Blocking chimney around woodburner flue

Hi
I have a wood burner and a flue that goes just a few inches up the chimney. The chimney was swept and tested and a liner was not required. The room as a vent in the wall too. My question is what to use to block the opening so the heat doesn't all escape up the chimney. Will plasterboard on batons and a collar around the flue be ok?

You need to create something called a register plate.....it gets very hot up there so the best material is mild steel sheet, fixed to the fire back with angled brackets. Timber is not satisfactory.
Personally I use 3mm BDMS to make these things - with a span like yours its self supporting without sag. I also cut an extra hole, plated later, so I can get a sweeping brush up there without shifting the burner.
In the absence of steel, vermiculite board can be used - its expensive but easy to work, and not particularly long lasting if disturbed.
You don't need a collar to fix the pipe to the register plate, fire cement or heat resistand silicone will do.
John

Hi,

Firstly, all solid fuel installations should be completed under Building Control approval or by a competent person registered as such (e.g. Hetas).

If the flue pipe is connected to the chimney and forms a sealed unit (smoke can not escape) then the plate is called a closure plate (not a register plate). It can be made of any non-combustible material. A non combustible material is one that is A1 fire rated. Plasterboard is classed as a combustible material believe it or not so should not be used.

If the flue pipe is not sealed to the chimney then the plate will be assisting in keeping smoke from the room and is therefore a "register plate" and needs to be metal.

in both cases anove there are a variety of building regs to follow: all available in ADJ (Doc J Building Regulations).