Sorry, a bit slow to see this thread.
I have successfully plumbed a wood-burner with a back (and top, and sides) boiler into our existing oil-fired central heating system.
The existing CH system was open and vented, stop here if yours is not!
It was done as follows:
Install a heat store. I used a bog-standard 210 litre direct hot water cylinder with five tappings: two on each side (one at top and one at the bottom) and obviously a single tapping at the very top.
Install this above the stove so that flow circulates by gravity convection using the tappings on one side.
Connect the tappings on the other side so that, in effect, they are in parallel with your existing oil-fired boiler. In other words tapping at top needs to run to pump suction side, tapping at bottom return from flow around system.
The tapping at the very top is your vent pipe and should run up to the existing CH header tank.
Install two motorised valves, one in the pipe from boiler to pump and one in the pipe heat store to pump.
Put a thermostat at the top of the heat store wired up so that when it is cold it connects to the motorised valve on the boiler, and when hot to the motorised valve to the heat store.
Rewire the existing "demand" from the CH timer so that it goes via the thermostat at the top of the heat store, meaning that when heat is requested it opens the boiler valve when the store is cold, and the heat store valve when the store is hot.
The pump should be switched on when either of the two motorised valves opens, so you need valves with built-in microswitches that make a contact when the valve is fully open. The switch in the heat store valve can connect direct to the pump, but that from the boiler valve needs to run a relay which, in turn, switches on both pump and oil-fired boiler.
Hopefully you've followed me so far, so it should now be clear how the system works:
If you light a fire you warm up the heat store, and when it gets hot enough the thermostat at the top switches so that when the CH system demands heat it can draw it from the store. If you haven't lit a fire, or the store isn't warm enough yet, then the system reverts to firing up the boiler.
The heat store also acts as a buffer in that it absorbs heat when the wood-burner is running but the CH system doesn't need it, and also stores it for when the wood-burner has died down yet the CH system needs to run.
To stop the water in the heat store getting too hot you also need an upper temperature limit thermostat at the bottom of the heat store, set at about 85 deg C. This needs to be wired up so that, regardless of what timers or thermostats elsewhere may be requesting, water is circulated from the heat store around the CH system. This is to dump the heat before the water in the store boils, and if you have an existing frost-stat in the system (which you should have) wiring it in parallel with this should do the job.
I'll get shouted at for saying this, but unless your wood-burner is absolutely massive and your house microscopic you really, honestly don't need to worry about boiling the water in the heat store unless you are catastrophically stupid.
This is because boiling 210 litres of water requires a *lot* of heat. The figures work out like this:
The specific heat capacity of water is 4200 Joules / kg / deg C, or in plain english it takes 4200 Joules to raise the temperature of 1 kg (= 1 litre) of water by 1 deg C.
Lets say that your store is already at 50 deg C, then to raise 210 litres by another 50 deg C to boiling point requires
210 x 4200 x 50 = 44.1e6 Joules, = 44.1e3 kiloJoules (kJ)
Stove outputs will be rated in kiloWatts (kW). 1 watt = 1 Joule for 1 second, so 1 kiloWatt = 1 kiloJoule for 1 second.
So let's say that your stove generates heat in the water jacket at the rate of 10kW, then it will take 44.1e3 kJ divided by 10 kW seconds to deliver all that heat, which is 4410 seconds, or 73.5 minutes, or about 1 and a quarter hours.
So you would have to blaze your stove at its rated full capacity for an least an hour and a quarter to boil that tank if it was already at 50 deg C, and for at least 2 hours if the water in the tank was at room temperature.
In practice the system will lose heat in pipework and elsewhere, meaning that it will take longer to heat the water, and it is pretty much impossible to run a wood-burner at full capacity for that length of time anyway since it consumes the wood too fast. So you would probably have to run the thing flat out for between 2 and 3 hours, refuelling it at least two times, while simultaneously not drawing any heat at all out into the CH system, for the tank to get anywhere near boiling.
If you *did* chose to do this you would get lots of audible warning from the stove itself as the water in its boilers started to hiss and bubble.
You would have to be completely nuts (and deaf) to get into this situation, and since a wood-burner has to be manually refuelled it simply is not a problem in practice.
As I said at the top of this post all of the above is based on actual experience: we have a wood-burner rated at 20kW, of which (nominally) 3kW goes to the room and 17kW to the water jacket, and an oil-fired boiler rated at 15kW. Last winter the wood-burner heated the whole house so long as I fed it enough wood.
That last point is worth considering: we burned about 1.5 tons of wood last January when it was really cold, and averaged about 1 ton/month through the winter as a whole. That's a lot of wood, and if you haven't got anywhere to store it you need to think long and hard about this whole exercise.
I hope this helps.