My friend,you have half a neutraliser and a oil /heating system tagged on the side with unessecary motorised valves in it.
Well I could only start from where I was at and yes, I've got a hybrid system ... which is exactly what I wanted. I've added two valves: one to isolate the oil-fired boiler and one to isolate the heat store. Hardly complex! Compare my diagram with the Dunsley one
http://www.dunsleyheat.co.uk/layout.htm
A true neutral point has all the flow and returns from every boiler and flow and return from it.
True. But that is not what I was trying to create. If I designed and built a house from scratch, and had unlimited money, I would have a truly massive heat store - probably underground because of the weight - and use underfloor heating because it will run at 25 - 30 deg C, run from a ground source heat pump. But I have an existing house that is 4 centuries old in parts; built mainly of stone, cob and wood; with a wet system using existing radiators. Short of adopting the "rip it all out" suggestion I have to work with what I've got.
Your earlier post that a 210 litre store is too far small for a house this size is correct. I'd love to have 2000 litres, or even 20,000 litres. But how could I possibly put that sort of weight into my roof structure? I bought the largest commercially available HW cylinder because it was the best compromise between cost, size and weight that I could achieve. In fact it holds about 1/2 hour's stove output at full blast, and that is a reasonable compromise.
You also state that if it was going to overheat you would have to be deaf and dumb not to hear a problem.But if you had a gravity heat leak circuit as I suggested then you wouldn't even have to be at home to ensure safe operation.
Again, true. But a wood-burner is a "hands-on" device, it can't be fuelled when no one is around. I could have bought a wood pellet boiler, but they start at £4000, require a hopper (another £1000), and pelleted wood is not much cheaper than oil. A silly solution in my view!
In practice the control system drops back to the oil-fired boiler when there is no wood-derived heat, or when we can't be bothered to light a fire.
Also I think that ideally you should have bought a solid fuel burning stove with a sensing phail in the water boiler to regulate the burning rate by automatically closing or opening the air inlet to control burning rate as needed. This would have negated the need for a thermal store in the first place and suffice to say a smaller neutral point was all that was needed.
As a matter of fact the stove has an "aqua-stat" which does exactly this. It is a flap over an air vent attached to a mechanical thermostat which is driven by the saddle boiler water temperature. I didn't mention this earlier because it didn't seem relevant to the flow problems.
It works, but the size of the air vent means that it only admits enough air to deliver about 1/3 of the max output when fuelled with hardwood. (Soft wood requires less air for more flame, and I suppose it is sized conservatively for this reason.) So it is not a complete solution.
In a funny way it may actually contribute to the reverse syphon problem by cooling down the stove when the store is hot!
Ideally running the system at a faster rate till hot water and heating hot then reducing air flow and refuelling just before bed to ensure constant steady heat till the fuel runs out and the oil boiler then kick in when needed.Now as well as heating up a five zone system you are heating up a thermal store and using extra fuel for no reason other than to distribute it into a very large system which has a lot of water in it so in effect only slightly warming the rads.
I have tried both "blaze up at bedtime" and "damp down to slumber overnight" approaches. The former seemed to work better, as low combustion rates not only produce too little heat to be useful but also tend to tar up the inside of the stove.
By the way the five zones have been there for 10 years. When we bought the house in 1996 it had only one zone, and a 15kW boiler which the builder said was undersized and should be replaced at vast expense. We deliberately broke the house into multiple heating zones, each separately timed and controlled, and I cannot understand why all houses are not organised like that. We still have that 15kW boiler, and by directing its heat only where and when it is needed we have saved a fortune in fuel. Zoning the system also reduces the volume of water you need to heat at any one time.
In practice the heat store will typically reach 60 - 70 deg C overnight, and will run the rads for 15 to 20 minutes in the morning before it gets so cool that the oil-fired boiler takes over.
You correctly state that for full efficency for a woodburner you must burn it fast to ignite the gases that form smoke.This means wood is o.k while getting the system upto temperature but after that a fuel such as coke or anthracite is better for a big system such as yours as it burns better when regulated.
Wood is £40/(wet)ton. Coal is £400/ton. Energy content of dry wood is 18 - 22 GJ/ton, energy content of coal is 27 - 30 GJ/ton. Money doesn't grow on trees, but wood does!
A lot of stove manufacturers make a wood stove,stick a boiler in it because they are scared to lose custom if they don't as everyone does it but a solid fuel burning appliance would have been better suited to your needs.
See cost comparison above. We have enough trees for about 20% of our needs, a dutch barn for drying it and a chainsaw, and I'm not afraid of hard work. Also I grew up in a house with an anthracite fired Aga, and I remember well the happy day when we converted it to oil. Coal is filthy stuff!
One thing to watch for if your water temperature on the return going into the wood boiler is too low it will rot the boiler from the fire side of the metal as the acidic gases condense on it .Lower water volumes are preferable or failing that a low limit stat to temporarily stop the pump from pumping cold water back to the boiler.Again wood is not ideal at preventing this.I have seen some boilers replaced within a year.
I'm well aware of this and, as you say, a smaller store is good in this respect because the return temp rises quite fast. Wet wood + coal is an absolute killer according to the instructions provided with the stove. Had I fitted a larger store I think it would have been necessary to organise a (pumped) loop around the stove to recirculate hot water purely to stop in the incoming flow being too cold.
Because I clear out the ash every day I have been monitoring the boiler state, and can check for any corrosion. So far so good... the key is to burn dry as opposed to wet wood.
I must go and get ready for the pub as it is time to go out.
Beware reverse circulation of beer, it can cause hiccups and other unpleasant symptoms!
You can make a tee with a cut off reducer slipped into the port of the tee which you want to be the injector . Did it myself in the 70`s