Rocket Mass Incorporating Water Heating?

[GrahamB]

Am I being completely stupid here, or are we over-engineering this? When I was kid we lived in an old Victorian house that had a coal fireplace in every room, including the bedrooms. In the kitchen we had a stove that sat back in a recess and to the left of the recess was a closet with the water tank in it. This supplied hot water to the kitchen sink, the washing machine and the outside privy. The electric hot water tank upstairs dealt with the bathroom supply, and the kitchen when we didn't have the stove burning.
Two pipes went from the stove to the tank. The pipe at the top (the hot pipe) had a pressure relief valve, like a big pressure cooker valve. The pipe at the bottom of the tank had a drain cock on it. I remember this because like most boys I used to play with the pressure valve and make it whistle, and I once got a whupping for opening the drain cock and letting the water out all over the kitchen floor. The tank was a tall cylinder type just like the electric tanks of today. The supply pipes just came out of the top of the tank. It was sat at the same level as the stove and pipes going into the stove were just normal black steel. It just relied on the siphon effect of the hot water to make it work.
All the time we had the stove burning, we had piping hot water. In the summer we used the electric immersion heater, apart from the time in the seventies when we had the power strikes and mother fired up the stove to heat the water in the middle of summer. That was a hot kitchen to skin rabbits in that year.

[pa_friendly_guy]

I think we are over thinking and over engineering what should be a very simple gravity flow system. The system that you describe was a pressurized system, as you drew water out of the tank to wash dishes or cook or wash your hands there was more water being pumped into the system to replace the water used. In his system the water is not pressurized but is in an open loop. He uses the water storage tanks as a heat exchanger and runs his drinking water system through the tank with coils to make his hot water for the house system. The water that will be heated by the RMH will never go to his kitchen or bathroom sink, it stays in the water storage tanks. To make the water move from the storage tank to the RMH cobb bench a difference in elevation is a huge help. The system may work if everything is on the same level, but it should work much better if the cold water has a way to fall to the RMH and the hot water can naturally rise back to the tank. The Hot water will rise no mater what, so even if his tank is level with the Cobb Bench the hot water should go to the top of his tank which will be higher than the bench I am sure. The cold water maybe sucked into the system by the hot water rising to the tank creating a void. Again I feel that having at least 1, and I think maybe 2 check valves to keep the water moving in the right direction would help the system work more efficiently.

[mannytheseacow]

Yep, I think you're right pa_friendly_guy. In fact, I think I have the water part of this RMH all figured out. I got bigger details to work out for the RMH, but I think that the water heating is pretty simple. I've already got zones of water lines running through all my floors. I can't raise the tank any higher just because it's already a big tank in a small space and there's limited room for accessing the controllers, pumps, manifolds as it is. However, the cold out to the stove can easily be linked into one of the floor zones which passes no more than 6 feet before going under where I'm planning my mass. This cold out line is about 3' off the ground where it exits the tank. I can run it out of the tank to the floor, accross the floor to the mass, and from that point up out of the mass, through the ceiling back to the tank or just continue it through the floors to heat... or both. Continuing through the floors probably won't raise the temp of the tank water much though, as the water will have cooled before it gets there.

GrahamB's system is much simpler, but I'd rather not be tending fires in every room. As much as I like a good saute au lapin, I like my little home naturally cool in the 60's and 70's all summer without using energy. GrahamB also had two things that I don't have: an electric company and a coal mine!

[matt walker]

[pa_friendly_guy]

If you have 3 feet of fall from the tank I think you are home free. Three feet is more than enough to make your system work on gravity flow with the thermal syphon. I was wondering if you insulated your storage tank to conserve the heat you gather in there? Is the tank metal of plastic? I was also thinking about a way to drain the system incase you want to go away for an extended trip in the winter. My neighbor just got back from a months vacation down south. He and his wife went to visit there kids and a few friends. As we get older the cold weather does seem to be harder to handle and some people just like to take a trip to a warm place for awhile to lift the spirits. I had thought about wrapping the water line around and around the exhaust smoke pipe, but that would not let the unit drain properly. Running the line back and forth from the floor area where you start from the floor area on 1 side of the smoke pipe, going up a bit with each run of pipe until you are at the top level of the smoke pipe. Then back and forth again through the cobb just above the smoke pipe to the point where you want to exit the stove and head to the ceiling. That would allow you to drain the water gravity flow and still give you a good bit of water pipe buried in the bench. I do not know how many Gallons of water would be in the cobb bench getting warmed, but I feel the more the better. Copper is great for dispersing heat all around and it might help with the thermal syphon, but I wonder if a much cheaper water pipe would work for you. Having a 1 inch water line through the cobb would greatly increase the number of gallons of water that you are heating at a time. I am very excited by this idea, I hope you take a lot of pictures if you decide to actually install it in your home.

[mannytheseacow]

Thanks for all the thoughts and insight on this. I've been away from my computer for a while and tinkering. I also got a copy of the RMH book from my library so I've been scouring it over. So many of my questions are answered in that book, so I'm trying to relieve this forum from my basic questions that can be answered elsewhere...

I found this video on thermal siphons: http://www.youtube.com/watch?v=5IRLVCJ1olA and I'm still working on how I'll incorporate this. I think you're correct, though, guy. I'll have about 3' of head and I think it will work. My tank is a copy of Gary's from builditsolar.com. It's a 2x4 framework sheeted with marine ply, lined with foil faced foam sheeting, then a EPDM pond liner. I didn't want any penetrations in the liner so I'm limited to draining it by siphon action with a hose, which would take a long time, but I shouldn't really ever have to. It's been operating just fine for years now. The cold outs go up over the side and then back down. The pump is located about 2.5 ft. off the floor and there remains enough head in the piping to keep the pump primed. This may limit the siphon action, though. Really, I don't mind running the pump anyway. It doesn't use much energy and it's running on solar anyway. The key to solar is keeping your energy usage down. Just like money, it's easy to keep it if you don't use it!

Right now I'm thinking of running a grid of copper in my mass above the ducting, kind of like the pex in the floors. I'm not sure how pex or other plastic will hold up to the temps of the mass, so I think I might stick with copper. I think I can dig some up. My current system is just copper wrapped around my stove exhaust and clad with mortar. It doesn't work great but it works. I figure it has only about .3 gal of water sitting in the heat exchange portion of it so there is plenty of room for improvement. If I could run a long grid of copper that is larger in dia. I could get much more exchange action going on. We'll see. I'm just going to do it one step at a time. Ideally I wouldn't have the cold out line running up over the side wall of the tank but I'll just get this RMH dialed in for now and worry about the tank later. While it's easy to improve and probably not too costly, I hate wasting anything or getting rid of something that works.

I spent some time last week making the casting mold for a 6" stove. Nothing innovative, just a copy of Matt's design scaled down to a 6" version. It's ready to pour, but the closest fireclay to me is 2 hours away. Sometime in the next couple of weeks I might make the trip, but I'm researching other materials that I could use which might be available nearby. I've been reading about some things that people use to make crucibles, which should be very durable, but everything comes at a price.

I'm taking pictures all along. One of these days I'll get some time to set up one of those photo site accounts so I can include them on here. I hate opening all these different accounts, but I've benefited so much from all the info in this forum I'll bite the bullet to include some for everybody else. I'll keep you posted!

[Lollykoko]

Manny, you may have already seen this video on coiling copper tubing. But with millions of selections available on YouTube, maybe not. http://youtu.be/AbDMITAMLio The videographer shows how to coil 120 feet of 3/4 copper tubing for a snug fit on the outside of an 8" stove pipe.

My higher level math skills haven't had enough coffee this morning, so I looked the answer up ... 100' of 3/4" tubing holds 2.3 gallons of water. At that rate, by the time you cook a meal, there should be enough hot water to do the dishes.

[matt walker]

[mannytheseacow]

Thanks for posting that video, Lollykoko! I haven't seen that one. That is very similar to what I currently use on my potbelly exhaust. I have mixed feelings about it. It works... not great, but it works. However, it cools the exhaust down significantly. I'm completely new to rocket stoves, but been using box stoves and potbellys for about 20 years now and I feel like I can gauge wood usage, creosote, and ash production pretty well with the different species of hardwoods we have here in the midwest. I noticed after I installed this system that my ash and creosote production increased dramatically. The bigger picture is that I think it's causing even more air pollution than my normal exhaust would. Maybe I'm completely wrong here, but that's what I've noticed. Anyway, that's what appeals to me about the rocket design: complete combustion and clean burning up front, then mindful use of the heat afterwards. It just seems smart.

Thanks for including that drawing, Matt. I would really like to get something like this working. I've been thinking along the same lines you suggest with the air space and hardiboard. I'm thinking I might run my bench tubes close to the floor (concrete slab) and heavily insulate the bench, burying copper lines close (1-2" above) to the exhaust tubes, and really going for long term storage in the mass.

I noticed in Evans/Jackson's book that they talk about using rock (or broken concrete) with a clay slip in the mass. Any advantage/disadvantage to this over the cobb idea? We have clay and plenty of limestone around here, easy enough to come by. This seems like it would dry faster too, without putting so much moisture into the house. Not sure about the long-term benefits, though. I guess cobb would be easier to mold and sculpt?

First things first, though. I'm planning on making the trip to get some fireclay Friday, so I'm hoping to have the core cast by the end of the weekend!

[matt walker]

The thing about the rocks or urbanite and slip I think is best thought of as rocks/urbanite with cob, rather than either/or. I used plenty of rocks in my bench along with the cob. It's that much less cob to mix. The dense materials add mass, and the cob couples them all together and provides the soft heat and soft seating that it excels at, to put it simply.