Flue calculation and material advice

A flue size calculation is calculated, not guessed

Every heating appliance requires its own dedicated chimney: the correct diameter, the proper height, the right material in the required wall thickness, and a draught that precisely matches manufacturer specifications. Our flue calculation sets all of this out in an advisory report costing 100 to 200 euros, which is fully offset if we install the chimney system for you.

The result: a design that works perfectly on paper before a single pipe is ordered, complete with technical drawing, bill of materials, and a fixed price quotation. Designed for home builders, renovators, DIY installers, and anyone who prefers getting it right the first time.

A flue design that fails on paper will never work on the roof: calculating costs 200 euros, fixing mistakes costs a multiple of that.

How our flue size calculation works

1. Step 1: mapping appliance and building

   We start with the appliance, whether chosen or planned, noting its nominal heat output, flue gas temperature, and exact spigot diameter. Next, we assess the building: the intended route, roof shape, and surrounding environment such as trees or taller structures within 15 metres. Building plans, photos, or a quick video walkthrough are often all we need for the initial calculation.

2. Step 2: calculating diameter and height

   Based on manufacturer data and total duct length, we calculate the ideal flue diameter and minimum chimney height. This accounts for the impact of every bend, horizontal section, and the average outdoor temperature along the route. An oversized chimney cools flue gases prematurely, while an undersized flue chokes the appliance: the calculation determines the exact sweet spot, down to the millimetre and metre.

3. Step 3: specifying materials and routing

   Single wall pipe for warm indoor zones, twin wall insulated flue systems outside, flexible liner through an existing masonry chimney, or a concentric system: we specify the materials, wall thickness, and fixing points for every section. The terminal location is planned strictly to building regulations, keeping 1 metre above the roof ridge as a standard rule for new installations.

4. Step 4: technical report, drawing, and fixed quote

   You receive a comprehensive report complete with schematic drawing, full materials list, and a fixed installation price. You can use this report to hire us, or to compare quotes elsewhere: the calculation remains your property, regardless of what you decide.

Who is this calculation for?

For self-builders and renovators wanting flue requirements specified before closing walls, for DIYers installing themselves who want total certainty on sizing and layout, for architects needing verified technical details for their specifications, and for anyone dealing with a problematic chimney with poor draught that needs a permanent, structural solution.

Chunk 2:

What does a flue calculation cost?

The complete advisory report costs 100 to 200 euros, depending on the complexity of the flue route. This fee is fully credited when we carry out the installation via our twin wall flue installation service or our chimney lining service. If you have us fit the system, the advice is effectively free. You can explore flue components in detail in our comprehensive guide to chimney flue parts.

3 core factors behind every calculation

1. The diameter: 80 millimetres for pellet stoves, 100 to 150 for pellet boilers, and 150 to 180 for wood burning stoves, always verified against the data plate of the specific appliance.

2. The height and routing: every metre of vertical chimney builds up draught, every bend creates resistance, and adhering to the maximum rule of 2 bends helps maintain proper airflow.

3. And thermal efficiency: an insulated twin wall flue pipe keeps exhaust gases hot and draught stable, particularly on cold external runs along walls or above the roof line.

Common sizing errors we rectify

The issues we fix most frequently: flues sized to fit existing wall core holes rather than appliance needs, using legacy chimney diameters when modern stoves require half that size, routing flues with 3 or 4 bends just for installation convenience, and terminals placed 0,5 metres too low.

Why manufacturers require a flue calculation

Warranties on modern heating appliances depend heavily on a correctly sized flue system: if the installed route strays from specifications, the manufacturer warranty becomes void at the first sign of trouble. Our report provides written proof of compliance for manufacturers, building inspectors, and insurers for years to come. You can review the regulatory guidelines we calculate against on the Buildwise standards page.

Self-installing using our design report

If you prefer to fit the system yourself, our report serves as your step by step blueprint: the parts list specifies every single item down to the last bracket, while the schematic details the exact assembly order. Many DIY customers choose to let us handle just the roof penetration and terminal flashings, which is the most challenging part, while fitting the remaining sections themselves. We can break down costs for this hybrid service directly in your report.

From initial calculation to completed flue installation

When you choose full installation by our team, the report becomes the official work order: exact dimensions, specified materials, and the agreed price guarantee. For pellet appliances, installation is handled via our pellet stove flue installation service, and for boilers via our pellet boiler exhaust service, both completed in a single working day with draught testing and certification included. Discover which flue fits your setup in our flue selection guide.