Hot water Thermosiphon Question

Our hot water is heated by an Aga via a thermosiphon, all very standard.

I want to add a second hot water tank to increase capacity - we have a spare one complete with coil etc. My thoughts are as follows.

Connect the outlet from the coil of the first tank into the inlet in the second, and the outlet from the second to the return to the Aga - the coils are in series. Assuming both tanks start cold, the thermosiphon will heat the first tank and then the second once the outlet from the first coil is hot. Will the thermosiphon flow through the two tanks in this way? I can site them next to each other or even so the inlet of the second is at the height of the outlet from the first.

With regard to the hot water system itself, the flow will be from the header tank into the second cylinder, and then out of the second cylinder into the first - for the chemical engineers among you think of a counter-current heat exchanger!

All this may sound complicated, but it avoids the need to balance the system (hot water or the flow from the Aga) and uses the two cylinders that I have. The purpose is to provide a larger inventory of hot water.

Hope this makes sense.

Harry

You can do this either with the coils and cylinders in parallel (which simply increases the amount of stored HW but will take twice as long to heatup. Alternatively you could do this with a pre-heating cylinder followed by the main heating cylinder in series as you outline. This latter might have the advantage that you will get some HW as fast as before but also end up with twice as much.

If you have the cylinder and space for it you might want to consider using the extra cylinder as part of a solar water heating system, instead. Indeed it might even supply most of the HW you need when the Aga is off for summer. These are ridiculously priced systems to buy but look quite cost effective to d-i-y.

HTH