Pluming is the name given by the industry to the visible flue products leaving an appliance terminal. When gas is burnt one of the products of combustion is H₂O (water). In a standard-efficiency boiler this leaves the flue terminal sufficiently hot enough to be invisible. However, even the latest standard efficiency boilers plume to a limited degree during winter months. In a high-efficiency condensing boiler the water leaves the terminal at temperatures down to 30°C and the water vapour appears as large droplets. These are now large enough to be visible as a mist leaving the terminal.

What are the issues with pluming?

Pluming can be a nuisance to householders if a terminal is sited near a window for example. A little extra care when siting a boiler and terminal however will resolve this issue. Similarly a terminal should not be sited too close to neighbouring properties. Something else to consider is the fact that the plume is a wet vapour which will condense on cooler surfaces. Therefore a reasonable distance must be left between building surfaces that may have a plume constantly on them, e.g. internal corners etc.

Do these only relate to condensing boilers - why?

Condensing boilers operate with lower flue gas temperatures and therefore pluming is more readily visible. However, as already stated, even today's highest efficiency standard boilers will plume to some degree on cold days when the flue gases are being cooled by the atmosphere more rapidly.

What are the basics that installers need to know about pluming (i.e. siting of flues etc.)?

Siting an HE flue is still governed by the same requirements of BS5440 as a standard-efficiency boiler. However, a little more care to ensure the plume does not cause a nuisance is required:

• Terminals should not be positioned alongside windows where the plume may be blown across.
• If sited next to a public footpath or regularly used private access or patio, the terminal should always be positioned so that it is a minimum of 2.1metres above the ground.

 

What are the issues surrounding neighbouring properties?

The plume can be a visible nuisance and therefore for good neighbourly relations a terminal should not allow the plume to be directed or continuously drift across a neighbouring property. When a boiler is sited close to the boundary, angled diverter terminals can provide a solution by directing the plume away.

Are emissions from high efficiency boilers different to those from standard efficiency ones? Why?

No, the same natural gas is burnt with air as in a standard boiler - producing the same flue gases. An HE boiler, however, extracts more heat from those gases thus making for cooler gases when they exit the terminal. In fact, many HE boilers use burners that produce lower levels of nitrogen oxides which is actually much better for the environment.

Can you use roof ridge vents to exhaust the gases?

Yes. Roof terminations in a variety of forms are available for HE boilers and these are excellent in removing the plume away from sightlines in most cases. A ridge tile terminal is also available which allows a very unobtrusive termination for a variety of properties. Ideal Boilers, for example, offer Ridge tile terminals, vertical terminals that can be put through the pitch of a roof or terminals suitable for flat roofs.

Should installers be concerned about distillation of salts from the flue stream?

No, this is not an issue as the flue gases are the same as a standard boiler.

How do you choose an appropriate retrofit boiler?

Selecting a suitable retrofit boiler with the range of modern HE boilers now available is simple. There is a wide range of flue systems available, which means it is also easy to avoid pluming nuisance. It is important to remember that with a condensing boiler, the condensate is readily managed via the standard small diameter plastic waste pipe. This can be led into an existing waste in the property with a suitable trap according to Water Regulations, or lead to a drain or soakaway (if necessary) outside. HE boilers are quite happy on a conventional system and a typical set of controls, programmer, room and cylinder stats will ensure that they operate satisfactorily and provide lower running costs. Greater control will produce even lower running costs.

What do you do if a condensing boiler is not appropriate in a retrofit situation?

It is difficult to think of a retrofit domestic situation where you would not be able to fit an HE boiler. Any issues with routing a condensate drain can be overcome by using a condensate pump. With such a variety of flue systems available to deal with pluming from the terminal - whether horizontal extensions, vertical routing or twin duct systems - different air inlet and flue terminal positions will resolve almost any problem.