Wood fired tub conversion, it works!

Wood fired tub conversion, it works!

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In our home, we use both electric and wood heat. Therefore, I decided to convert the tub to wood-fired by constructing an outdoor fireplace/wood stove for it. I preferred not to have the stove inside the tub like some commercially available units, so I built my own stove with a heat exchanger inside. With a 250A MIG welder and basic fabrication equipment at home, I was able to accomplish this. The stove is located about six feet away from the deck, connected by foam-wrapped hoses running under the river rock.

I commissioned a laser cutter to fabricate the pieces for the stove body and door, which I welded together to create a 30x20x18 firebox. I supplied a sheet of 3/16" steel plate for the task, and they cut all the necessary parts for $100. It was a great deal! The door features strap hinges and a BBQ-style draft control with three 1-1/2 inch holes, along with a round bar door handle secured with a large plate washer that "camlocks" to the body.

The heat exchanger element was TIG welded at a friend's house using two 1-1/2" Sch 40 stainless pipes, with nine 1/2" Sch 40 stainless pipes spanning between them, each about 10 inches long. Imagine the shape of an "H" with nine horizontal pipes. Drilling the 18 holes of 7/8" on the sides of the large stainless pipes was the most challenging task; patience and plenty of oil were required. The unit features a plate flange that bolts into the stove from the inside, allowing for easy removal and servicing if necessary.

To ensure proper airflow, I placed some scrap square tubing lengths at the bottom. This setup allows air to circulate under the fire, creating a swirling effect that leads to clean burning. I checked with my neighbors about any smoke odor, and they were unaware that I was even burning wood.

Don't forget about the firebricks! I purchased enough firebricks from Mutual Materials to line the bottom and sides for about $45, and fortunately, the sides remain touchable without burning oneself even when fully open.

I bought two 4-foot sections of 8" Heatfab stainless chimney liner and a chimney cap kit for my stack.

During my initial test run, the pump circulated the water too quickly to effectively absorb significant heat from the unit. After three hours with the fire blazing, the water only reached around 80°F.

To address this, I inserted a 1/2" fender washer into the hose connecting to the exchanger unit to restrict the flow. The pump draws the water through the unit, eliminating the risk of a hose blowout from the restriction.

This modification worked nearly perfectly. If I turn the pump off for more than 15 seconds, the water begins to boil in the pipes, though the outlet temperature in the tub remains hot but safe. My second test with the restriction was encouraging, achieving approximately 100°F in two hours. With the draft halfway closed, it maintains the temperature very well.

Next time I drain the tub, I plan to install a 1-1/2" ball valve with a 1/2" hole drilled through it in the closed position. This will allow me to open the valve to increase flow to the jets once the water is heated to the desired temperature.

My appreciation for the energy required to heat water has grown significantly. This stove burns hot and clean, and it takes a full wheelbarrow load of firewood to heat the water. Once heated, it consumes about four sticks of wood per hour; I use smaller seasoned alder, about 4-5 inches in diameter. Thankfully, I have a free wood source, which is more economical compared to the five-hour heating time and approximately $20 cost associated with our kWh pricing each time I heat the tub.

Years ago, I purchased an old 350-400 gallon Apollo spa in fair condition for a very low price. It came in a rotting wooden enclosure, which I dismantled using a sledgehammer, then dropped it into my deck.