Sorry about taking so long to reply, it's spring on the homestead and everything needs to be done yesterday. The good news is I was able to talk about your system with a master maintenance guy that has been in the field longer than I've been alive. Maybe one day I will learn not to do drive by thinking like I did above. So lets start on the list I came up with:
1: Pipe material. I've been doing a lot of guessing about pipe strength at the boiling point and above in relation to hydronic heating, so I took the time this week to run some experiments and I put them on YouTube:
http://www.youtube.com/watch?v=L2L2DExEPXg
Just remember, that cement will dissolve copper pipe when it's placed in direct contact. I saw this a lot in old houses in Detroit where they embedded copper pipe in the cement. If you're building with cob or other clay sand based material, the chemistry is different so you
may be ok.
2: Expansion Tank. The expansion tank is the most important part of a hydronic system. It prevents the system from going boom when the water expands from heat. It will be the highest point in the system. The pipe coming out of your stove and the pipe coming out of your pond will go into the bottom of this tank. These two pipes can be hooked together with a T so that the third leg goes up into the tank. An overflow/ vent pipe needs to come out of the top. It would be good to wrap the pipe with screen to prevent insects and rodents from invading. You need to keep it just barely not empty. This is usually done with a float valve that automatically maintains a minimum level. This is particularly critical with gravity feed systems. A gravity feed system will stop working if the top of your loop is dry. You should expect some boiling water and percolator action to occur.
3a: Pump Driven Systems. The vast majority of buildings in this country are hooked up to public power, so a pump circulated system will be the easiest to install. Water pipes are incredibly effective for transferring heat from one place to the other. I was the building maintenance guy for eight years in a couple of buildings with 2 million BTU boilers and they pumped all of their heat through 3 or 4 inch water pipes. One of those buildings transferred the heat to an air duct, but it took an air duct 3 feet high and about 8 feet wide to transfer the same heat. Taking that into account, a 1/2 inch or even 3/8 inch pipe should be sufficient. There are quiet energy efficient pumps for circulating hot water through buildings available, and they last for years if not decades. I would also put in a valve to shut off the water just to make sure that water doesn't flow when you don't want it to. Hook both up to your thermostat and place them just before your water reaches the stove. No special effort needs to be taken with height of the RMH in relation to the pond nor any special consideration with how you run the pipe, as your pump will overpower any gravity concerns, however if you run your pipes close to the way you would for a gravity system, even a pump system will be happier.
3b: Gravity Driven Systems. I like gravity based systems for many reasons. Not having a pump makes for no needed electricity, no pump maintenance, and no running/ maintaining wires for the electricity. Gravity never stops working. They are more difficult to install, however. The make or break part of your system will be whether you can find a thermostatic valve. My head says there should be a thermostatic valve that requires no electricity to operate, but I've had had parts counter guys look at me like I just grew a third head when I ask for a part that does something I think they should have, so look for this part first. I think there is a valve for the old radiators houses and apartments used. It will need to be submerged in your pool. If your local plumbing or heating supplier can't help you, search in an old city for heating and plumbing guys that work on the old and large buildings. The master maintenance man says you need at least a 1 inch pipe to get the needed flow on a gravity feed system. Be meticulous about how you run the pipe. The lowest point in the system needs to be just before it enters your stove. Imagine an air bubble inside the pipe. It needs to be able to rise continuously all the way through the pipe on your stove and into the expansion tank. The same goes for the pond side of the system. Any air in the system will lock it up so it won't work. Bubbles
will happen as you will without a doubt end up with some boiling. Gravity never stops working, including when you don't want it to. Your cold water is heavy, so it will want to push down and shove the light hot water out of the way. The pipe is there to contain and control it's path. If your heater is below your pond and gets colder than your pond, then the water will stop flowing unless your water gets near 39F. As I recall, water starts re-expanding when it gets colder than that.
4: The RMH. I haven't built a RMH yet, but I have just finished Ianto Even's book. I recall him mentioning masonry bell type heaters and I've seen them on YouTube. They aren't good for instant heat, but instant heat really won't benefit you much in a green house/ pool set up. If you are building a gravity driven system you should put a slope the top of the "Bench" or mass portion of the heater so you can get a continuous rise on your piping. You may find it easier to branch your pipe so you have several parallel pipe going straight instead of winding back and forth. I recommend wrapping the pipe to the outside of your heater as the pipe will expand when it heats and it also makes repairs 1000 times easier. Wrap your heater in insulation. If the insulation is mounted in removable panels then repairs are easier again. If you wrap your heater with r30 or r50 insulation, then pretty much all of it's heat should go into the pond. If you want some of the heat for the green house, then you can either let it come from the pond (humid!), or reduce the thickness of your insulation.
5: The Pond. Water is very good at transferring heat, but is lousy at retaining it. If you have lots of surface area on your pond, it will do a great job of dumping heat into the air. The greater the temperature difference and the greater the surface area, the faster it will dump heat. Hot tubs have insulated covers because of this. If you can cover your pond at night, there will be benefits. My brother has been in the hot tub business for 20 years and will tell you that insulation on the bottom of a hot tub is useless because heat goes up, not down. This is not entirely correct. Heat conduction cares little about up and down. Wrap the underside as well as the sides of your pool with insulation, but keep in mind any plumbing you may need to get to and repair. You don't want to have to dig it all back up to fix a leak. Aerators that pump air into the water are another source of cold for your pond just as they are for hot tubs.
I'm probably forgetting something or confusing someone here, but these are the basic design principles to work with. Feel free to ask more questions, ask, I may even shoot another crappy video for you.
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