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Essential for Newbies: Wood gasification boiler - pressurised expansion vessel v. open system
I am planning the installation of a wood gasification boiler and have been feeling fairly confident having tackled oil-fired systems in the past but they have used an open feed tank in the loft to keep the heating circuit 'topped' up.
The oil-fired systems have used copper for the hot-water cylinder and the large accumulator tanks for a gasification boiler are generally steel with a greater risk of corrosion.
It seems as if continental European systems are closed circuit with a pressurised expansion tank, presumably this means that the water is not in contact with the atmosphere with less potential for oxygen becoming dissolved and corroding the system. Is my interpretaion correct and what size of pressurised expansion tank do I need, 10% of system volume? Also, where should the pressurised expansion tank be located, near the boiler/accumulator or as high as possible?
Inhibitors greatly reduce the problem of corrosion, as does best practice system installation, ie good pipework with no minor weeps that allow air into the system. You shouldn't have need to be constantly "topping up" the system via the filling loop, thereby bringing in fresh oxygenated water, if you do, in my book it's been poorly installed. 10% of the whole system is the general rule of thumb for pressure vessel capacity. I have mine next to the accumulator where I can keep an eye on system pressure etc. I have two nice large 4" pressure guages one on the boiler and one on the PV. The feed to the PV is usually taken off the accumulator/boiler return pipe, it seemed sensible for me to put it where it is; less pipework and I liked the idea of everything being easy to hand. Mike
Mine is the European version (and it does auto-top up via a pressure valve, which also tells me what the pressure is). Happily invested in an inhibitor at extortionate cost - with a 2,000 litre tank (less the 320 litre DHW tank inside it - with separate 25 litre expansion vessel) it needs a lot! And my 4x25 litre expansion vessels sit next to my tank - in theory they can be anywhere but it is better to have as little between them and the tank as possible because one wouldn't want a valve turned off now would we.....
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