Plumage is the term used for the flue gases produced by condensing boilers.
Because the "plume" is much lower in temperature than the flue gases produced by a less efficient boiler, and is concentrated in water vapour, it is much more noticeable (appears similar to steam). To avoid the potential nuisance of plumage boiler manufacturers have developed highly versatile flueing options. So instead of replacing the boiler in the kitchen for example, we can resite the boiler almost anywhere in the house (airing cupboard, loft, garage etc). This means that in almost every home the nuisance can be avoided.