Woodburning Stove, Fire back boiler, Thermal store and DHW integration.

[julian]

On a related note, im kind of surprised that there are not more 'drop in' flue gas heat exchangers about.

As dhaslam says, my concern with back boilers is that they strip too much heat from combustion, and make it inefficient, however, the flue gas heat is only needed for one thing (the draw) and im sure a lot of temperature could be stripped from that before the fire did not draw well.

I suspect it may be that, in most installs, there is not enough physical space above the unit for a heat exchanger.


wh00sher:

Do you find that the fire burns equally well now that it has a sub-100c lump strapped to it?

[clivejo]

Regarding your drawing, is there a reason for the flow of the WB Boiler to be entering the tank so low in the tank?  (to stir it up) Also, is it a good idea for steam etc to have to make its way into and through the tank before being vented??  Also is there need for the second pump.  As the heat exchanger removes heat the cold water should fall, creating a thermosiphon, if the exchanger is kept as high as possible?  Do you really need a heat exchanger??  So many questions!!

I've defaced your nice drawing to try and show you what I mean.

[wh00sher]

Clive, the Back boiler return is at the top of the tank, the drawing I used as a `template` had it entering near the bottom, but I don`t.

I haven`t tried it without the `2nd` pump. I can easily do so though. Hmmm, I`ll try that. I understand your theory, but I think the heat exchanger may be too flow restrictive for a gravity circulation ?

I can`t do as you have in your second drawing. The heat store cylinder above the fire is vented to atmosphere. My DHW cylinder, UFH and system boiler is all pressurised. Your drawing doesn`t separate the 2.

Julian, what I did was weld to pieces of 10mm tube to the face of the back boiler, that helps `space off` the timber which helps it burs efficiently. The only time I find an issue is when I have a load of small bits of wood. ie 3/4" cubes from my supplier. They tend to build up at the back of the fire if it is set on `half`. If I open up the air fully, its no problem. If it does build up with cooler wood at the back, once the wood at the front has burned away it `falls forwards` away from the back boiler and then burns nicely.

Thats why I initially made a `half height` back boiler and fitted it at the top of the stove, but it wasn`t efficient enough.

If I had to buy everything from scratch and had a larger budget, I`d look at a large cylinder with once of those `finned` coils for the DHW take-off, instead of having 2 separate cylinders as I have now, but this was all about using parts I`d accumulated over the years. I`m glad I had, the current price of copper would have made the install pretty expensive !

[clivejo]

I haven`t tried it without the `2nd` pump. I can easily do so though. Hmmm, I`ll try that. I understand your theory, but I think the heat exchanger may be too flow restrictive for a gravity circulation ?

Yes, if its restricting the flow a pump would be a good idea!

I can`t do as you have in your second drawing. The heat store cylinder above the fire is vented to atmosphere. My DHW cylinder, UFH and system boiler is all pressurised. Your drawing doesn`t separate the 2.

That explains why its connected to the mains and why you need an exchanger    I'm not a fan of pressurised systems, but they have their uses!