DIY Fireplace Heat Exchanger
Step-by-step guide made from Indy Farm Life's YouTube video.
A custom heat exchanger for a 42-inch fireplace, built from 1.5-inch black steel pipe. This system captures wasted heat and uses an inline fan to blow it back into the room, increasing the fireplace's efficiency and heat output.
- Difficulty: intermediate
- Finished size: Fits a 42-inch fireplace; manifold is approx. 24-26 inches wide
Disclaimer
- Fireplaces, heat exchangers, and combustion systems can be dangerous. Do NOT copy this build without understanding local codes, safety requirements, and proper materials. Never use galvanized pipe or toxic materials.
Safety
- Do not use galvanized pipe, as it can release toxic zinc fumes when heated to high temperatures.
- Do not use any pipe dope or thread sealant on the fittings inside the firebox. It will burn off and create unpleasant or potentially harmful fumes.
- This project involves a live fire. Build and operate at your own risk. Always have a fire extinguisher nearby and a carbon monoxide detector in the room.
- The outlet pipe will become extremely hot. Ensure it is positioned where it cannot be accidentally touched, especially by children or pets.
Must know before you build
- Never use galvanized pipe; it releases toxic zinc fumes when heated.
- Avoid pipe dope/sealant on firebox fittings; it burns off, creating fumes.
- Minimize 90-degree elbows; too many restrict airflow, reducing efficiency.
- Use a strong fan; a weak fan won't push enough air for effective heat transfer.
- Elevate pipes above coals with a tall grate to prevent deformation and failure.
- Source intake air from outside to prevent negative room pressure and cold drafts.
Tools
- Inline Fan (4 inch)
- Fan Speed Controller (Rheostat) (optional)
- Pipe Wrenches
- Infrared Thermometer (VEVOR) (optional)
- Leaf Blower (Cordless, DeWalt) (optional)
Materials
- Black Gas Pipe (1.5 inch diameter)
- Black Pipe Fittings (multiple, black gas pipe fittings, 1.5 inch)
- Flexible Ductwork (4 inch diameter)
- Fernco Couplers (2, rubber coupler, Fernco, 4 inch)
- Firewood
- Fireplace Grate (1, Heavy-duty)
Step by step
Step 1: Preparing and Lighting the Fire
The heavy-duty fireplace grate is placed over the heat exchanger pipes, and a fire is started using a gas log lighter and kindling.
- Position the Fireplace Grate. Place the heavy-duty grate into the fireplace, positioning it next to the heat exchanger pipes.
- Tip: Using a tall, heavy-duty grate elevates the fire, which protects the heat exchanger pipes from direct, prolonged contact with the hottest coals and improves their lifespan.
- Light the Fire. Use a gas log lighter or other fire starter to ignite your kindling and establish a fire on the grate.
Step 2: Initial Performance Test
With the fire just getting started, an infrared thermometer is used to measure the baseline temperatures of the pipes and the air coming out of the exchanger.
- Measure Pipe Temperatures. Use an infrared thermometer to measure the surface temperature of various points on the pipe system inside the firebox.
- Measure Air Output Temperature. Turn on the inline fan and use the thermometer to measure the temperature of the air being blown out of the exchanger's outlet pipe.
Step 3: Testing After 30 Minutes
After the fire has been burning for 30-40 minutes and has a good coal bed, the temperatures are measured again, showing a significant increase in heat output.
- Re-Measure Pipe Temperatures. With the fire now much hotter, use the infrared thermometer to re-measure the surface temperatures of the pipes inside the firebox.
- Measure Hot Air Output. Measure the temperature of the air exiting the outlet pipe. In the video, the temperature has increased to over 225°F.
Step 4: Modifying the Output Nozzle
To prevent the hot air from being drawn back into the fire, a 90-degree elbow and a short pipe are attached to the outlet to direct the airflow upwards.
- Attach Elbow and Extension Pipe. Carefully thread a 90-degree elbow followed by a short length of pipe onto the hot air outlet. Use gloves as the pipe is hot.
- Watch out: Make sure you use gloves as the pipe is very hot
Step 5: High-Airflow Test with Leaf Blower
To demonstrate the impact of airflow, a leaf blower is used to force a high volume of air through the system, resulting in a massive increase in heat output.
- Connect the Leaf Blower. With the fire burned down to a hot coal bed, connect a leaf blower to the intake of the heat exchanger system.
- Measure Maximum Temperature Output. Turn on the leaf blower and measure the air temperature at the outlet. In the video, the increased airflow pushes the temperature to over 360°F.