I am wondering if I could use a radiator, mounted on the wall behind my wood stove, to send heat downstairs to a storage tank. Could this work by thermosiphon, or would it require a pump? If latter, would a submerged hydroponic pump be OK, inside the heat store ?

Thanks

gg

1. Does the "storage tank" contain potable water, and were you envisaging transferring heat to that water directly, or indirectly via a coil? I'm guessing direct and non potable.

2. As Nick says thermo syphon is usually "up" but if the water is hot enough it will eventually go horizontally quite easily, but down is be a big ask, especially as the rad may not get too hot.

3. That leaves the pumped circuit to which you refer. The hydroponic pump is unlikely to cope with the potential high temps, should your experiment work well. I think they have lots of plastic bits.

4. One problem may be that should you put in a pumped circuit, you will end up with your rad acting like a radiator. It could draw the heat up from your storage tank to the back of your stove, and quickly "empty" your store of heat.

5. It would also need to be vented or with an expansion vessel of some description. Then you'd need a filling loop etc. etc..

How much hassle are you prepared for?

Hello, gentlemen, thank you for the fast responses !

NICK: Yes, this is a straight "dry WBS", rated at 7 kW, and right again, it would be in the hot zone, behind the stove.

I have 6 inches of clear space, the wall is solid-block, faced with earthenware tiles.
This area it gets far too hot to touch (well over 60°C, which is all my thermometer reads...).

We run the stove from end of September through end of March, most days;
generally a quick burn for an hour or so of a morning, and a longer (4 or 5 hrs) of an evening, light it before sunset, give it a good blast, let it run down and out. Never run it overnight (too much hassle, it is not a continuous-burn model).

The radiator could be anything - single- or dual-panel, steel or aluminium.
Steel is much cheaper, so would probably start there, 60 cm x 60 cms, for "discretion"

And thanks for the "prototyping method" would never have thought of that (-:

====
OWLMAN, thanks also !

(1) Like you said, direct and non-potable. The intention is to BRANCH my CW mains through the tank (and NOT the rad!), to preheat DHW and save money in winter, when the wood stove is on. The RAD is on an open circuit, so per intent, I would not want a pressure vessel or relief valve etc.
The tank will also be uninsulated, so losing heat all the time anyhow, into the basement (which is part of the intent).

(2) Apologies if I got confused between thermospiphon and gravity siphoning, I actually meant BOTH / EITHER !
But I get the drift (I think !).

(3) Re the hydro pump, I just happen to have one lying around; maybe I could use it for experimenting with, then if it looks hopeful, invest in a proper boiler or CH pump.

(4) I envisage having 240 liters of CW to start with: compared with my DHW tank, that represents around 14 kW when charged. I don't expect my 7kW wood stove to fill that tank with HW in anything under two or three sessions, at least, so hopefully your fears will not be realized !

Many thanks again !

gg

Avoiding any calculation, my instinct is that the rate of heat transfer to the radiator will be a lot less that you may be hoping. The wall behind gets nice and hot because it does not conduct heat away very well, but once you start pumping cold water there...
Ok, while the water temperature is low the rate of heat transfer will be higher, but it will still be quite low unless the rad is very closely coupled to the stove wall IMHO.

Why not just run a length of copper pipe in the flue, then pump it.

Find the point in the flue where the temperature is about 90°C and put the pipe above that.

Posted By: SteamyTeaWhy not just run a length of copper pipe in the flue, then pump it.

Well, the flue is twin-wall flexible stainless steel, I would not want to be puncturing it to install a pipe !

The flue is routed in a RC cage.

It is planned to install air pipes in the cage, so space is at a premium. Also, I prefer to avoid a multiplicity of heterogeneous systems... Penetrating the RC is somewhat of a complex and expensive operation...

So the radiator is the only feasible way (at present...)

Thanks anyhow !

gg