This is a short edited guide from MAINSTEAM MODELS "The Complete Guide to Miniature Steam" which is available on CDROM or as an internet download.
Steam blowers are a very necessary item on a coal fired steam boiler. On full size steam locomotives the steam blower is essential to prevent blowback when the engine is stationary or coasting with the regulator shut. Accidents were quite common in the days of steam when drivers & firemen could be badly burnt by a sudden blowback of combustible gases from the firehole. On model steam locomotives, I have inadvertently singed the hair on the back of my hand on a few occasions by forgetting to turn on the blower when coming to a halt.
A steam blower is simply a jet of steam that blows up the chimney, the volume of steam being controlled by a suitable valve. The speed of the steam is important, so only a small hole in the end of the pipe is necessary. If the outlet hole is too big, the blower will just waste steam. Blowers are very easy to make & are also very effective on gas fired boilers, a more complete combustion takes place with the extra air admission over the fire. Too much blower will suck too much cold air into the tubes & is not recommended & will actually drop the steam pressure. I use a steam blower on a 4 inch diameter vertical boiler in a radio controlled steamboat, with very good results. If you have a gas fired boiler & it emits a bad smell whilst the burner is running, consider fitting a steam blower.