The HTH is a made of a single continuous rectangular steel tube (4″ × 2″, sections ~24″ long) formed into a C-shaped or vertical inverted-U configuration. These can be used in multiples to increase total heat output. It serves dual purposes:
- Structural grate: The lower horizontal section (and any cross-bars) supports the firewood exactly like a traditional andiron/grate. In the photos you can see 3–4 logs resting directly on or against the lower tube.
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Heat exchanger: Cool room air is forced through the interior of the tube by the high-output 90 mm muffin fan. The tube walls are heated by the fire via:
- Strong infrared radiation from the flames and glowing coals.
- Hot combustion-gas convection.
- Conduction through the steel wall (very low thermal resistance).
Heated air then exits the open end(s) of the tube into the living space as clean, smoke-free convective heat.
In the vertical orientation, the tall riser promotes natural buoyancy-driven flow even if the fan is off (useful in a power outage). With the fan running, forced convection multiplies output significantly. The modular design lets you combine multiple units side-by-side for larger fireplaces and more total heat output.
Estimated heat output with a well-established 3-log fire + fan
Sensible heat delivered by the air stream is given by the standard formula:
\[ Q \approx 1.08 \times \text{CFM} \times \Delta T \]
(where \(\Delta T\) is the air temperature rise in °F).
- \(Q \approx 75.6 \times \Delta T\) BTU/h.
Realistic sustained \(\Delta T\) values for a single-tube unit of this size sitting in a good 3-log fire (fire heat-release rate typically 20–40 kBTU/h total, much of which still goes up the chimney):
| Air ΔT (°F) | Approximate useful output | Notes |
|---|---|---|
| 150 | ~11,300 BTU/h | Conservative / moderate fire |
| 200 | ~15,100 BTU/h | Typical good performance |
| 250 | ~18,900 BTU/h | Strong fire, good tube exposure |
| 300–400 | 22,700–30,200 BTU/h | Peak / optimistic (air very hot) |
Most probable sustained range for this single modular unit: 12,000–20,000 BTU/h of useful room heat under a well-established 3-log fire.
This is lower than multi-tube commercial claims (35–70+ kBTU/h) because those usually have 4–10 parallel tubes and higher total airflow. This modular design scales nicely—two or three units side-by-side would push the higher end of the competitor range. Actual output varies with fire intensity, tube surface area exposed, fireplace draft, and how well the tubes are enveloped by the fire.
