Using water coil on barrel to extract heat and reduce size of cob bench and still get cool exhaust

the title just about says it all,, my 6 " system has a bench 6' long12" high 22" wide --10' of pipe, three elbows, up 8' two more elbows as a cap-- exhaust is cool enough to keep my hand in it forever--guessing less than 110-115

I have approx 100 feet of 1/2 inch copper pipe wrapped around top of barrel--goal was to get instant hot water,, and with fire going strong, using the water to heat the radiant floor, often i can run the circulating pump continuously and maintain water temps approx 125 or so

if i switch system to heat hot water in the "water heater" the cold water in is 30 degrees cooler making it necessary to wait a few min between batches of hot water

the pipe is wrapped as tight as i could enhancing direct conduction from barrel to copper so my hypothesis is that this might be a design possibility for people with limits on the size of their thermal mass due to support restrictions or what have you----

the obvious downside to this is the reduction of available heat radiating into the room,, since the hottest part of the sides of the barrel is mostly cool enough to touch

leaving only the cooler lower 1/2 of the barrel and the top surface to radiate heat--and top surface has been red hot until i started keeping a big pot of water on it

i also theorize that this actually should increase the pushing effect of the exhaust gasses by cooling the hot gasses at the top quicker, thereby enabling longer horizontal runs/ stronger exhaust pressure

my cob bench barely gets warm most of the time unless i run the stove for several hrs at a time

This might be dangerous. Any direct heat to copper is in risk of explosion. Even with good safety values you can trap air.. There are multiple you tube videos with people showing there mistakes. And a video on premise with a safe indirect water heating method. I think the term is squish boom. As in the water goes boom.

Yes, i should have put a safety warning about that part of the system, and for the record i have experienced times when the coil started to boil water--i wrapped the pipe around the barrel so there is no place where an air bubble might be trapped in the way you are referring to, also i keep the system unpressurized so if there is a failure of thermostat or pump, any hot water or steam is exhausted into the shower or back into the hot water tank

This is very much an installation that is experimental in nature and as such should not be considered a finished /easy / foolproof design

Anyone attempting to duplicate this experiment should be aware of the risks

I would say more likely than an explosion is the very likely possibility of steam blowing out a line creating a scalding jet of hot water--THIS HAS THE POTENTIAL TO REALLY RUIN YOUR DAY

This post is intended for more advanced experimenters with good experience in plumbing and hot water protocols,,

I don't know the physics but air buble can get trapped in the pipe it will expand at a different rate and burst the pipe, steam expands 7 times that of water even an open system can explode I won't say anything else but you may want to get the video or reasesch the exploding thing. This site is great because you can find all the I can kill you info easy, you may you have to contribute so money to get specific plans.
Good luck
Mikael

Hi Mikael,,

first, thanks for your concern for my safety, i've been working a thermosiphon water heater with wood stove for several years now, although this is the first rmh i have played with, i am taking appropriate precautions.

I would like to thank you for sparking my curiosity, as i have been looking at google results for "dangers of wood stove water heaters"

interestingly enough i found lots of plans for water heaters as well as a couple pictures of "explosions" -- the worst one there was a comment that it would have probably been approved by an insurance policy since it was a factory model and not a retrofit

also, that water heaters for woodstoves are not as popular because of the difficulty of extracting heat from a conventional wood stove without creating a "dirty" burn so most systems would fail epa guidelines

I even learned that thermosiphons work better when the storage cold water supply is higher than the heater intake--something that would have been difficult to do with my older system, but it was so robust it worked pretty well anyway with the cold water supply slightly lower than the intake on the water jacket.

so even though my post was more concerned with the dynamics of the rocket stove, and the two principal effects extracting heat from the barrel might have ie: greater exhaust pressure/push to facillitate getting the exhaust out of the building without a costly draft system and the need for less concentrated thermal mass in the bench, allowing installation where weight is a consideration

it would seem you redirecting my focus to looking at conventional woodstove systems as they have been utilized with heating water has opened up a whole other amazing sort of benefit rmhs have over conventional woodstoves--namely the easy ability to extract heat after the combustion process is complete


in most woodstoves the combustion process involves the entire stove and water jackets directly impact the burn, but in a rmh a water jacket is after the burn and actually potentially improves the performance of the rmh overall

so thanks for the inspiration to take another look

Bob Day-
The simple water bath heat exchanger you have is about as safe a way to get hot water off a RMH as possible.
I have been contemplating my water heating options for quite some time. Started a thread just for that idea. Using a RMH to heat water
Maybe we could get that thread going again with some of your ideas.
To give you one comment here, I think using a water heat exchange coil on the outside bottom of the barrel would work better than the outside near the top. If you cool the exhaust gases at the top it seems that it would slow down the pump effect of the cooling /falling gasses. As opposed to cooling them at the bottom which would seem to speed up the pump effect. However unless we're not to interested in heating a cob mass post barrel in the exhaust stream any extraction of heat will reduce heat flow down stream in the exhaust.
Experimentation is in order to draw a conclusion.

of course it is a good idea to verify my findings,

first of all, the extraction from boiling water is way too slow to do any meaningful work in my situation--maybe if i give up this house and move into a much smaller space i could get by with a pot on top of the stove and a coil running through it, but i doubt i could fill even a small water tank with hot water before i blew myself out of the house with excess heat

when i first started this experiment i used 60' of coil at the bottom of the barrel to extract heat, but it was much slower than 60' at the top

too slow for what i wanted to do (hot water for showers and kitchen and radiant floor heat supply)

another iteration was having 100' of coil somewhat spread over the top 2/3 of the barrel, which was a poor test because the coil had loosened direct contact with the barrel

this last test has the coils tightly wound close together over the top 1/2 of the barrel and the heat difference between the different coil spacings is suggesting to me that probably the best heat extraction would consist of coils spaced an inch or so apart as tight to the barrel as possible.(assuming you can't afford enough copper tubing to cover the entire barrel, and don't mind working with that weight and clumsiness of the coils--even 100' is a very tedious difficult project)

starting the coil low on the barrel to use the full surface of the barrel is probably going to turn out to be optimal

starting lower on the barrel would also maximize the thermosiphon effect --i did try this system as a thermosiphon at first but was never happy with the transfer rate to the water tank,,

I do plan a more permanent 8" rmh where the water tank is within a foot or two of the rmh, and the thermosiphon is the main capture of heat, with secondary lines from the water tank running to the radiant floor system

This system should give a very large window of response time in case of pump failure, allowing the fire to go out before dangerous pressure blew out emergency systems

As to the question of relative exhaust pressure, i have no real observations on that since having the tubing at the bottom gave such a poor heat return

my rmh exhaust has always been fairly strong --with 8' of rise at the final that may actually have a partial draft effect anyway--

and of course extracting the heat early on does require far less cob in the bench, although in order to keep final temps down i do believe those original distances specified should be followed, but they could have far less cob around them, as temps would be lower

To John

Just to clarify the water bath is mine Mikael69. I am not that good at posting my grammer and spelling are bad and I am using an Iphone to log on most of the time from the shed an the auto correct is hard to catch. I came in to type up on my desktop and can see alot more of the web site and it is easier to type but most of my pics are on the phone so. Thanks for the link to the other discussion on RMH and water heating where info is cleary stated and with the right math. I think I said steam is 7 times that of water and I believe Big Al correctly states it 1700 times I will just repost that link as I believes it is a great balenced attempt to clearly guide people to not blow themselves up and not blow up the craft with it. I made the water bath after seeing many warnings here about the safe way and things I did not consider, and needed to. people are going do things that are experimentle that is part of the greatness of this sight, people sharing in real time what they have found, I just try not to muck up the info or stear people into unsafe teritory with the proper warnings or at least I try not to but I am sure I will learn more as I go. Some of the designs are so complicated that I would not even begin to calculate all the variables so to comment on them other than to share what I did is well not my place I will let others with better voices do the math and warnings.

Bob Day : The Actual hot water to hot water Flashing to Steam is actually 1600 Xs . As Built one of your biggest problems is keeping your cold water tank full !
A failure here could just melt Down the Entire System ! Big AL

Bob Day : Have you experimented with a Toilet tank Float valve ( A New One ! :p ) To automatically refill your Cold Water Supply tank !?!!

I have friends who Run their Wash and Rinse Water from their Washing Machine to a grey water holding tank, and top off the tank Auto - Magically
they Have the Fill tank float valve in line from their rain water collection tanks to to their Holding tank to keep things topped off as this is the only
water line that runs into to ether a settlement basin or as a direct supply to their garden !

If they can use a totally Mechanical Float valve that works thousands of times with no problems, I would expect you could use Clean water from
your House supply to keep that tank filled to within a 1/4 of an inch ! Big AL

Hi Big AL,,

actually, i replumbed a hot water heater so the supply side of the rmh system is directly connected to the drain valve opening --and that connection is always left open as a water supply and also acts as a secondary pressure relief if i forget to open the valve at the top of the heater in between batches of hot water (yes, i know myself well enough to protect against operator error)

using the shower head as the steam escape i can just turn off the cold water supply, open up the hot water side of the shower to depressurize and it works very well, as i have let the water in the copper coil reach boiling many times

and you are right, even if it's just air in the pipe, there really is no such thing as a system with no pressure on it,, the key is to make sure the pressure is well below what would cause a serious problem

with this system there have been times when i was not sure what it would do, and since i am the only one here i have a great deal of freedom to throw the switch and take cover (figuratively of course)

just to see what might happen without worrying about others who might start to play with it without knowing the protocols

and as always, the real issue is to get a system that runs and maintains itself as much as possible, so us dumb operators can't mess it up

i'm pretty far from that, but figuring stuff out all the time--for instance, i just figured out how to replumb the return from the radiant floor through the water heater so that all the air bubbles automatically are eliminated before they hit the pump---i'm smiling and patting myself on the back--but in reality i'm still just reinventing the wheel--this is my first radiant floor system, and i tend to do everything wrong that can be done wrong at least once--sometimes i like to learn more thoroughly than that

Big Al, sorry, i didn't understand your post completely, i think you must have been looking at all those pictures posted by Mikael long-- my system has a copper coil wrapped around an rmh barrel as described in my other posts

To Bob, Big Al and anyone else viewing this tread.

I believe I made it more confusing by including pics of what I had done (the copper tube in water bath) rather than discirbe it. I did not properly note that fact and I think that made it unclear weather I was saying that the pic was dangerous insted of what was being proposed by Bob. Not understanding the dept of Bobs knoledge I tried to share with him what I thought might be dangerous then quickly realized Bob was well into his process. I only made note of what Bob was suggesting doing because it seemed like a question he was possing and because a person had made a series of you tubevideos claiming success heating water with a coil ontop of the barrel only to find out later that same person had all most burnt his friends shop down and had a massive fallure. It did not go into why it failed but i stopped checking that youtube page because well he made it seem as if he figured it out and was so sure of himself and a little cocky and was really doing some unsafe things. Sorry for the confussion. I should be a little less ready to comment on every little thing just really entusiastic I quess. I hope that clears things up.

Mikael Long
Asheville NC

no worries, it's all good

Like i said, your response prompted me to do some additional research which i gathered some knowledge from--or at least was reminded of a few details i had misplaced