Insulating a balanced flue fire

[A1W]

My balanced flue fire is draughty from below and the hearth is freezing cold.
I noticed open cavity with some loose pack fibreglass insulation above and to the sides of the fire, but nothing below, when the fitters had it out. Is cavity closing likely to be the problem area?
How should the installation be insulated?
Wonder if an RGI expert from the combustion chamber group might be looking in and could advise...

 

[durhamplumber]

Is cavity closing likely to be the problem area?

Find the manufacturers fitting instructions.

 

[A1W]

Thanks, I have them, but they don’t help. I downloaded some instructions from another manufacturer which help ish.. says the cavity must not be bridged, then advises putting 100mm rock wool in it above and to sides, with a drip tray above. That would be an improvement on mine. Doesn’t say Anything about the cavity at the bottom, and can’t really tell from the pics whether there is anything behind the box.What do you reckon should go there?

 

[durhamplumber]

Thanks, I have them, but they don’t help. I downloaded some instructions from another manufacturer which help ish.. says the cavity must not be bridged, then advises putting 100mm rock wool in it above and to sides, with a drip tray above. That would be an improvement on mine. Doesn’t say Anything about the cavity at the bottom, and can’t really tell from the pics whether there is anything behind the box.What do you reckon should go there?

No expert sorry,but from experience balanced flues are usually sealed all the way around,otherwise it is defeating the objective.

 

[gasbanni]

A balanced flue fire is room sealed and draws its air from outside, no ventilation is required.
The outer part of the flue should remain cold and the inner flue is how the products of combustion are exhausted and is hot.
Combustible materials shouldn't be used so they can contact the flue.

You could use expanding foam around the back / bottom of the hearth or stuff it up with rock wool.
You could use a thick stove rope seal stuck around the flue at the back of the fire to stop draughts from the cavity pushing the fire back would compress it against the wall forming a seal.

It's a pretty easy fix just don't have Combustible material in contact with the fire flue.

 

[A1W]

Thanks guys for the good steers. Gonna find a RGI to whip the fire out. Afore I go, can I take the opportunity to ask one more dozy question. Can I assume that the flue is the only hot area and that the back of the fire - the firebox? - doesn't get hot?

 

[gasbanni]

The manufacturer's instructions will specify this .......I've seen plenty of wall mounted fires mounted on wallpapered walls, not recommended but seen loads with no visible signs of distress (scorching) a non combustible surface is recommended. Marble natural / conglomerate, slate. Google fireplace back panel.