And can anyone clarify what a powaflue is - I've seen an nice looking
model which says that whilst it doesn't need a chimney, it does have a
twin walled flue, so I'm wondering where the twin walled flue actually
goes!

OK, the following types of flue are available for gas appliances.

1. Flueless. The oxygen comes from the room. The fumes go back into it.
Advantages: Very cheap, simple.
Disadvantages: Possible death. Limited to low power. Requires massive
ventilation. Condensation.
Examples: Gas hob. Portable gas space heater. Cigarette lighter. Plumbing
torch.

2. Open flue. The oxygen comes from the room. The fumes rise up a chimney
flue using convection.

Advantages: Chimney may already exist. Removes combustion products, like CO
and water. Cheap appliances.
Disadvantages: Ventilation still required. Chimney problems may lead to room
contamination. Flue expensive to construct or fix if broken.
Examples: Traditional range cooker. Boiler from 1970s. Traditional gas fire.

3. Balanced flue. The oxygen comes from the flue terminal. The fumes are
ejected from the flue terminal.
Advantages: Room sealed. No specific requirement for ventilation. Cheap flue
to install.
Disadvantages: Flue length limited to short horizontal length through wall.
Must be situated against outside wall. More expensive appliance.
Examples: Balanced flue fire. Boiler from 1980s.

4. Fanned flue. The oxygen comes from the flue terminal. The fumes are
ejected from the flue terminal. A fan is used to enhance this process.
Advantages: As balanced flue. Additionally, flue length can be long, and
involve bends and vertical and horizontal sections. More reliable flow than
by convection/expansion alone. Good for high power applications.
Disadvantages: More expensive appliance. Long vertical flues may be more
expensive than an existing open flue. Fan may be noisy.
Examples: Fanned flue fire. Modern boiler.

Christian.