Stove for a housebus (revisited)

[jgreen]

Hey Ya'll.

I was finally able to get the stove to a state that I could do a full test run. Everything went great (except for some leaks in the plumbing of my hot water heat exchanger caused by using old pipe thread compound. its been fixed). Even with wet mud in the combustion chamber and along a lot of the path of the flue gases and some key areas left un-insulated, the stove started drawing immedietly and burned very cleanly after some initial warm up time. I was very pleased. Actually, the hot water heat exchanger worked too well. The water near the top of the tank heated up to "too hot to touch" temperature very quickly and I was worried that there would not be adequate circulation inside the tank to keep my P/T release valve from opening. So, I've decided to try using another water heater as a storage tank and attempt to create a thermosiphon between the two tanks (the one inside the stove and the storage tank). FYI, old water heaters are suprisingly easy to obtain. One craigslist wanted ad yielded 10 or more responses within a few days and I ended up picking one up today for free. In working condition. And the shells of the larger ones (40-50 gallons) seem to be the perfect diameter to hold insulation in against a heat riser and leave the right gap when a 55 gal. steel barrel is placed over top (for stoves that use that design).

I understand the basic principles of thermosiphons but don't have any actual experience setting this up. Due to the circumstances of my structure (a bus with a low ceiling), I can't easily locate the outlet of the storage tank above the inlet of the heat exchanger. I'm posting a picture/diagram to help illustrate this. I've heard and read mixed responses about whether a thermosiphon can work in this arrangement. A post on these boards a while ago (donkey32.proboards.com/index.cgi?board=experiment&action=display&thread=94) about a rocket stove powered water heater gave me hope that this could work, as it seemed to be working out for them. My heat exchanger outlet will rise a bit to the storage tank hot inlet and my theory is that the pressure from a full storage tank with a height of around 5' will push the cold water up to the heat exchanger cold inlet (only 3 feet-ish high). My plan is to supply the water + pressure for this whole setup from a 55 gal barrel on the roof. It feels kind of experimental, but I have enough confidence to think its worth the effort to test it. I would of course put the storage tank higher if it were possible in the space I have. If anyone has any thoughts on this, especially those who have experience with thermosiphon systems, I would love to hear them. I'll post the results after the weekend when I have a chance to set it up and test it.

Also, does it strike anyone as a bad idea to tap the hot water coming from the hot water outlet of the heat exchanger (on its way to the storage tank)? I figure that the rocket will have to be running for quite a while to heat all 52 gallons in the storage tank to a hot temperature... so if I need hot water sooner it seems to make sense to tap it on it's way out of the fire. But I'm unsure of how this might affect the thermosiphon effect.

Diagram of my setup (plumbing not installed yet)

[canyon]

Use the largest diameter pipe you can, it makes a significant difference. Glad to hear you have a p/t safety valve. I would also put a tee at the high spot on your hot water to storage so you can bleed air. The rise and fall depicted creates a high spot that can collect air and stop the thermosiphon. Not sure about the tapping hot water direct question. It strikes me as a bad idea but as I think about it... why not? Try and let us know!

[woodman]

jgreen in the pic. it looks like you installed a barrel over the riser.

With the heat exchanger next to the storage tank is the same challenge I have of building one next to a 450 gallon hot tube on ground level.

[jgreen]

woodman-

there is a barrel over the heat riser, but its function is purely to hold insulation. Rather than being inverted and acting to reverse the flow of gases and radiate them outward, its installed with the "bottom" of the barrel down and the open top facing upward. I cut a hole in the bottom the size of my heat riser pipe and slipped it over the riser. Then filled with loose perlite (a 55 gal. barrel takes a lot of perlite!) I did this to avoid having to mix clay slip with all that perlite and create a cob enclosure to hold it all in.

You're building a rocket fired hot tub? Cool. Somehow I missed that thread..

[Donkey]

Since there's not much mixing action and the hottest water will rise to the top... It's probably gonna be less hassle just to tap off the top and not worry about it.
In short, you won't need to heat the whole thing to draw hot off the top.