Is the fireplace fitted with a refractory flue liner, only a proper refractory flue can be used in the combustion of solid fuels. Many gas fires are fitted with a flexible insulated flue liner, which is suitable for use only with gas fires where combustion gases are low in temperature and condensation is an issue 9thus the insulated flue). Th refractory flue liner is brown / orange in colour (like a terracotta pot). If in any doubt a builder or someone with experience in building will readily confirm the suitability of your fire place.