Coil for preheating ECS

[d2taf]

Bonsoir.
I have recently been traveling forum, a mine. I do not doubt to get some wise advice, some tracks to follow. And some others to avoid ...

My wish :
On a wood stove like Deom that heats my house in parallel with a PSD, I envisage a water recovery by external coil at the beginning of the steel duct (vertical, D 150 on L 1000, before interlocking on stainless steel double skin). This recovery must go into a 250L flask to preheat the water, which then goes into a plate heat exchanger for instantaneous DHW (400L flask of heat transfer fluid -ECS and PSD-, heated by solar panels and electric back-up ). The goal of the game is to dispense with this electric booster in winter.
The whole installation is functional, it remains for me to put the ball (which I already have) and to make the recuperator.

Precision: It is possible for me to circulate this water in a plaster like earth and hemp on a thick stone wall, in order to increase the inertia of my stove and to cool down if necessary the water circuit which will dive into the balloon. This option is not in place, it's just an idea.
Of course I'm planning a regulation, an expansion tank, multi-way valves, in short, I see a little how it should work.

My questions :
Does such a system make it possible to heat 250L with water at 40 °? Even though preheating to 20 or 30 ° would be nice.
On the contrary, if this system heats up too much, the buffer wall is a good idea.
The advantage of both, preheated water and calorie accumulation in the wall is it possible?

If all that is not very clear or precise, I answer all your questions.
I am a buyer of all advice, all warnings ... Thank you in advance.

[dedeleco]

Some elements have been realized on econology with detailed photos, search with the search function or google.

[d2taf]

Yes, I have read some topics, but nothing that really answers my questions. For example, a Deom transformed into a boiler, with a very mixed record of performance and ultimately a shutdown of the system. If I understood, the dissatisfaction came mainly from the fact that the boiler was dedicated to this use. I saw another Deom with coil for a balloon and a radiator, all in thermosiphon, for a total satisfaction in appearance. Two very contrasting experiences so. And in either case, the coil was wrapped around the heater, not just on the smoke outlet.
In short, my questions remain: Considering that the stove operates on continuous fire, is a coil only on the duct at the exit of Deom realistic for the preheating of 250L of water; is duct-induced cooling a big problem; Is a "radiant" wall a good idea or will the heat recovered be insufficient?

I can not imagine the temperatures of the system. Hence my question of ignorant.
My solar installation works very well, I am only very reluctant to use an electric booster for hot water in winter. I would also like all the system (circulators) can work on photovoltaic. I think, read to you, that the preheating of water is feasible, but what about a radiant wall? If this wall idea is realistic (and useful?), Should it be connected in parallel, regulated as a discharge, or simply following the balloon?

In summary, I have trouble getting a clear idea of ​​the possibilities of such a DIY. The main question remains the capacity of a coil on the flue.

[loop]

Bonjour,

The essential difference between heat recovery in a stove or around a flue is that in the first case, it is mainly radiation that turns into heat in contact with the inner wall of the flue. exchange and in the second case it is conduction between the hot gases and the casing.
To get an idea, a fireplace that would release 10KW heat, loses 2KW in the flue if the overall yield reaches 80% (which is already not bad).
Recovering those calories circulating in the conduit is another matter. On the one hand because the gas is bad conductor / heat accumulator, so it would require a very large exchange surface (not just a piece of tube wrapped around the duct) and that cool too much smoke reduces the draft and can cause the formation of bistre (it is already a problem in my insert-boiler yet planned for !!)

For the rest of your project, we would need a sketch, it would be easier to understand.

A+

[dedeleco]

The temperature of the flue gases at the outlet (without excess air for combustion) indicates the yield, probably around 60% to 70% and therefore there remains 40 at 30% for a heat exchanger at the flue gas outlet which will therefore heat the circulating water in to a flow rate automatically adapted to the stove power available by thermostat controlling the circulator if the water before going out is hot enough.

So that it is the T of the fumes at the exit (pipe at the exit of the fumes isolated over the thermometer?); 200 at 300 ° C ??? given the shape of the exchanger stove apparently not very big.

A priori there is enough to heat water well before having condensation of steam fumes.
A tee on the fumes can collect this condensation for more security, if lack of luck and avoid that it falls back into the stove (it deteriorates) and also collect ashes deposited by the fumes, and thus improve the heat exchange.

For more precision, it is necessary to quantify the radiant wall (surface, thickness and T) and the T of the desired balloon, but it should work.

[loop]

By recovering 1 KW of power on the duct, which would be good, it would take 1h to heat 20L of water from 10 ° to 50 °.
It gives you an order of magnitude.
I will try to find an experience of this type that I read on the web.

Some reading

https://www.econologie.com/forums/recuperer-de-la-chaleur-de-son-conduit-de-cheminee-t10217.html

A+

[dedeleco]

the second case is conduction between the hot gases and the casing

is exact but this is calculated (charts on econology) and saw the T fumes often raised 200 300 ° C, gives a variable power output depending on the valid combustion force that is usable if a thermostat adjusts the water flow to maintain its output temperature at the desired value and stops this flow if this T is too low.
This is crucial, because then we heat water whatever the quality of the exchanger, more or less quickly obviously, avoiding excessive condensation and the associated bistre.
Too often we use a circulator at constant speed (or 3 speeds) without ever being able to stop it, which is absurd.
The bistre exists further in the chimney and is translated by a regular sweeping on any chimney.
The bistre increases dramatically if the combustion is incomplete and bad (wet wood and poorly distributed air, poorly designed stove).
A tee below the exchanger harvests this bistre by shaking and scraping with a hedgehog from time to time !!
The heating time depends on the power of the burned wood (1Kg per hour = 4KW) of the yield which fixes what remains in the fumes and therefore 240l at 40 ° C with a residual 1KW in the exchanger will take 12h approximately.
The exchanger must be big enough on the surface (10W / ° Cm2 about 2 or 3 near and so for 1KW with smokes to 200 ° C it takes a surface 1000 / (10x200) = 0,5m2 of smoke pipe, I repeat to a factor 2 or 3 close !! (in turbulence the laminar layer of air to a few mm thick which slows the heat exchange by its conductivity on about 3mm !! Then the calculation with the air at 0,28W / m2 ° C becomes a biblical simplicity comparable to that of double glazing, which have an immobile air thickness comparable factor 2 or 3 comes from the fact that this laminar boundary layer decreases in thickness with agitation)

[d2taf]

These few figures and info help me to see a little clearer.
I said again, it's a coil outside the duct I'm considering.

In the absence of sketches, a small summary of my current installation:
12m2 of collectors which heat 400L of fluid in a stepped tank (slabs, DHW). An electric back-up takes over from the sun if necessary. In practice, I only use this extra for DHW. To do without it completely in winter, I want to heat with wood (a Deom type stove in service all winter) the 250L of water from an annex tank already equipped with a plunger. The water from this "wood" tank then passes into the solar tank via a plate heat exchanger and is then distributed.


If I am fine: I wind up the maximum of copper around the linear meter of tube (diameter 150, so surface 0,5m2).
I circulate the water or the fluid (?) Thanks to circulator regulated by thermostat.
I test the results. If it heats more than necessary, I add a discharge flow in a wall. I have two walls of 4m2 each, something to dissipate (usefully) I think.
does that seem like a good approach?

About the copper heat exchanger: Is a thermal paste useful in copper / steel interface?

Thank you for your opinions.

[Philippe Schutt]

To know how much you can still recover, you should know the temperature of the fumes, and the nature, the section and the height of your duct. Because if it begins to sound, you will not have won anything.

[d2taf]

To know how much you can still recover, you should know the temperature of the fumes, and the nature, the section and the height of your duct. Because if it begins to sound, you will not have won anything.

No idea of ​​the T °. I'll try to see that. To get an idea, it's a Deom type stove (plunging pipe for air inlet at the base), "Green Hedgehog" brand, internal diameter 42cm, useful height 60cm. Front loading by small glass door (30x30). Small oven on the hearth. Given for 14Kw.
The diameter 150 pipe is a stainless steel double skin over 4 or 5m, connected vertically by a single steel tube of 1m. The draft is good, without being "voracious." Little soot in the duct, except at the top (wood sometimes not dry enough) House in the Alps, at 1300m.