Old chimney pieces can be very valuable but people should know the danger of being "ripped off" when selling or buying them, according to Mr Kevin Scott who specialises in fireplaces and sculpture at Bonhams Auction House in London.

Mr Scott warns: "It's a minefield. Watch you don't get ripped off for price. . . It's a very difficult thing to price a fireplace."

One of the difficulties is that it is relatively easy to interchange the pieces of marble that make up a high-value 18th century Irish or English fireplace. None of the pieces should have been replaced.

He explains: "All of the pieces must be original to each other and not just a collection of 18th century parts put together."

For example, the shelf must be original. A shelf from between 1760 and 1800 would only have been six or seven inches deep. But this was often taken out during the 19th century and a shelf of some 12 to 15 inches inserted. This was to provide space for the Victorian fashion of keeping a mantle clock and candelabra on the fireplace shelf.

The original height of an 18th century fireplace may have been increased or reduced. Height can be increased without damaging the fireplace by placing it on block feet but it should never be reduced by cutting the jambs (the uprights), says Mr Scott.

Regrettably, it seems many 18th century fireplaces in Ireland and England were cut at the jambs which can significantly reduce their height and value.

Mr Scott says an 18th century fireplace was sold in the mid1990s at a London auction house for £170,000. Nineteenth century fireplaces of which there are many more can still command attractive sums. For instance, Mr Scott sold a 19th century chimney piece for £25,000.

A Dubliner, he regularly comes to Ireland to value chimney pieces. He recently valued an 18th century fireplace in a 19th century house on the south side of Dublin for £35,000. As well as the impressive sum, this shows that houses can retain older and more valuable fireplaces than the age of a house suggests.

Mr Scott says: "Magnificent fireplaces were made in Ireland. Unfortunately it's not that well documented. In England you can attribute fireplaces to certain designers or manufacturers which tends to enhance their value."

Even cast-iron grates with tiles from the last century can fetch up to £1,500. He says tiles indicate that a the fireplace dates from the 1850s onwards. Before the introduction of tiles, the cast iron tended to be more ornate.

He advises anybody considering buying or selling a valuable fireplace to "find somebody you can hire on a professional basis to advise you".

"There are a lot of dud 18th century fire surrounds around. When it comes to fireplaces, a little knowledge is a dangerous thing."

Moreover, because fireplaces are relatively simple to tamper with, it is all too easy to pay too much at auction. Unfortunately people can also part with a fireplace for a fraction of what it is really worth, he says.

Prices fetched for a range of quality fireplaces sold at Bonhams includes: £20,000 for a fine mid-Victorian Statuary marble (a type of white marble) chimney piece from about 1860; £6,500 for a late Victorian Gothic revival (about 1880) carved Portland stone chimney piece; £5,000 for a Louis XVI Pavonazzetto marble (a creamy yellow marble with black and yellow veining) chimney piece from 1780; and £7,000 for a George III-style statuary and Siena marble chimney piece, circa 1890.