Wood stove pumpless water heater project

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MNwr786

Demi-God
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I finally built my dream water heater. Wood stove heat, closed system to keep calcium buildup inside the heater element down (distiled water), no circulation pump, and somehow include temperature control. Its been a big headache, but it is getting there.
The heat exchanger tube inside the stove (will it be big enough?)
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The top pipe is the center tube of the heat exchanger (hot water/steam output), the bottom tube is the jacket (cold water input). The 90° elbow connecting to the water reservior has a ball bearing inside to act as a check valve (with a metal wedge keeping it from going down the pipe). The downward pipe is to capture sediment, purge the lines with water when needed and drain the system.
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The hot water leaves the stove in the top pipe and the returning cold water is fed into the reservior. When the water in the heat exchanger boils, it pushes hot water toward the coil in the water barrel because the check valve keeps it from going toward the reservoir. After the hot water goes through the coil in the water barrel, the returning cold water encounters another check valve. When the void created by the steam collapses, it pulls more water in from the reservoir and the cycle repeats.
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The reservior is open on top (plan to add dust cover) and the level of distilled water in the system is monitored here.
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Getting everything insulated was not fun.
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Big lights for a bedroom, I know. That room doubles as a plant greenhouse in the spring when its still frozen outside and a place to dry herbs and spices in the fall.
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If you have hard water, check out mine! The crap in this barrel represents about 20 barrels of water being heated using the old system (which causes the calcuim to precipitate out as it traps iron). I chose not to clean it out for the initial test run after removing the filter and stirring it up thinking I was draining it out. The second check valve is in the left pipe in that orange barrel in the removable section touching the water. It was made using a piece of smaller copper tubing soldered inside that tube and a ball bearing to act as a check valve (with lots of fiddlin around with the seal surface and making a plug of titanium hobbycraft wire to keep the ball from shooting up the pipe.
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As the water in the stove begins to boil, the check valve at the barrel starts to raddle. Then the raddle gets a bit louder (nothing loud enough to keep someone awake though), then snaps shut and then the one on the stove almost immediately raddles afterward as it takes in more cold water. Right now, for example, the water in the barrel is 75°, the left pipe is also 75° and the right pipe is 155° (probably hotter during strong pumping cycles). The black tape on the fittings is for the IR temp gun.

I learned a few things... Most importantly, the heat exchanger tube is too small. I have to get the stove very hot if I want it to make 35 gallons of water hot within a couple hours. That is definitely getting upgraded. Also, just using one check valve (at the heat exchanger) isn't enough. I thought it would be, but even after purging all the air our of the lines, it would get hot for one or two shots of hot water only then cool down not pumping anything after that. After a day and a half of messing around (with one check valve and getting nowhere and finally adding a second check valve), it occured to me that all I need is a magnet to hold that ball bearing off the seat in the second check valve (perhaps based on the water temperature with an electromagnet) as a means to stop the heating. Exactly how does that stop the system, I do not know, but she don't pump but once or twice without it.. My guess (from observations on failed starts) is that there is an equilibrium between the formation of steam and the condensing of it further up the line (perhaps even at the barrel) as it pushes water back uphill toward the reservoir. As the steam condenses (without 2nd check valve), it just pulls back water from the line instead of from the reservior. The heat tube would exceed 400° even with the 2 feet of water pressure in the reservior trying to feed it, so it just maintained steam for however far and the liquid front would move back and forth and nothing would pump. Cracking the drain at the bottom would often induce an energetic pump cycle, and afterwards the return line flow would oscillate almost unnoticeably. That second check valve stops this oscillation and makes the pump cycle repeat reliably.

Now I get to go scrub that orange crap out, fill it up fresh, then feed the dragon good before bed. There should be a free hot shower waiting for me in the morning (figuring I'll run out of wood before the barrel boils if I don't hear the meat thermometer beep :) ). I can't wait to automate the temp!
 
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A loose compression fitting let some of the reservior water drain into the barrel last night exposing the end of the return pipe at the reservior to air. I tried topping it off and running it without purging the air from the system and noticed about 50% reduction in pumping efficiency. Having all air out really makes a difference. The 20' of copper coil in the barrel is not enough during the fast part of the pumping cycle, should bave been twice the copper because when the barrel approaches shower temp (108-110), the return water is warmer than the barrel water by about 15 degrees. Without making the house 100°, the pumping cycle happens every 3 to 5 minutes, so I figure there could be about 10x the surface area at the ehat exchanger. Going from 48° to 108° currently takes about 6 hours, so I think I will add 20" to the coil in the barrel so it keeps up while hot water shoots through it and swap out the stainless heat tube for a 20'of copper line inside the stove with a preheater coil wrapped around the burn box.
 
I like it.
Wouldn’t have been easier to take a radiant boiler heat system and adapt it to wood fueled.
Not knocking your engineering just another perspective. I still like it.
That would have worked too. Just working with the stove we had. The cost of the copper alone had the boss mad so I wasn't about to buy a new stove. I think she was relieved to see it finally work.
 
Now that the lines are in place and the valves work, i think i will tee in a line outside for the summer and give the fresnel lense a try. It would be nice to make this work all year.
 

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