If you are in any doubt about the maintenance of the chimney, have it checked internally and above the roof. Typical problems with chimneys are fumes, a cracked or leaning stack, spalled brickwork and damp.
If a chimney has been damp for a long period, the acids in old soot can eat into the mortar in the flue and, in the worst cases, allow fumes into other rooms and an adjacent house. A stained or, when a fire is established, a hot chimney breast is an indication that the mortar is in poor condition.
If smoke is emerging from another fireplace above, then it is likely that some of the feathering inside the stack has come away; these are the bricks which separate one flue from another. You should also check the upstairs fireplaces for failed pointing which may let smoke in. If the problem proves to be the feathering, you will need to have the flues lined. Evidence of smoke in the loft would confirm this.
To be certain, you can have a pressure smoke test carried out in accordance with BS 6461 and Appendix E of Approved Document J of the Building Regulations. However this can be costly if the problem is self-evident.
It is not compulsory for pre-1965 houses to have the chimneys lined but it is a fact that many properties built about that time are showing signs of deterioration within the chimney. If there is evidence of smoke leakage it is imperative the flue be lined in order that it be made safe. The smoke contains carbon monoxide which is very dangerous in a closed space.
Where rooms have been knocked through, you may find smoke and fumes being drawn down the second flue. Fitting a chimney fan is not a recommended solution; ideally consider lining the flues in case the cross-flow is due to failure of the feathering. Alternatively, cap one chimney, leaving minimal ventilation. The design of the fire opening has no impact on this issue, however the size of the opening is significant in relation to the flue size, especially when taking into consideration the final size following lining. Openings which are too large for the eventual flue size will likely cause smoke to blow back into the room from the fire itself. You may therefore need to reduce the fire opening by installing a replacement 400mm wide fireback.
Most old stacks lean or curve slightly. If you are concerned, ask for advice from a structural engineer or surveyor. A chimney which leans excessively, and certainly one which is cracked, needs urgent attention. Strong winds can cause such a chimney to fall and this will usually damage the roof and can cause death and injury to anyone below. Remember that a stack is typically two metres high and is very heavy.
The use of cement pointing, as well as extreme weather conditions, can cause spalling of the brick surface when the surface cracks away exposing the soft and porous core of the brick.
A common problem is the ingress of damp, with staining at the junction of the ceiling and chimney breast, or lower down around the fireplace. However it is often difficult to trace the route the moisture has taken. The possible causes are: