Hi Coul
the flue sizing will be determined by the type and size of boiler - plus factors such as the flue diameter and to an extent possibly whether there are buildings nearby. On my boiler for example if you use a 200mm flue it can be shorter than a 150mm diameter flue. Even if you need say 7 metres it is possible to strap the top of the flue with stays to hold it stable in adverse weather.
You can fit a control to your heating system to make it work at low temperatures but with a wood gasification boiler you need to take a slightly different approach to the way gas and oil boilers work. The boiler is usually installed with a themal store / accumulator tank. This is simply a tank of water that 'banks' the heat that the boiler produces. When you need heating (or hot water) you pump this heated water around you radiators or UFH using a normal heating control programmer. So it can be cold - but your boiler can be off. Coversely it can be a mild winter day but you can be firing the boiler because the store has cooled to the point where it needs 're-loading'.
Wood boilers often produce water at 80-90 degrees and that's why they aren't usually directly connected to the heating system.
Wood boilers burn logs, chips, or pellets of compressed sawdust. A log boiler uses the cheapest fuel but must be loaded and lit manually. It may also require reloading with logs during a burn to heat the store fully. Chip and pellet boilers can achieve a greater degree of automation and can be self igniting. The fuel (especially pellets) are more expensive than logs.
So frost protection is perfectly possible with a wood gas boiler - but there are some details to work out first and this can then be a function of the heating control programmer - unless you go for a fully automatic self igniting hopper fed pellet boiler.

Thanks Julian, helpful comments.
 
My concern about frost concerns the boiler. If we are away from the house for a couple of weeks and it get really cold, the boiler, being in an outhouse, could freeze. I guess there are three approaches:
 
- put antifreeze in the system
- have some sort of electric heater in the outhouse to keep the temperature above freezing
- drain the system, but that's a lot of water, corrosion inhibitor etc..
 
Anyone else tackled this issue?

Well, anti freeze will turn down the effiziency of the heating system by a few percentages, similar to ST systems where it is used.As well the antifreeze will deterioate in a boiler system due to high temperature exposures. Regular checking and replacing is a must-and costs even more percentage points on the efficiency of the heating system(energy input versus output)
The situation is bad enough with the positioning of the boiler, so be carefull when bringing it down even further.
 
Is there a reason why opting for a wood gasification boiler ?
Have you contacted a heating engineer who has figured out for you the most cost efficient/environment friendly heating method? What did he/she recommend?
See, the internet is a bad advisor on real world questions. Everything can be laid down in a logic way, but this needs real knowledge of the situation, information is of little help.... Facts and numbers are a must for this aproach. Contact a professional, get his/her advise signed and guaranteed.

Before i make a comment i wish to say that i work for Eco Angus Ltd as one of the Directors and i add my comment purely for the sake of the thread and to add information to the debate as this is how people gain knowledge.
Flue size is dependant on the size of the boiler as Julian has commented. Ours range form 180mm to 210mm and we suggest stainless steel twin wall flue (unless you are fluing to a chimney stack).
The height is governed by the need for invariably 20Pa Chimney Thrust. Usually the vertical climb of the stack will be around 6 to 8 metres.
However (and forgive me as we offer these) if you have a supplier who can offer a chimney fan this will reduce the vertical climb of the chimney stack greatly.
Most of the time with gasification boilers the chimney flap is shut (only open when you are re-loading or starting a fire). When the chimney fan is open you can turn on the chimney fan to suck the gases up the vent reducing the need for a high stack.
We have a number of installations where all works well with a 3 to 4 metre stack but the longer the stack the better the draw as a rule of thumb.
As far as the building of the outhouse you want to reduce your pipe runs to a minimum if possible. There are several companies that offer specialist piping (Uponor/Watts Industries + Flexenergy) that minimise the velocity in the pipework with specilist fully insulated pipe carriers that can go underground (excellent low thermal loss readings on these).
Lastly frost protection...there are some systems (like ours and others!!!!) where the probe in the water jacket that is linked to the microprocessor on the back of the control panel can tell when the water reaches a critical low point (for ours it is 4 degrees).
At this point it would start the pump om the Laddomat thermoregulator (21-60 or 21-100) as anti-freeze protection.
Hope that helps.