Builders opening fine but catchment space HUGE!

[Groovejet]

Two chimneys in house arn't built the same.
Both builders openings are within MI's, but 1 chimney has larger opening area inside the builders opening.
Im concerned when the fitter installs DFEF fire, the products wont immediately go.
So i need to brick up sides, but as i get higher , just below flue direction change. It wont be possible to just brick up and .....job done.
Because the flue is large, if i bricked directly up, id end up laying bricks straight through the 9 x 9 flue itself. So somehow the new brick walls need to be built with the need to close off the sides completely..
Talking about its 1 thing, doing it is another.
I know it may be hard to get a visual pic, when not infront of it, but has anyone got any tips? Maybe i should lay brick by brick in a half lap?, which will give strength and also change direction, creating a smaller builders opening.

The 2 images show differand sized openings.
Sorry bt no photos of internal flues,

 

[footprints]

Best to speak to your installer before making any alteration you may end up pulling it all out again!

 

[Groovejet]

Best to speak to your installer before making any alteration you may end up pulling it all out again!

Ohh yes of course. Im in contact alot. Thing is the customer found it hard to find a fitter to do job, start 2 finish. Guys would fit a fire & not a surround. And guys would only fit surround. madnes.
So he told me to brick directly up both sides, turned out 3 blocks high.
Its still a grey area ive never dealt with before.
Ive constructed many builders openings. Done everything apart from this issue before.

 

[Madrab]

It all depends on what look the customer wants at the end of the day.

One option would be to use a specific fire back with built in chair/knee and sides and install a liner on a registry grate/plate and build the rest out. As suggested though you would really want to have whoever is installing the fire on board as there are only certain things that can be used to ensure compliance.

This page is for a solid fire and the diagram is for an inset open fire but for a DFE the principle would be the same.

http://www.soliftec.com/openfires.htm

 

[sbgasman]

DFE are very inefficient, then there’s the 100 cm square vent that the wind will be howling through

 

[allfives]

DFEs are not efficient anyway but depending upon the type used you might be able to get a galvanised dfe catchment box to reduce the opening, with respective flue spigot 5- 7-inch diameter then pack the anular space with Rock wool to prevent chilling/condensation, around the sides take the correct size flue liner if required into 9" brick chimney by 1mtr or direct to the chimney making sure the draught pull is good and meets MI when fitting the fire

 

[FiremanT]

When you say a DFE, what do YOU mean?
Have you the fire already?
A DFE can be a in a square firebox, a tapered box, or just a tapered tray designed to sit into a chair brick, normally with 18” or 16”. Although you will not find any part that measures 16/18”!

A photo of the new fure or a model name/GC No would help

 

[allfives]

Dfe = Decorative Flame effect
Lfe = living flame effect as firemanT says there are different types, some do not need additional ventilation and others require 135 cm2 minimum ventilation another aspect which make them less efficient depending on KW/h and type of fire using