Get Started

What happens inside a pellet stove or boiler, from hopper to flue

Pellets move through a fixed sequence, from hopper to auger to combustion chamber to heat exchanger to flue, and each stage decides how much heat reaches your rooms and how much ash you clear out. A room stove, a boiler stove and a full biomass boiler run that sequence differently, and the differences show up in what space and plumbing each one needs.

Run the flue height and draught checker See what running and maintaining one costs
Mechanism

How the fuel becomes heat, stage by stage

A pellet stove or boiler moves fuel and air through a fixed sequence of stages, each one automated by a control board. Knowing what each stage is meant to do also tells you what a fault message or a smoky start-up is actually pointing at.

  1. The hopper stores the pellets nearest the burner

    The hopper is the fuel store built into or beside the appliance, holding the pellets that are drawn down into the burner as the fire runs. A stove hopper typically holds enough fuel for one to three days at normal firing rates, so it needs refilling by hand at that interval. Pellets that have taken on moisture during storage swell and can bridge across the hopper outlet, stopping fuel reaching the auger below even though the hopper looks full from above. Storage conditions before the pellets ever reach the hopper affect how often this happens; see the guide to pellet quality, storage and delivery for what keeps pellets dry.

    Hopper capacity, not boiler output, is usually what decides how often someone has to think about the fuel at all.

    Stove hopper

    A small integrated hopper that the household tops up by hand, usually daily or every few days.

    Boiler day-tank

    A smaller hopper automatically refilled from a larger external store by a secondary auger or vacuum system, which is what lets a boiler run for weeks unattended.

  2. The auger doses pellets forward in short bursts

    The auger, the screw mechanism that feeds pellets from hopper to burner, turns in short, timed bursts set by the control board. How long and how often it turns is set against the heat demand the thermostat is calling for, which is how a modulating appliance turns its output down as well as up. A jammed auger, usually caused by sawdust fines settling at the base of the hopper or an oversized pellet fragment lodging in the throat, is one of the most common service calls on a pellet appliance.

  3. An electric igniter lights the first dose without a match

    Ignition is handled by an electric heating element, usually a ceramic or cast rod, that glows hot enough to set light to the pellets fed into the burner pot at start-up. A fan drives air across the igniter during this sequence, which typically takes several minutes and is why a pellet appliance can smoke slightly at the flue terminal when it first fires. If the control board doesn't detect a flame within its programmed window, usually because the igniter is worn or coated in ash residue, the appliance faults out and shuts down.

  4. The burner pot burns a small, closely fed fire

    Combustion takes place in a steel or cast burner pot, where a dedicated fan supplies only as much air as the current firing rate calls for. This forced-air design is what lets output modulate up and down automatically, and it's the main mechanical difference between a pellet appliance and a wood-burning stove fed by hand. Air is usually introduced at two points, under the fuel bed and above it, and the balance between the two affects both how completely the pellets burn and how much particulate reaches the flue.

    Whether that particulate level matters legally depends on where the appliance sits. Try the smoke control area self-check to see whether your address falls inside a designated smoke control area, and what that means for the appliance and fuel you'd be allowed to burn there.

    Primary air

    Fed up through the burner pot, sustaining the base of the fire.

    Secondary air

    Introduced above the fuel bed to burn off remaining gases before they leave the chamber, which is what keeps particulate emissions down when it's correctly set.

  5. The heat exchanger takes heat out of the gases before they leave

    The heat exchanger is a set of metal passages, finned plates or tubes, that the hot combustion gases cross before reaching the flue, transferring their heat into the room or into water. Soot and fly ash settle on these surfaces over time and reduce how much heat they transfer, which is why most boilers run an automatic cleaning cycle that scrapes the tubes on a schedule, and why a stove's glass and internal surfaces need periodic attention too.

    Room stove

    Gases pass across a finned casing that warms room air directly, by natural convection or with a fan-assisted convector.

    Boiler

    Gases pass across a water jacket, transferring heat into the central heating and hot water circuit.

  6. A powered fan pushes exhaust gases up the flue liner

    The flue liner, the inner sleeve of the chimney or flue system, carries combustion gases safely outside, and a pellet appliance uses a powered extraction fan. That fan is why a pellet appliance can run on a smaller-diameter flue than an open fire needs, but it also means the appliance depends on mains electricity to operate safely, and won't fire during a power cut. A blocked flue liner or a failed extraction fan trips a safety cut-out, because the control board won't let flue gas build up inside a sealed appliance.

    Flue routing and liner sizing are specified at installation, not chosen by the appliance itself; see the installation requirements page for what that involves. Before a surveyor visit, you can run the flue height and draught checker with your appliance's output, the proposed flue diameter and the planned height, to see whether the combination looks like it clears the usual draught and terminal requirements. It works from the figures you give it, so it can only flag an obvious mismatch, not replace what a surveyor finds by looking at the actual chimney.

  7. Ash collects in a tray beneath the burner pot

    Ash collection happens in a removable tray under the burner pot, catching the residue pellets leave behind after burning, which for good-quality certified pellets is a small fraction of the fuel's original weight. How often that tray needs emptying depends on how many hours the appliance has run and the ash content of the pellets burned, more with lower-grade fuel, less with ENplus-certified stock. An ash tray left to overfill blocks the air intake at the base of the burner pot, starving the fire of air, which shows up as excess smoke and pellets that char.

Hopper capacities, feed rates and cleaning cycles vary by appliance and manufacturer, so treat the figures above as typical. None of this replaces a competent installer's assessment of your actual flue and fuel store, and any installer's HETAS registration should be checked against the HETAS online register before work starts. See the running costs and maintenance page for what routine servicing of each stage involves.

Three system types

Room stove, boiler stove or full boiler compared

Three types get called a 'pellet stove' or 'pellet boiler' depending on where you look, and they suit different houses. This sets out what each one actually heats and what it asks of your home before a full survey.

System type What it heats How it connects Space it needs
Room stoveIf your address is in a smoke control area, the appliance must be on the current DEFRA-exempt list. The smoke control area self-check walks through what that means for a specific address and appliance before you commit to one. The room it stands in, through radiant heat from the stove body and convected air. Nothing, in most cases. A back-boiler version can feed a handful of radiators or a hot water cylinder alongside the room heat. A hearth, a lined flue running up through the building, and floor space beside the stove for the hopper.
Boiler stoveOutput splits between the room and the circuit, so a stove sized for the whole house can overheat the room it sits in. The room it stands in directly, plus the rest of the house through an integral boiler built into the stove. Your existing radiator circuit and hot water cylinder, in a similar way to a combi boiler attached to a stove. The same room footprint as a standalone stove, with added pipework to the cylinder and a flue sized for the higher output.
Full biomass boilerThe hopper and auger feed need clear access for filling and ash removal, and the pellet store is a real space commitment on top of the boiler itself. Whole-house space heating and hot water; the boiler unit itself gives out little useful heat to the space around it. The full central heating system, usually through a buffer tank that absorbs the boiler's combustion cycles so it isn't switching on and off constantly. A dedicated plant room or outbuilding, a pellet store sized for a season's fuel, and a flue run built to match.

This sets out how the three types typically connect and what space they need in general terms; it does not replace a site-specific assessment from a competent registered installer. Read the installation guide for what a survey actually checks, and the pellet quality, storage and delivery guide for how much room a season's fuel takes up. Where a flue run is already in mind, the flue height and draught checker tool shows whether it meets the minimum draught for a given appliance output before a surveyor visit.

What the controller actually manages, and what it leaves to you

A modern pellet stove or boiler runs on an electronic controller. It manages ignition, adjusts how hard the appliance burns while it is running, and shuts the system down safely when the heat is no longer needed. Understanding what it handles automatically, and what it does not, tells you how much day-to-day attention the system will actually want.

Ignition and modulation

Ignition is automatic on virtually every pellet appliance sold today: an electric heating element ignites the first pellets fed by the auger (the screw mechanism that moves pellets from hopper to burner), and a fan establishes draught before the controller settles into normal running. Whether that draught is strong enough in the first place depends on the flue's height and internal diameter relative to the appliance's rated output, which is exactly what the flue height and draught checker works out against the relevant BS EN 15287 thresholds, so it's worth running before a surveyor visit. Once alight, the controller does not simply run the appliance flat out or switch it off. It reads a sensor, usually flue gas temperature or a return-water sensor on a boiler, and adjusts the auger feed rate and combustion fan speed to hold output somewhere between a low simmering rate and full load. This is called modulation, and it is what lets a pellet boiler track a house's heat demand through the day.

Thermostats and how the appliance gets its instruction

A room stove usually works from a simple room thermostat or the appliance's own built-in temperature sensor: reach the set temperature, and the controller drops to a low modulation rate, because pellet appliances are not designed for frequent full restarts. A boiler is different, because it is heating water. It typically takes its instruction from a room thermostat, a boiler thermostat set on the unit itself, or a weather compensation sensor that adjusts flow temperature against the outside air temperature, and it can be wired into a wider heating system alongside a hot water cylinder thermostat exactly as an oil or gas boiler would be.

Programming and scheduling

Most controllers allow a weekly schedule, so the appliance can run to a lower background temperature overnight and step up before the household wakes, in the same way a conventional central heating programmer works. Frost protection is standard on boiler-type units, holding a minimum temperature even when the programme calls for no heat, to protect pipework in cold weather. Beyond the schedule and the thermostat setpoint, there is usually little for the householder to adjust day to day: the controller is managing the combustion process itself.

What automation does not do

The controller cannot refill the hopper, and it cannot empty the ash tray. Depending on pellet quality and how hard the appliance runs, ash removal is a manual task on a cycle measured in days or weeks, and the hopper needs topping up from bulk storage or bags on a similar rhythm. Automation also does not replace an annual service: sensors drift, seals wear, and a burner that is quietly running dirty will not necessarily flag itself through the control panel.

It is also worth being clear that none of this touches the legal position on what you are allowed to burn and where. A controller manages combustion; it has no bearing on whether your address sits inside a smoke control area, or whether the specific appliance you are considering is on the DEFRA exemption list. That is a separate question from how smoothly the unit ignites and modulates, and it is worth settling before you commit to a model. The smoke control area self-check walks through your address, the appliance and the fuel in turn and names the Clean Air Act 1993 provision behind each outcome, though it does not replace a determination from your local authority.

What running costs and cleaning actually look like over a season is covered on the running costs and maintenance page, and what a competent installer will specify for your particular appliance and flue arrangement is covered in the guide to what installation really involves.