Instant water heating?

Ok, so heat up one end and pour some cold water down the other end.  See what happens.  Maybe steam will exit at both ends.

Ever pour water into a hot pan?
It forms steam.
Pour enough,  and you cool the pans surface down and the water stops evaporating,but it still gets hot.
I would be inclined inject a drip of water into the coil,knowing it will steam up.
A tiny 1/4" or smaller line delivers water into the coil, controlled by two valves,  one to dial in the rate of flow,  the other for on/off.
The water hisses into steam and travels upward at an angle, away from the stove pipe,cooling as it goes.
Arch it up and over,  and finally back down, ending above a sink.
Collect the water that condenses in the arch if you want distilled and the water that exits the bottom of the coil, if you just want it hot.
Could it produce dangerously hot or pressurized steam?
It might,  but it seems very unlikely.
Open at both ends with the amount of water introduced controlled by your hand,  it seems safe enough to try.

I might try this as a way to move heat around, only my condensing device might be a water jacket/thermosiohin loop.
The key revelation for me is the scale of water to heating vessel is very malleable.

Well it was nothing exciting.  I put my 15 mm/ .5 inch copper pipe into the batch box for 15 minutes.  Pulled it out and then poured a cup of cold water down the cold end.
Just steam shot out the bottom but it certainly wasn't that much more than from a boiling kettle.  No steam came out the cold end, and no water emerged from the hot end.  The water was all vapourised.
So, I think this would work. Just has to empty the steam into another vessel or to force it through coffee grounds.

Ok, hot water does come out.  It's not all steam!

This is the simple version of the way to heat water with an unregulated gas flame or flue gas transit, that will be safe any time.  A simple stainless cooking pot with a coil in it.  

Also, for anyone wanting to use 1/4” pipe, the recommendations i’ve seen are to use 3/4 or above, as narrow pipe like 1/4” has a higher propensity for the water to flash to steam, due to larger surface area in relation to volume of water contained.

 Could be ok in areas of the stove you know will never reach over 212F, as well as in a directly heated open ballast tank.

As for how a thermosyphon works, I believe it works better the higher your tank is above the coil, as there is more weight of water in the lines to and from the coil, which increases the differential of weight and thus pumping pressure.

 An exit for coldest water is at the bottom of your tank, feeding to the loop... and a return from the hot end of the coil at the top of your tank.  

(I cannot remember if having cool water travel down the coil from the top, or up the coil from the bottom is more efficient, but one is, so look it up if you try this)

 As water heats in the coil, it expands and becomes lighter by volume than the colder water coming from the tank.  This weight differential die to thermal expansion causes the hot water to “float” and rise up it’s tube and enter at the top of the storage tank, while cooler water “falls” out the bottom.  

 A thermosyphon also depends on larger diameter pipe to maintain sufficient flow, especially with little elevation difference between the height of the tank and the coil (tank can never be below the coil, and runs should never change direction past horizontal...  always some inclination.

 Google will bring up tons of discussion of solar thermosiphon systems, and the same rules apply, except that you are dealing with a MUCH more variable and dense heat source.  Especially the closer to the riser one goes.

 Consider an open tank, buffered heat exchanger like the one in Donkey’s video above. no boom, no scalding jets of steam...  and it can be scaled up or down, to heat just a cup of coffee (on a very small rocket) or be able to heat a whole house...  

You could do a sheet metal or masonry bell with insulation around the sides and two 16 quart stainless cooking pots (Walmart had them for $12 each in my neck of the woods a little while back)

A 1/2” thermosyphon coil in each one, hooked in parallel tees at each end to 3/4” line connections to your hot water tank and you could safely heat your household hot water tank with wood.  Cold water out from the drain valve, and hot water in at a tee on the hot water out line just above the tank.

If we are still talking instant here.  Small diameter coil of soft copper bends easily without having to be annealed. Batch heating is no where near instantaneous. Physics has some crazy constants. Heat rises, volume matters, not only when we're talking heat exchange but changes from liquid to a gas state. My Toyotomi on demand water heater not only heats domestic water but also has a closed loop radiant floor heat system. If it's been offline (radiant heat portion)  as it is in the warmer months it takes a bit of time to get back online it is not instant heat like a conventional forced air heater or wood.  When working with hot water always think relief valve, better to know where it is going to pop-off than have an unintended failure in a surprise location.

Robert Ray wrote:If we are still talking instant here.  Small diameter coil of soft copper bends easily without having to be annealed. Batch heating is no where near instantaneous.

No, but your tankless water heater controls output temperature of the water either by modulating  water flow, or the size of the gas flame, based on the output temperature, which also depends on flow through the pipes, which depends on a pressurized or pumped supply of cold water to be heated, and an open valve at a faucet or hydronic loop. Your tankless NEVER puts out more than a certain amount of BTU’s and has safety’s to shut the flame off if the water gets too hot too quickly.

 It MIGHT work out to be safe to do a closed loop of small diameter at the end of his gas stream, with a pumped, pressurized, thermosyphoned, or gravity fed supply of cold water to heat... but if not, the results CAN be hurtful or even life ending.  And for an installed system, one must ask “what happens if someone who has NO IDEA how this thing works comes and builds a fire in it, without knowing to do X,Y, and Z first”

Are you leaving a potential bomb for someone else?   Having talked to a few trades-people about steam pressure accidents, including one who had  seen a man literally cut in half by 900 degree steam, after a crack developed in a high pressure pipe next to him, I don’t take heating water with uncontrolled heat sources lightly.

With a shallow pan as the thin, conductive surface on top of a bell, batch heating can be VERY fast... and would be safer than anything using a pipe, or other tube setup, if the water is 1/2-1” deep it should heat rather quickly, given the exhaust temps, and will be safe even if there is no water in the pan.  Adding water when it’s already hot would produce a lot of steam, but it would not be directed in any direction forcefully.

 There’s a reason you never see  wood boiler systems exposing small diameter piping full of water to the fire or flue gas stream...  unlike your gas combustion driven tankless water heaters.

Shallow pan of H2O
|~~~~~~~~~~|
|_____________|
|                       |
|                       |
|                       |
>>>                 |          |     |
Hot Gas in        |          |     |
|                       \_____/      |
|              Out to Chimney    /
|               >>>  ________/
|                       /

And I don’t mean to discourage anyone entirely from experimenting with “instant hot water”.  I think you and Graham both are intelligent enough to successfully experiment with the idea, I am partly posting my warnings and suggestions to make aware, the many others who might read this and be inspired, but are potentially unaware of the serious dangers of closed and semi-closed water heating systems with uncontrolled input and offer them safer alternatives.

I was going to mention that it might be easier to put the copper pipe in the riser itself so less bending to do. A straight vertical run from the top of the riser to the bottom where it bends out of the riser to your cup.
Now the water is going to be subjected to potentially 900 deg C of heat. But would need 10 m of vertical pipe to produce 2 atmosphere of pressure at the bottom but espresso needs 9 atmospheres so that sounds like an experiment outside a high rise building!

Copper will deteriorate rapidly inside a well insulated riser.  Even stainless steel.  Definitely plan on that being a “consumable part” in that location.

I think we are talking two entirely different things when we say boiler and heat exchanger.
A system where the feed resevoir is above the heated tubing and a valve introduces water is what initiated my voice of concern.
When talking scavenged heat from a flue I just don't see where we could stretch to the idea of heating a house but a coffee maker creative and doable. With the desire for instant heat small diameter tubing would be faster and far superiror than trying to create a thermosiphon with 1/2 inch tubing. A feed resevoir with an open loop to the recieving vessel would be safe and  easy to fabricate.
An open pan as has been described seems to me to be cumbersome if the desired end result is a cup of heated water. Why not a kettle on top of the stove?
Running  the tubing inside the flue presents another set of concerns, flue cleaning, acidic residue on the stainless or copper might be better served if monel tubing was used but then why make it so complicated. You have this free heat available outside the flue.
There is a reason that boilers are in the basement.

Matthew Goheen wrote:Also, for anyone wanting to use 1/4” pipe, the recommendations i’ve seen are to use 3/4 or above, as narrow pipe like 1/4” has a higher propensity for the water to flash to steam, due to larger surface area in relation to volume of water contained.

To be clear,  I meant to suggest using a 1/4" or smaller line to feed a 1/2" or larger coil.
I should have specified that.