I seem to have stirred up a bit of a hornet's nest here! Many responses ask for more detail, so here is some.
Firstly the system design. Here is the original schematic diagram which was how I designed it and, in fact, is pretty much how it got built. (Sorry about the size, I'm still using how to learn this forum software, but I think it enlarges if you click on it.)
To recap a little: the heat store is a bog standard 210 litre direct HW cylinder with 2x tappings 180 degrees apart at top and bottom (ie 4 holes in sides and one at top used as a vent here). The flow from (top of cylinder) and return to (bottom) are on one side, and the flow/return to CH system are likewise on the other. All pipework is 28mm.
You'll see from the diagram that there is no direct connection between CH system and stove, but rather that the stove heats the store by gravity convection (over a height of 2 floors), and the the CH system draws hot water from the store when it is available. The actual water and header tank are common, but I'd say it was next to impossible for the CH pump to induce any flow through the stove.
Now for the control system. I designed and built this myself, and I won't bore you with a circuit diagram, but I think a picture of the control panel may help:
The ideas behind this are:
+ Heating should be available on demand at any time, drawing from the heat store if it is hot, and from the oil-fired boiler if it is not.
+ Switchover between heat sources should be automatic, based on the output from a thermostat at the top of the tank. This is set to about 50 deg C, and when this temperature is reached the store is deemed to be "hot".
+ There is also safety circuitry so that when the heat store gets too hot, defined by a 2nd thermostat at its base reaching about 75 deg C, then an override switches on to pump water through the selected CH zone(s) regardless of the state of room stats, timers, etc.
+ The CH system in this house is fully pumped, with 5 zones: 1 being domestic hot water (through a separate indirect cylinder), and the other 4 being various areas of the house. All five zones have separate timer circuits and thermostats controlling their own motorised zone valves.
+ When adding the heat store and wood-burner we added two further motorised valves: one to the oil-fired boiler and one to the heat store. This was to apply positive control over where heat was drawn from, and also to stop heat from the store being pumped through a (cold) boiler, and vice-versa.
I hope you'll all note that there are currently
no valves or pumps in the circuit between wood-burner and heat store. Circulation here is purely by gravity convection.
My theory that reverse convection is happening is based on the following evidence:
+ The return pipe from store to stove is significantly hotter than the flow from stove to store. This based both on the evidence of my hand when touching the pipes, and from the dual-channel digital thermometer I have attached to them.
+ If I blaze up the heat store in this state then the return pipe gets even hotter, the flow colder, and before long I get bumping noises from the boilers at the bottom of the stove.
This has only happened twice in 3 months of operation, but on both occasions it was when:
+ The stove had been loaded up in the mid-afternoon, heating the store.
+ At that time of day the CH system is not on, so the store simply heated up.
+ Once it ran out of fuel (after about 2 hours) the fire died right down, and the stove cooled down too.
+ I then refuelled the stove and blazed it up.
Watching the digital thermometer readout (in real-time) I could see the following happening:
(1) Flow temp (from stove to store) rose slightly, starting normal convection, in the range 40 - 50 deg C.
(2) Return temp (from store to stove) then rose sharply to about 70 deg C as - my assumption - hotter water was drawn down from the store.
(3) This effectively stopped and - my assumption again - slightly reversed normal convection flow.
(4) Yet another assumption: this reversal drew hot water from the stove's boilers back up the return pipe, reinforcing the reverse flow. I could see the return temperature rise and the flow fall as this occurred.
(5) Obviously this flow was poor, since it was working against nature, gravity, the thermal gradient in the system (and everything else!) so heat could not escape properly from the stove, at which point water started bumping away as it boiled.
So my original question was "how could I add a pump to the gravity system in order to restore the correct flow direction?". I would want this to be normally off and to switch on in two circumstances:
(1) When the outlet temperature of the stove exceeds about 70 deg C, as experience shows that it is capable when run flat out of raising the water temperature across itself by 30+ deg C under normal gravity flow, causing boiling, therefore increasing the water flow rate would extract heat more quickly and prevent this.
(2) To restore the correct flow direction when the reverse convection I've described above occurs. This would be switched manually, although in fact putting a thermostat on the return pipe set to 70 deg C would do this automatically.
From the suggestions made so far I think an injection Tee will allow me to plumb a pump in parallel with the flow pipe without adding any valves or other impediments to gravity flow, and this is what I'm going to investigate. However I shall experiment first by making up a tee from plastic drinks bottle and pipe, and pumping inky water around, since my gut feeling is that the detailed geometry of the design (see my picture in an earlier post) will be crucial to its performance.
Finally, since I've been rabbiting on for so long I may as well be hanged for a sheep as a lamb, so here is the rationale behind the system.
The original system was heated from the oil-fired boiler only, and the goal of adding the wood burner was to save money and be eco-friendly.
To heat this house through 5 months of winter requires about 3000 litres of oil, which currently costs about £1100 at 35p/litre + VAT. Heating the house with wood requires about 5 tons thereof, and a ton of green (freshly felled) timber which I can log, store and season myself costs about £40. Also we have some trees ourselves which are free, and I have planted over 100 trees so far this year with plans for many more next winter.
As I'm sure you gentlemen (and possibly ladies) know, the heating oil price pretty much tracks the crude oil price and the $/£ exchange rate, reaching over 60p/litre last summer when oil was $150/barrel. In my view it will head that way again sooner rather than later - do your own sums.
The total cost (stove, parts, labour, etc) of installing this setup was about £4750, and I reckon we will save about £750/year at today's prices.
Also there is no denying that a barn full of wood will give me a nice cosy feeling that - regardless of the antics of Bankers, Browns and others - we will at least be warm in winter.
'nuff said!