retrofitting a wood stove to heat water

[Redoverfarm]

CREATIVE1 a friend of mine who is off grid has one. Uses it daily.  His is an older model and he said he needs to re-order the wooden stays. He bought his 2nd hand several years ago.  If you can make a cover with rigid insul it will keep the heat better and required less time to heat up. 

[CREATIVE1]

Came with one of those too, plus about 20 spare staves, etc.   Have to fix the inside bench.

[MountainDon]

I added a water heater to my wood stove this past weekend.

[Ernest T. Bass]

On the topic of wood-powered water heating...

We have a simple thermal-siphon system set up on our rocket stove (http://www.rocketstoves.com). Our water heater tank is elevated on a platform, just a few feet from the stove. We put a tee on the drain valve of the tank and ran a line up to a copper coil on top of the stove. From there it goes up to the top of the water tank and tees into the hot water out line. We made an anti-siphon device to prevent sucking cold water through the coil from the bottom of the tank when the stove is not in use.

The system is nice because we didn't have to modify or compromise our new water heater in any way. BUT, there is a big drawback... All of the steaming hot water likes to sit right on top of the tank (naturally, since that's where it's coming in). So, we get about 5-10 minutes worth of scalding hot water, and then nothing but cold.

The simplest fix is to come into the side of the tank with the hot water, rather than the top. But, we're not about to cut a hole in the tank... So, can anyone think of a solution to this problem, short of cycling the water with a pump?

[glenn kangiser]

May work - may not but how about custom making an extended nipple to go down inside the water heater about half way -maybe solder or braze a piece of copper pipe into the cold inlet of the heater and put the hot from the stove in there to mix with the other water.  The hot would still go out the hot side of the heater.

Going from how I think it is setup, Andrew - pix or drawings could help if I don't have it right.

[cordwood]

 You did it again!! Thats what I was thinking, Just continue the tubing down into the tank rather than terminate it at the top. Provided it's smaller tubing.

[Ernest T. Bass]

Doesn't the cold inlet usually extend down into the tank like you're describing? We tee'd into the hot line, based on some instructions I read.. Not really sure why.

Would the hot water actually go down through the pipe, or would it just stay at the top since it's less dense than all the colder water around the pipe? Or perhaps it would get sucked down regardless, since water's leaving at the bottom of the tank and has to be displaced?

If you want some visuals, here's the site that we got most our info from: http://www.green-trust.org/2003/fireandwater.htm

[glenn kangiser]

You need to get up a couple minutes earlier or type faster cordwood....  

[cordwood]

 Perris High School and dial up = SLOW. Not my fault
If the heated tubing line still terminated higher in the tank than it started it should still siphon. And if you came in the hot outlet opening in the top of the tank but did not obstruct it it should still allow hot water out to the rest of your plumbing. A tee in the top of the outlet you could sweat a reducer to the tubing to seal small tubing inside larger tubing and then go through the tee.

[MountainDon]

Your hot line from the stove should be tee'd into the cold supply line to the water heater tank for this to work. Glenn is on the right track but the work was already done for you at the water heater factory. The cold line in the tank has what's called a "dip tube".

[glenn kangiser]

I haven't researched it that well, Andrew.  I'm just trying to figure out a way to get it to mix better in the tank.  It stratifies in the tank if not mixed well enough.

There you go -- Don has the straight poop on it.

[glenn kangiser]

Perris--- I worked there at least once, cordwood.  Putting Roll Bars on Caterpillars -

[Ernest T. Bass]

Your hot line from the stove should be tee'd into the cold supply line to the water heater tank for this to work. Glenn is on the right track but the work was already done for you at the water heater factory. The cold line in the tank has what's called a "dip tube".

Yeah, that's what I thought... Sure seems smarter to feed the hot water into that line. Any idea why that site has you go through the hot? It was the most thorough guide I had read on the subject, so I figured the author knew what he was talking about..

[glenn kangiser]

Maybe when you experiment with it you will be able to come back and tell us.

It may slow the thermo-siphon as the heated water will be closer to your drain but it should immediately rise mixing on the way as it is so hot,  but again it may not want to pass through the cold.

[cordwood]

Your hot line from the stove should be tee'd into the cold supply line to the water heater tank for this to work. Glenn is on the right track but the work was already done for you at the water heater factory. The cold line in the tank has what's called a "dip tube".

Teeing into the cold line might work good when there was demand on the hot side circulating the water, But at rest I think it would be too close to the bottom for a good heat siphon effect.
By making your own dip tube on the hot side it wouldn't cool as fast on entry either.
What about the blow off valve? Could you go in there? Is it seperate?

[Ernest T. Bass]

Yes, it's separate. Judging by the pics it looks like the hot line already has a little dip tube, but perhaps that varies by model.

The hot and cold nipples on our heater have little plastic strainer type things that would have have to be removed in order to extend a tube down inside the tank. Not knowing anything about water heaters, we didn't want to mess with them at the time since it looked like they were supposed to be there. It sounds silly, but do you have any idea what I'm talking about or if we could remove them?

[glenn kangiser]

I've seen them - I don't think they are removable.

I don't think the T/P valve would work as it needs to be there unrestricted for safety.

[glenn kangiser]

How about a separate tank on the ceiling with switching valves to cut it out when there was no heat or ...  You could tee it into the existing lines - maybe just send your cold to it first then let the regular heater finish the job if necessary.

Send the output of it to the cold of the main heater.

[cordwood]

 If I remember right those are corrosion isolators and I have actually had to remove the one in the tank we have now because it deformed and blocked the flow
I am not sure how it would work with the blow off valve because a tee would take the thermal part of the valve out of the water if an air pocket were to form in the tee/valve. For a dip tube to work there it would have to go straight down and that would put the valve on the horizontal of the tee, Witch should still work but it would have to have a nipple to make room for the thermal probe on the valve.
.
.
. I see a scalding experiment in my future

[Ernest T. Bass]

Thanks for clarifying what those things are, cordwood. Another mystery solved... (We live in a cordwood house, btw. )

The tank is only a foot or so from the ceiling, so we don't have room for your idea, Glenn. Thanks for the help, though! Seems like there is no easy fix for our situation.. I can't really complain about the system, though, considering how simple it is.

[MountainDon]

My thinking was, and is, if the way it is plumbed at present produces the problem with very hot stratified at the top, something must be wrong. The simplest thing seemed to be to switch the position where the hotter/warmer water re-enters the tank.

If it was me, I'd plumb a new Tee into the cold, leaving the Tee at the hot, but plugging it. Then switch the hot line from the stove to the cold inlet Tee on the tank. Let her rip and see what happens, all the while monitoring the temperatures.

I notice the article makes verbal mention of a tempering valve, but it is not illustrated. Could be the source of the problem you are experiencing?      I don't really think so, but I'm grasping at straws here.

[MountainDon]

Here's another straw... If the hot water is stratifying at the top, it's too hot there, you don't have enough circulation within the system. The model T Ford used a thermosyphon cooling system. So I believe this must work and work as drawn, but your flow rate is too slow to mix the water in the tank well enough to temper the temperature throughout the tank.

Bigger pipe? Does the cold loop between the tank and the stove have a downward slope (from tank to stove) with no dips or rises that would impede the flow? 

[cordwood]

Here's another straw... If the hot water is stratifying at the top, it's too hot there, you don't have enough circulation within the system. The model T Ford used a thermosyphon cooling system. So I believe this must work and work as drawn, but your flow rate is too slow to mix the water in the tank well enough to temper the temperature throughout the tank.

Bigger pipe? Does the cold loop between the tank and the stove have a downward slope (from tank to stove) with no dips or rises that would impede the flow? 

I've worked on a few old tractors with thermosyphon, Usually a blown headgasket from over heating  
The drawings say not to go up,over and down,........But all the drawings go up,over and down

[MountainDon]

... or there's not enough vertical separation between the tank and stove to get a good thermosiphon action happening. 

[Ernest T. Bass]

Here's a very basic sketch of our setup:



The coil position is not ideal due to the height of the rocket stove, but we got the tank as high as possible and should have enough separation based on what that article says.

Unfortunately, the water heater is in a very cramped closet and is difficult to work on (house design boo boo), otherwise I would tie into the cold line just for the heck of it and see what happens like you suggested. I do have a feeling that the thermosiphon action would suffer from that though, depending on how far down the dip tube reaches...