What a flue is actually doing
A pellet stove or boiler burns fuel in a sealed chamber, and the gases from that combustion have to go somewhere. The flue is the pipe or liner that carries them up and out of the building. What makes it work isn't just the pipe itself but the pull of air moving through it, which is what people mean by draught.
Draught is created by the temperature difference between the hot gases inside the flue and the cooler air outside it. Hot air rises, so a column of hot flue gas inside a cool pipe creates an upward pull, and that pull is what drags the exhaust out of the appliance and keeps fresh air moving in to support combustion. Get the flue wrong and the appliance has nowhere good to put its exhaust, whatever else about the installation is right.
Why height is the main lever
A taller flue generally produces a stronger draught, because a longer column of hot gas creates a bigger temperature difference with the outside air and a stronger upward pull. That's why height is the figure most installation guidance comes back to: the manufacturer's instructions for a specific appliance, and the Building Regulations document covering chimneys and flues, both set out minimum heights and termination clearances for a reason.
Height isn't the only thing in play, though. A flue that terminates close to a chimney stack, a nearby wall, or below the ridge of a roof can sit in turbulent air that works against the draught rather than for it, even if the height figure is technically met. The exact clearances for your roof shape and termination position are set out in the manufacturer's installation instructions and the Building Regulations document for your appliance type, and they're worth checking directly.
Where diameter and routing come in
A flue liner, the inner sleeve that the hot gases actually travel through, has to be sized to the appliance it's serving. Too wide and the gases cool and slow as they spread out, weakening the draught and encouraging condensation inside the flue. Too narrow and the flow is restricted, which can cause the appliance to struggle on ignition or trip into a fault condition. Bends and long horizontal sections before the flue rises vertically both work against draught too, because each one adds resistance that a shorter, straighter run wouldn't have.
How a weak or a strong draught shows up day to day
A weak draught tends to show up as symptoms you'd notice without needing to measure anything: smoke drifting back into the room when the door is opened, soot building up faster than expected on the glass, or an appliance that's slow and reluctant to get going on ignition. In more pronounced cases it can push an appliance into a fault lockout, because the control system reads poor flue performance as something it can't safely continue running through.
An overly strong draught causes a different set of problems. It pulls heat up and out of the flue faster than the appliance can make use of it, which wastes fuel, and in some cases it can over-fire the appliance by pulling more combustion air through than the burn is designed for. A flue that's taller or straighter than it needs to be isn't automatically the safer choice; it can just move the problem from too little draught to too much.
Common mistakes in flue height and routing
- Shortening the vertical rise to avoid roof work, which keeps the flue below the height the appliance actually needs
- Routing several bends into the system before the flue reaches its vertical run, adding resistance that a straighter path wouldn't have
- Running a long horizontal section from the appliance to an external wall before turning the flue upward
- Leaving an external flue run uninsulated, so the gases lose heat to the outside air faster and the draught weakens in cold weather
- Fitting a flue liner sized for a different appliance or a different fuel type
Checking your own flue arrangement
The flue height and draught checker lets you put in the figures for your own setup, such as flue height, run length, bends and termination position, and see how that arrangement is likely to behave. It works from the same relationships described above: height against draught strength, and the routing factors that work against it.
The checker can only work from what's entered into it, and a figure that's slightly off, an internal diameter estimated rather than measured, for instance, can give a reading that doesn't match what's actually happening in your flue. Treat it as a way of understanding the shape of the problem before you talk to an installer. You can try it directly on the flue height and draught checker.
None of this replaces an assessment from a competent, registered installer looking at your actual chimney or flue run. Flue and draught problems are specific to the building they're in, and a figure that works on paper can still behave differently once it's fitted.
Where to go next
For how the flue connects to the rest of the appliance, including the auger that feeds pellets through to the burner, see how pellet stoves and boilers actually work. For what a flue installation involves in practice, including the standards an installer works to, see what installation really involves. If soot build-up or draught performance is something you're already managing on an existing system, running costs and maintenance, honestly covers what ongoing upkeep looks like. Before taking on any installer's work, it's worth checking their registration on the HETAS (Heating Equipment Testing and Approval Scheme) online register yourself.