Heating cabin with pipe, tank and water?

I have a small wood cabin that is easy to heat and also easy to cool. Anyway, it's freezing inside when stove is not working, like during the night, or when I come for some short time and don't want to start fire, etc. I had the idea of having a hot water container inside the cabin. Since it's small space with small thermal mass, but well insulated from outside, I guess some small hot water container could change temperature enough. I only need power source for heating the water.

Then I started thinking, I find some skrap plastic pipe, paint it in black or find a black one, place it on the sun and attach it to the water container. I'm sure it won't work that way, water has to circulate, so I need water circle. Ok, if I dug another water line in the ground, or leave it on the surface but paint it white, water will heat in one pipe, cool in other, and convection wil start if pipes are near the ground, and water barrel is above that level.

Slept over and imagine what will happens with water in the pipes when it's cold outside. I figured out already that when sun heats the black pipe, convection start water circulation. Since water is not heated inside white pipe, obviously it is cooled. I'm cooling my heating system. Now I'm thinking, what if I place white pipe inside the black one?

Does picture explane what I'm thinking?


Let's say it's freezing outside, and my water container is warm. Water wil not go down, for sure, if the pipes are cold. But when sun heats the black pipe, it heats the water between 2 pipes. When this water is warmer than in the container, it goes up and replaces it. Water from container goes through thin pipe down, but it doesn't cool since it is "insulated" with hot water. Once it hits the bottom point, it is heated again and goes up Of course, black pipe is closed on the bottom. If I can guess, I would say I can build quite efficient system with pipes and a bucket or barrel? To make it more soficticated, I would probably place water container inside insulated locker and whenever I need heat, I would open the door, or something like that. But what you say about the system itself?

Its a very interesting idea Milan and one worth experimenting with. I think that making the pipe more of a loop would create more circulation and thus more thermal transfer to the tank. Your idea is essentially is how an evacuated tube solar thermal collector works but there are usually about 20-30 tubes tied to a manifold to get enough usable heat plus they use refrigerants, solar glass, black painted metal and an air vacuum for insulation. I also think your cabin shell would need to be ultra efficient to be heated by the standby losses of a hot water tank.

Brian Knight wrote:I think that making the pipe more of a loop would create more circulation and thus more thermal transfer to the tank.

I agree. I only don't know how much flow I need. Need experiment.

Brian Knight wrote:I also think your cabin shell would need to be ultra efficient to be heated by the standby losses of a hot water tank.

It's nice issue. I guess thermal loss of metal water tank would be huge, I can't imagine that when water is not heated anymore, it will stay warm for long time. I need actualy to check does a water have enough energy to cover thermal losses of my cabin. I calculated thermal losses of 50W, more or less. But to be sure let's say it's 100W, depending on temperature and many other factors. During 24 hours it is 2.4 kWh or 8.64 MJ. Water heat capacity is 4 J/gK. If my tank is 100 litre, water weight is 100.000 g. If water is heated to 30C, before it reaches 15C it will loose 6 MJ. So to keep cabin heated to 15C(60F) I need more than 100 l tank (26 gallon) heated to at least 30C (86F). Right?

put loops of black hose on roof.

thermosiphon should stop after tank is hotter than hose.

There are others here who are better suited to doing the math but I can guess from experience that a 26 gallon tank will be woefully inadequate especially at the luke warm tank temp of 86 degrees. Although 86 sounds like a fair tank temp to expect with a homemade system like that. This setup would be great for domestic hot water but expecting to get space heat is probably too ambitious. There is quite a bit of physics and relatively complicated math involved with solar hot water, especially when space heating. Ive never heard of a solar thermal space heating setup that used standby tank losses only. Usually the solar heated water is run to radiant tubes, space radiator or a coil in the air handler (fan).

I think the bigger variable is your cabin heat loss calculation. As you mentioned, there are many variables that affect this math. There are lots of programs available to do the calculations but I suspect with your simple cabin shape, there should be a free, simplified spreadsheet out there to get close enough. Here is a great, new article that explains how to go about sizing your heating equipment; http://www.greenbuildingadvisor.com/blogs/dept/musings/how-perform-heat-loss-calculation-part-1

I commend you for trying to go solar Milan, but are there reasons you are avoiding a woodstove or the rocket mass heaters that are so popular on this site?