Flue terminals, plume and condensate basics

A flue terminal must let products of combustion disperse outside without being drawn back in. Choose the clearance for the actual terminal, opening and position, using the applicable guidance and the appliance instructions.

Terminal position

Clearances from openings, corners and the ground exist so products disperse outside and cannot re-enter or be obstructed.

• Diagram 34 of Approved Document J gives 300 mm for a fanned-draught balanced-flue terminal below, above or horizontally beside an ordinary opening window, air vent or other ventilation opening. Other arrangements have separate entries, including an opening in a carport into the building. Natural-draught distances vary with position and input and are not always larger. Check the diagram’s notes and the manufacturer’s requirements.
• A room-sealed boiler's flue and air duct brings combustion air in from outside and takes the products back outside.

Terminal guard

Fit a guard suited to the boiler where the terminal could be touched, damaged or obstructed, such as less than 2 m above ground, a balcony or an accessible flat roof. It protects people from the hot terminal and the terminal from damage while the flue keeps working. It does not keep out rain, redirect the plume or reduce the clearance to an opening.

The flue run

• From the boiler, set the horizontal flue at the upward angle stated by its manufacturer. Cooling exhaust leaves water inside the flue; the slope carries it back through the appliance's condensate trap. A flat or outward-falling run can let water collect or drip from the terminal.
• Support it at the manufacturer's stated intervals. A sagging flue can hold condensate and pull its joints apart.
• Additional bends use up the flue’s resistance allowance. Apply the maker’s equivalent lengths, checking whether the initial elbow or adaptor is already included in the permitted run.
• For an existing room-sealed, fan-flued run hidden in a void, inspection access normally allows the joints to be checked. Some circumstances permit a specified CO monitoring system in the void with safety shutdown instead; an ordinary room alarm is not that alternative. Without access or a qualifying alternative, assess the situation as At Risk if no immediate danger is found, explain it and turn the appliance off with permission, using the required label and warning notice. Evidence of leaking products or another immediate danger needs its own classification and action.

Model examples: check direction as well as distance

Flue condensate travels back towards the boiler; the separate condensate drain falls away from it. The numbers for those two pipes are not interchangeable.

For the Ideal Logic+ Combi2 C24/C30/C35, October 2023 instructions, an extended horizontal flue rises 26 mm per metre from boiler to terminal, equivalent to about 1.5°. The terminal outlet duct needs at least 25 mm from combustible material. Where the flue is concealed, the manual places inspection hatches within 1.5 m of the joints. The inside-fitting method calls for accurate coring and applies to walls no thicker than 600 mm; thicker walls need the appropriate alternative method.

The 2021 Vaillant ecoTEC plus VU/VUW ..6/5-5 instructions distinguish their standard terminal kit from extended flues. The standard kit incorporates the fall needed for drain-back and may be fitted level between the appliance and termination. Other extended runs require at least 44 mm fall per metre towards the boiler.

In that Vaillant manual's terminal table, a vertical terminal needs 1,500 mm to combustible building material and 500 mm to non-combustible material. Its separate minimum to a roof skylight or another flue is 300 mm. These are terminal-position entries, not a clearance for every part of a vertical duct. Select the entry that describes the actual installation.

Plume and condensate

• Plume: visible droplets as water vapour condenses in the exhaust. How it looks tells you nothing about combustion quality, and the discharge also carries flue gases. Resolve nuisance or re-entry risks by compliant terminal positioning or the maker’s approved plume arrangement.
• Condensate: mildly acidic. Drain it to a suitable point and protect external runs from freezing.