, water-to-air coil to add hydronic heat to a forced-air system without rebuilding my whole plenum. The GPOAS 12x12 coil looked like the right mix of size, materials, and straightforward installation. I’ve run it off an outdoor wood boiler and also pulled a few experiments with chilled water to see how it would behave for shoulder-season cooling and dehumidification. After a full install and a couple of weeks of real use, here’s how it performed.

Build and materials

The 12x12 coil uses three rows of 3/8-inch seamless copper tubes and a fin pack that’s 12 aluminum fins per inch. That’s a solid, middle-of-the-road fin density for residential air handlers: dense enough to transfer heat efficiently, but not so tight that it clogs instantly or strangles airflow. The case is steel and feels stiff enough to protect the core. All the joints and contact points are brazed, which matters if you’re running higher temps and pressures from a wood boiler or pressurized hydronic loop.

A detail I appreciate: four 1-inch MNPT ports. Ports on both sides give you flexibility to pipe from the left or right depending on your plenum and service access, then plug the unused pair. I used brass plugs with PTFE tape and thread sealant. The copper had a few water marks out of the box; that’s normal from quenching during manufacturing and doesn’t affect performance.

Fins arrive straight, but like any coil, they’ll bend if the package takes a hit. Treat the fin pack with care during handling and fit-up. I keep a small fin comb in the truck; it’s worth having.

Installation notes

I slid the coil into the supply plenum just downstream of the air handler. The 12x12 core is compact, but measure your actual case dimensions and leave room for service. I framed a cutout with 1-inch angle, added a simple support rail so the coil doesn’t sit on the fins, and sealed the perimeter with foil tape and mastic to prevent bypass air.

Piping is straightforward. I ran 1-inch oxygen-barrier PEX from the primary loop to the coil with full-port ball valves and unions for service. A purge tee at the high point made bleeding easy. If your boiler or buffer tank runs hot (170–190°F), I recommend:

• A mixing valve or injection pump control to temper supply when needed
• Isolation valves on both supply/return
• A strapped-on aquastat that locks out the pump if the air handler isn’t running to avoid overheating the plenum

If you plan to use it for chilled water sometimes, add a drain pan and trap below the coil, because this unit doesn’t include an integrated pan or drain. Insulate the supply/return lines to prevent sweating.

One caution: 1-inch MNPT threads are chunky. Don’t over-torque into the copper headers; use two wrenches and support the header so you’re not twisting the core.

Performance in heating

With 180°F supply water, about 3 gpm on the water side, and roughly 1,000 CFM across the coil, I consistently saw a 30–35°F air temperature rise. Translating that:

• Air side: 1.08 × 1,000 CFM × 34°F ≈ 36,700 BTU/h
• Water side: 500 × 3 gpm × 20°F ΔT ≈ 30,000 BTU/h (typical ΔT under these conditions)

Those numbers line up well for a 12x12, 3-row coil. If you push more airflow or hotter water, you’ll get more capacity; if you drop to lower water temps (say, 140°F from a condensing source), expect proportionally less. Balance matters: too much water flow with too little air can make the coil sizzling hot but not transfer that heat efficiently to the airstream.

Pressure drop across the coil was modest in my system; the blower didn’t need a speed bump, but if you have a marginal fan, check total static. The 12 FPI fin density is a nice compromise for airflow.

Subjectively, it delivers a comfortable, even heat under continuous fan operation. No pinging or ticking, and no odors beyond the first warm-up of the metal.

A quick test in cooling/dehumidification

I ran a short chilled-water trial using a small recirculating chiller and a pan underneath. With 45–50°F water and a box fan mounted to the face, it pulled moisture effectively and took the edge off a hot, humid room, though the 12x12 face limits airflow compared to a full air handler coil. If you plan to use this for regular cooling:

• Provide a proper drain pan and trap
• Insulate the plenum section
• Expect lower capacity than a purpose-built A-coil of larger face area

It’s a handy hybrid option if you’re already moving water, but not a replacement for a standard AC coil if cooling is your main goal.

Efficiency and control

The coil itself draws no power; the system’s efficiency comes down to your pump curve, blower watt draw, and control strategy. I had good results with:

• Continuous low-speed fan during calls for heat to reduce stratification
• Outdoor reset on the boiler to lower water temp in mild weather
• A simple on/off pump tied to a call for heat with a fan interlock

This setup maintained steady comfort without short, high-temperature blasts.

Maintenance and longevity

• Keep a filter upstream and change it regularly. Twelve fins per inch will accumulate dust over time.
• Flush the hydronic side at season change if your water quality is questionable. Use inhibited glycol if freeze risk exists.
• Ensure your loop has proper air elimination; a gurgling coil transfers less heat and can corrode over time.
• If you ever do bend a patch of fins, a fin comb restores airflow quickly.

The brazed construction and copper/aluminum combo are standard for durability. The steel case is solid enough for residential use. I’d expect a long service life if water chemistry is maintained.

What could be better

• Packaging protection: The fin pack is vulnerable if the box takes a hit. A cardboard face guard or foam spacer would inspire more confidence. Handle with care on arrival.
• Documentation: A simple one-pager with recommended flow rates, typical pressure drops, and example piping diagrams would help homeowners and pros alike.
• Drain pan: No pan or brackets are included. That’s normal for plenum coils, but if you intend to cool, you’ll need to fabricate a proper condensate solution.

Who it’s for

• Hydronic homeowners with outdoor wood boilers who want to add forced-air distribution without a big air handler swap
• Shops and garages needing a tough, compact coil that can deliver steady heat with a modest blower
• Hybrid tinkerers pairing a small chiller or cold well loop for occasional dehumidification or spot cooling

If you need large-capacity whole-home cooling, jump to a bigger face area coil designed for AC. If your existing blower is underpowered and static pressure is already high, confirm that adding a coil won’t tip you over the edge.

Practical tips from the install

• Use unions and full-port valves at the coil for easy service
• Plug the unused ports with brass plugs; seal threads, don’t over-torque
• Support the coil; don’t let it rest on fins or hang from piping
• Seal every bypass gap to force air through the fin pack
• Verify air and water temperatures with probes on first fire to dial in flow and comfort

Bottom line

As a compact hydronic-to-air solution, the GPOAS 12x12 coil hits the right notes: copper tubes, a sensible fin density, a sturdy steel case, and flexible 1-inch ports that make piping clean. In my installation it delivered roughly 30–40k BTU/h under typical boiler temps with comfortable, even heat and manageable static pressure. It’s simple to fit, easy to service if you plan ahead with valves and unions, and tough enough for shop or garage duty.

Recommendation: I recommend this coil for anyone adding forced-air heat to a hydronic system—especially outdoor wood boiler owners—who needs a compact, efficient, and serviceable coil. It’s well built for the price, performs predictably with standard controls, and rewards a careful install with steady, comfortable heat. Just mind the packaging on arrival, add a proper drain plan if you’ll run chilled water, and give your blower and filter a quick reality check before you button up the plenum.