Chimney Chemistry
Chimney Chemistry, Access Hatches and Register Plates
Register Plates
One modification for such (to us) overlarge chimney voids is to fit a hood or a stove, and in each case, the chimney has to be closed off so that air from the room does not go around the stove or hood.
This is achieved by fitting a false ceiling called a 'register plate', which is usually a horizontal sheet of metal that is pierced by the neck of the hood or the stove fluepipe.
It has to be mentioned that this sheet should be made of metal, because occasionally such plates are made of wood planks, plywood, plasterboard or other unsuitable material which, being flammable, have an excellent chance of igniting. Fireproof materials which resemble asbestos sheet (but without the health risks) are available, but are not in practice suitable. They are vulnerable to rain falling down the flue, they can have a tendency to crack if they get hot, and above all they are not always strong enough to resist having brushes and rods pushed through their access points.
Again, these plates should be made of metal and be thick enough to resist sagging and the inevitable corrosion. They should NOT be supported on wooden battens, but on metal supports, and where these are fixed to the wall, plastic rawlplugs can melt.
Access Hatches
These MUST be big enough, ideally 9" x 9" as a minimum.
They should be closed off by lift in, lift out loft door type access hatches held in place by gravity. (PLEASE, no screws, no bolts, no catches, no hinges, no runners, just square, simple lift out hatches).
Threads freeze solid with heat, as do sliding doors, and we risk damaging the register plate if we have to use force to open them. On most plates, two hatches are needed, but where the plate is particularly wide, a third one can be useful. If you only have one, it can be difficult if not impossible for us to remove the soot each time, and it there is no access at all, then soot can build up until it catches fire.
This plate fire can turn into a chimney fire with all its associated problems. Worse, there will be no way for the fire brigade to deal with such a problem without ripping out the entire plate with all the mess that can entail.
Chimney Chemistry
The binding agent in cement mortar is cement which is a calcium atom with two hydroxl groups attached. It is written thus: Ca(OH)2.
Sulphur from the fuel burns in the fire to give sulphur dioxide (SO2), which reacts with water in the flue gas to give sulphuric acid (H2SO4). This reacts with the cement thus:
H2SO4 Ca(OH)2 > CaSO4 2H20 (ie acid plus base equals salt plus water)
The calcium hydroxide is turned into calcium sulphate, and similarly into calcium nitrate and calcium carbonate (chalk). These salt groups are bigger than the OH groups they replace, so they swell, and because they are weaker than the cement particles, the liner expands and softens, crumbling away. The whole stack actually gets taller, and where one side is cooled by the wind, it bends like a bimetallic strip in reverse (ie the colder side expands).
This chemical reaction can also lead to staining on the chimney breast. (See below)
Staining on the Chimney Breast
The salts formed in the chimney (See 'Chimney Chemistry') don't stay in the liner, but are the victims of a tug of war. They make the lining hygroscopic (ie the salts pull at the water at the liner flue interface) as does the stream of gas/air passing up the chimney. Normally the stream of air is strong enough to keep the liner comparatively dry, and there is not enough water to soak the salts further into the chimney structure.
The real problem starts when a once used Victorian chimney is closed off, and the steady stream of air stops. Instead the air from outside enters from the top, bearing with it a cargo of moisture. The salts pull at this, and gradually the salt solution soaks further and further between the bricks and out into the plaster on the chimney breast which can discolour badly if the acidic salt solution is contaminated by tars.
There is little real cure for this save the removal of all contaminated material, for example stripping the breast of plaster and replacing it with waterproof plaster. While not a permanent cure, it can effectively be if the problem takes long enough to work around the repair. This is why the repair has to be radical. Simply removing the stained portion and replastering will most likely soon show a ring of staining as the salts creep past the repair.