Cozy Heat O Thru-Wall Wood-Burning Sauna Stove – 12 & 18 kW

A clean round stove inside. A fire door in the adjoining room. The O Thru-Wall keeps the cylindrical stone cage in view and moves the loading door through the wall. That is a practical fit when you want a softer silhouette and a sauna floor free of the active woodpile.

Cozy builds structural-steel firebox walls about 67% thicker than a typical standard-stove wall, then joins them with clean, precise welds. It is made for years of real fires, not dressed up for a showroom.

Get the room and wall opening right together

On a phone, swipe the table sideways to compare every column.

Use finished dimensions. Add about 4 cu ft per square foot of glass, tile, brick, concrete, or other uninsulated surface. Multiply a massive-log room by 1.5. Between sizes? Call before ordering.

Put the fire door on the other side

The firebox extension passes through an opening in a concrete or brick wall so wood and ash stay in the adjoining room. Cozy’s manual (section 2.5, Models COZY TW) sets the opening and the wall:

• 12 kW: opening at least 10.0 in wide and 17.3 in tall, measured from the floor (255 x 440 mm).
• 18 kW: opening at least 12.4 in wide and 18.5 in tall, measured from the floor (315 x 470 mm).
• Wall thickness: 5.9 in (150 mm) maximum. The through-wall extension adds 5.9 in (150 mm) to the stove depth.
• Non-combustible zone: keep combustible material at least 23.7 in (600 mm) from the sides of the opening and 37.8 in (960 mm) above it, so the concrete or brick has to extend at least that far around the opening.
• Ventilation: with a through-wall model, Cozy wants the sauna’s exhaust vent near the floor on the wall opposite the intake, kept open while bathing.

High efficiency without a fussy fire

Combustion air enters in three paths, including secondary air above the fuel. Cozy says the design uses about 25% less wood than comparable heaters; 40% occurred only in ideal tests. The manual reports 73.9% efficiency for the 12 family and 81.3% for the 18 family. The practical difference is just as important: load dry wood, light the kindling, regulate the air, and add a couple of pieces as needed. There is no specialist firing routine to learn.

The O Thru-Wall carries 243–353 lb of exposed stone. That reserve stores heat and gives water a broad surface for steam, but it is a specification to plan around rather than the reason to choose the stove.

Before the stove arrives

• Minimum side clearance to combustible material: 6.7 in for 12 kW; 9.1 in for 18 kW.
• Rear clearance: 9.1 in. Front clearance: 19.7 in.
• Clearance above the stove: 44.5 in for 12 kW; 52.8 in for 18 kW.
• Required chimney draft: 12 Pa. Minimum smoke-flue cross-section: 12.2 sq in.
• Stones, chimney, and any required floor or wall protection are sold separately.

Cozy builds these steel heaters in its Estonian facility. The U.S. warranty is 24 months. European EN 15821 and BImSchV Stage 2 documents do not replace local code review. The current manual and exact drawings below control installation and stone loading.

Want us to check the room math or chimney plan? Call or text (844) 565-1113 with the finished dimensions.