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RMH water BOILER for hydronic radian floor heat

 
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Has anyone designed a RMH to include a water boiler and how do you retain the RMH design without taking away too much heat?
 
pollinator
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You need to extract the heat after the combustion zone, so that the temperature of the burn chamber is not affected. I'm sure it has been attempted before. I personally wouldn't go down that route.

Partly because the RMH design is aimed around high thermal mass and slow release of heat, and hydronic heating systems tend to be designed about rapid response to temperature changes in the building, then rapid distribution of that heat to the final heat distribution system. Eg - turn a boiler on for ten minutes to get the floor to the right temperature, then turn it off. If you don't turn turn the boiler off what happens to that heat in the system? If you simply shut off the flow valves you still have water in the system but now potentially trapped in pipes and boiling. With a RMH you can't turn it off once it is up to temperature, so I suspect you will end up over heating your floor.

Partly, mixing water and fire is a recipe for DIY bomb making. I'm sure it is possible to design a safe system, but the learning curve is steep and dangerous.

Can you find a way to duct the exhaust air from your RMH directly under the floor?

 
Rocket Scientist
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First off, you wouldn't want a boiler for in-floor radiant heat, just hot water.

There are many variables to consider, and a concise answer is not possible without narrowing the discussion space a bit. Are you talking about new construction, or retrofitting? Can the RMH be in part of the space to be heated? How severe is the climate? How constant is the need for heating when it is needed?
 
gardener
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I've seen a few different RMHs used to heat water (for a shower system) and the key to me would be to have an open system, not pressurized. A pressurized system is just one failed pressure valve away from BOOM-SQUISH (death due to explosive force and/or steam scalding). Then run your water lines through that heated water, keeping the water in the lines totally separate from the water you heat directly (sort of like a double-boiler?). But if the water-flow in the pipe stops while you still have an active fire going, then that section of pipe in the tank will be heated up quite a bit, and I'm unsure how you manage that heat buildup. That seems like another single point of failure, if that pump fails. I expect a person would try to automate the wood feed or at least use a batch box, and that automated or unattended wood burn could lead to the water being overheated.

Wheaton Labs has a water heater where the J tube heat riser sits under a water tank and heats that up, and then a water line runs out to showers. It has a thermometer and those who fire it know how much wood to feed it to hit a safe temp. I would be incredibly cautious about any direct water heating system, making sure the water never gets close to boiling temperatures. The imaginary lawyer in me also says "none of this is intended as professional advice or recommendations, any risk you take with such a system is yours alone and could result in injury or death".
 
Glenn Herbert
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The big safety factor in these pressurized-tubing-in-atmospheric-hot-water-tank systems is that pressurized water has a higher boiling point than atmospheric water. Thus, even if the tank boils, it will not be quite hot enough to make the pressurized pipe boil. This does depend on not running the tank dry, which might allow too much heat to reach the piping.
 
pioneer
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Although it's far from finished, my design will use copper pipe laid up in a serpentine path in a layer of my thermal mass bench. The idea is that when I am burning, a thermistor will trigger a pump to begin moving water when my bench gets to a point where it is uncomfortably warm to the tooshie.  The water will move through the serpentine path and take away the excess heat and deposit it in storage barrels that are also connected to my solar water heater system.  I like the idea of being able to warm my space yet still moderate the temperature of the bench.  Plus, I can always burn extra if I want to bank up the hot water storage side.
 
A new kitten. What are we gonna name it? How about tiny ad?
Rocket Mass Heater Jamboree And Updates
https://permies.com/t/170234/Rocket-Mass-Heater-Jamboree-Updates
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