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Rocket Mass Heater that can heat a "pool/pond"

T Stan


Joined: Mar 31, 2012
Posts: 2
Hello Permies,

This will be my first post on these forums. I have been actively involved in "greening" out my life for some time now and I've been subscribing to many Youtubers that continually keep talking about permies.com! So here I am with some Rocket Mass Heater questions for all you pros out there.

I am going to be building a fairly serious Greenhouse this summer. It will be 12 x 24 when finished, although I am building it in stages. The first section of the Greenhouse will house a fairly large water tank, at least 250 gallons. This water tank will be used for Aquaponic purposes, it will house fish.

I have some ideas about heating this pond with a Rocket Mass Heater. My theory is that I would build an insulated box on all 6 sides. Although the top of the box will be a lid (naturally...) In the bottom of this box I was thinking I could lay the duct work for the RMH to vent through, pour sand all around the ducting to make it flat and then put pond liner on top of the sand in the box and fill it with water. My theory is that a Rocket Mass Heater could be heating the sand, which would then heat the water of the tank. My concern is that I can't see the weight of the water wouldn't crush the ducting and I can't figure out how to make this work.

So I'm sure there must be ways of strengthening the ducting so that it could handle the weight of the water, perhaps big iron pipes or something instead of tin ducting? Concrete or Cob instead of Sand? Also any other tips for me would be a great! Thanks a lot for all you guys do
Yone' Ward


Joined: Feb 14, 2012
Posts: 135
Location: Springdale, WA USA - Cold Mediterranean Climate
If you put your RMH under your pool, you will have no way to regulate how much heat you are putting into your pool. It will be very easy to cook your fish accidentally as there will be a significant time delay between when you light the fire and when the heat gets to the water and if your water is getting too hot, there would be no way to shut off the heating.

However, if you use a masonry bell type RMH, wrap it in insulation, and run pex tubing between the insulation and your heater and then under the pool, then you can transfer the heat to the pool with water that can be thermostatically shut off when your pool gets too hot. If your RMH is below (not under) the pool, and you take care how you run your pipe, then gravity will pump your hot water for you and won't circulate the water if your RMH is colder than your pool.


Just call me Uncle Rice.
17 years in a straw bale house.
T Stan


Joined: Mar 31, 2012
Posts: 2
Yone' Ward wrote:If you put your RMH under your pool, you will have no way to regulate how much heat you are putting into your pool. It will be very easy to cook your fish accidentally as there will be a significant time delay between when you light the fire and when the heat gets to the water and if your water is getting too hot, there would be no way to shut off the heating.

However, if you use a masonry bell type RMH, wrap it in insulation, and run pex tubing between the insulation and your heater and then under the pool, then you can transfer the heat to the pool with water that can be thermostatically shut off when your pool gets too hot. If your RMH is below (not under) the pool, and you take care how you run your pipe, then gravity will pump your hot water for you and won't circulate the water if your RMH is colder than your pool.


Wow, that sounds brilliant! So would I put regular pex tubing in the insulation inside of the inner chimney? And then plumb that into the pool? and you think that gravity and the heat will pump itself?
M Ploni


Joined: Apr 22, 2012
Posts: 9
Yone' Ward wrote:. . . a masonry bell type RMH, wrap it in insulation, and run pex tubing between the insulation and your heater and then under the pool, then you can transfer . . .


Where can I see/get details about this online?
Roy Clarke


Joined: Feb 05, 2012
Posts: 120
Yone' Ward wrote: then gravity will pump your hot water for you and won't circulate the water if your RMH is colder than your pool.


The circulation may not be fast enough to maintain the temperature in cold weather. The circulation could go backwards if the stove is colder than the water, unless you include a suitable trap.
Yone' Ward


Joined: Feb 14, 2012
Posts: 135
Location: Springdale, WA USA - Cold Mediterranean Climate
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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