Ok so I want to use a RMH to heat water for radiant floor heat. I have done a bit of research on my options and have done a lot of reading on here to see the similar conversations that you folks have already had on this subject. We haven't even bought the land yet so I am in the research part of all of this. I want to use an open system with a large storage tank that I can heat up every day or so that uses pex tubing to then transfer the stored heat to the cement floors in a basement and a main floor. I plan on using a Dragon Heater brand RMH with bells so here are my questions that I hope you all can help me with. I think I would want to put the heat exchanger coils at the top of the first bell in this one riser two bell system. What do you guys think? Is that the right spot? I wouldn't want to install on top of the riser would I? Is it too hot here, or too cold? Will my colors really be brighter with All Temperature Cheer?
I know it has been talked about on here a few times but it seems like a few people get started and I haven't seen anyone with a finished project of this kind, has anyone done this?
If you're going to put a coil inside the heated containment, the top of the first bell is the place I would choose. Near the riser is too dangerous - it would be likely to sputter steam frequently at the very least, assuming you had the outlet to the storage tank in open air above the tank, and only a few feet from the bell. Any obstruction that happened somehow would cause an almost instant steam explosion there. The bell would have similar but much milder risks. I saw a video of a hot water setup similar to this with hot water and a few steam spurts coming out. Okay if you plan for it properly.
The safest method, as described in a number of threads around here, would be an open tank set into the bell with a coil going through it; this prevents the water in the coil from ever getting above boiling point, and if it is pressurized at all, it could never get to the boiling point (pressurized water has a higher boiling point than atmospheric water).
This setup will require the RMH to be lower than the reservoir if only by a bit, of course. Are you also planning on a solar water collector for heating the reservoir when the sun is out? The combination of sources will give you a lot of edge days where you don't need to light a fire at all, and it can take care of most if not all of your domestic hot water as well. For maximal freeze-safety, I would advise a drain-back system. It requires a pump to circulate the water to the collector, but in a power failure the water will just go back to the reservoir leaving nothing exposed to the cold, and not requiring antifreeze. The collector in this system needs to be above the reservoir by at least a few inches - more is not helpful.
posted 4 years ago
Thanks for the reply Glenn, I do plan on using solar evacuated tubes that this system would be a backup for, but possibly not right away depending on what the budget works out to be at the time. I was also thinking drain back system with a pump powered by a little solar panel so when the sun was out it would kick on. I wanted to put the RMH and water storage in the basement and the solar tubes on the roof or something, but it seems like that would be too far away from the water storage?
So I had pretty much thought against a closed pressurized system for sake of safety, but you mentioned it, is it possible to do it safely? I really would much rather have it closed in a perfect world, I just didn't think I should.
Location: Upstate NY, zone 5
posted 4 years ago
A combination open/closed system is what has been worked out by a number of experimenters. A largish open tank in the heating stream, with pressurized coils running through it to take the heat to use or storage, is said to give good safe results.
Re the RMH in the basement, unless it is also a living space where you will be spending time, that is not a recommended location. A J-tube needs regular attention to keep it fueled, and you will find it burdensome to interrupt your daily life to go check on it. A batch box does have a longer untended runtime, and may be practical there.
It would be possible to have solar collectors two stories above the storage tank, it would just take a larger pump and more power to get water up there. If it is solar powered, that is not necessarily a problem. The possible drawback to a simple system that sends water through the collector when the sun is shining would be bitter cold days. If the pump sends water up to a collector that is still below freezing, it would be conceivable to have it form an ice plug and jam up the system. Then the sun gets hidden by clouds for the rest of the day, and the whole collector freezes up. It is also just not beneficial to send water through the collector until it is above the reservoir temperature.
Rocket Mass Heater Plans: Annex 6" L-shaped Bench by Ernie and Erica