What happens inside your gas boiler
A gas boiler burns natural gas or propane to heat water, which then travels through pipes to radiators or underfloor heating throughout your home. The process starts when a thermostat signals that the house temperature has dropped below what you set. A valve opens to let gas into a combustion chamber, where an igniter spark or pilot light sets it alight. The flame heats a metal heat exchanger — a coil or series of tubes — and water flowing through those tubes absorbs that heat. Once the water reaches the target temperature, the gas valve closes and the flame goes out. A pump keeps the heated water moving through your system until the thermostat signals that the house has warmed enough, then the cycle stops.
The whole sequence happens automatically and repeats many times a day. You do not have to do anything except set your thermostat and make sure the boiler has fuel. Most modern boilers are condensing boilers, which means they capture heat from the exhaust gases that would otherwise escape up the flue, making them more efficient than older models.
Key Takeaways
- A gas boiler ignites fuel in a combustion chamber, uses the flame to heat water in a heat exchanger, and then pumps that hot water to radiators or heating pipes throughout your home.
- Your thermostat controls when the boiler fires up and shuts down by measuring room temperature and comparing it to the setting you choose.
- A pump circulates the heated water continuously while the boiler is running, and a pressure relief valve prevents the system from building up dangerous pressure.
- Condensing boilers recover heat from exhaust gases and are significantly more efficient than older non-condensing models, which is why most new boilers sold today are condensing.
- Annual servicing by a may have access to engineer keeps your boiler running safely and catches problems before they become expensive repairs.
The thermostat: your boiler's on-off switch
The thermostat is a temperature sensor mounted on a wall in your home, usually in a living area away from direct sunlight, drafts, or heat sources like radiators. When the room temperature drops below the number you set — say 68°F — the thermostat sends an electrical signal to the boiler telling it to start. The boiler ignites and heats water. As the room warms up and reaches your set temperature, the thermostat cuts the signal, and the boiler shuts down.
Modern programmable and smart thermostats let you set different temperatures for different times of day. You might program the boiler to heat to 70°F on weekday mornings, drop to 62°F while you are at work, and rise again before you come home. This saves fuel because the boiler does not run when you do not need heat. Older manual thermostats require you to adjust the dial by hand each time you want a change.
The heat exchanger: where fuel energy becomes warmth
The heat exchanger is the core of the boiler. It is a metal vessel — usually made of cast iron, steel, or aluminium — with tubes or passages inside. Hot combustion gases from the burning fuel flow around these tubes on one side, and cold water from your system flows through the tubes on the other side. The metal conducts heat from the gas to the water, warming it up. In a condensing boiler, a second heat exchanger captures additional heat from the cooler exhaust gases before they leave through the flue, wringing out extra efficiency.
The water temperature inside the heat exchanger can reach 160°F or higher. A thermostat inside the boiler (separate from your room thermostat) monitors this water temperature and tells the gas valve to close once the water is hot enough. This prevents the water from overheating and protects the system from damage.
The pump and pressure relief valve: keeping water moving safely
Once water is heated in the heat exchanger, a circulating pump pushes it out through pipes to your radiators or underfloor heating. The pump runs continuously while the boiler is firing. As water flows through radiators, it releases heat into the rooms, cools down, and returns to the boiler to be heated again. Without the pump, hot water would sit in the heat exchanger and cool down without reaching your home.
A pressure relief valve is a safety device that opens if water pressure inside the system gets too high — usually above 3 bar on the pressure gauge. If pressure rises, the valve opens slightly and lets some water escape into a drain, lowering the pressure back to safe levels. This prevents pipes from bursting or the boiler from being damaged by excessive pressure. If you see water dripping from a pipe near the boiler, the pressure relief valve may be opening repeatedly, which signals that the system pressure is too high and needs adjustment by an engineer.
The flue: where exhaust gases leave your home
Burning gas produces exhaust gases — carbon dioxide, water vapour, and other byproducts — that must leave your home safely. These gases travel up a pipe called the flue and exit through your roof or an external wall. In older boilers, a lot of heat energy escapes with these gases. In condensing boilers, a second heat exchanger cools the exhaust gases enough that water vapour condenses back into liquid, releasing more heat that gets transferred to your heating water. This condensed water drains away through a small pipe, usually into your kitchen sink drain or a dedicated condensate drain.
The flue must never be blocked. If it is, exhaust gases back up into your home, creating a carbon monoxide hazard. You should never cover or obstruct the flue opening, and you should have the boiler serviced annually to may support the flue is clear and the boiler is operating safely.
Ignition and fuel control: how the boiler starts
When your thermostat signals the boiler to fire up, an ignition system lights the gas. Older boilers use a pilot light — a small flame that burns continuously and ignites the main burner when gas is released. Newer boilers use an electronic igniter that creates a spark only when needed, saving fuel because no flame burns when the boiler is off.
A gas valve controls how much fuel enters the combustion chamber. When the boiler is running, the valve stays open and gas flows to the burner. When the water reaches the target temperature, the valve closes and the flame goes out. If the igniter fails or the gas valve sticks, the boiler will not start or will not shut off properly — both situations require a service engineer to diagnose and repair.
Why annual servicing matters
A may have access to heating engineer should service your boiler once a year, ideally before the heating season starts in autumn. During a service, the engineer checks that the ignition is working, cleans the heat exchanger to remove dust and debris that reduces efficiency, tests the pressure relief valve, inspects the flue for blockages, and checks for gas leaks. They also verify that the boiler is burning fuel cleanly and efficiently.
Regular servicing keeps your boiler running safely and efficiently, extends its lifespan, and often maintains your warranty — many manufacturers require annual servicing as a condition of cover. If you notice unusual noises, a drop in heating performance, water leaking from the boiler, or the pilot light going out repeatedly, contact an engineer rather than trying to fix it yourself. Gas boilers involve pressurised water and combustible fuel, so repairs should always be done by someone may have access to.
Frequently Asked Questions
Why does my boiler make a banging noise?
Banging or kettling usually means mineral deposits have built up inside the heat exchanger, causing water to boil unevenly and create steam pockets. This is common in hard water areas. An engineer can flush the system or add a water softener to prevent it. Occasional banging when the boiler first fires up is normal, but persistent loud noises warrant a service call.
What does the pressure gauge on my boiler mean?
The pressure gauge shows how much pressure the water in your system is under, measured in bar. Most boilers run at 1 to 1.5 bar when cold and rise slightly when hot. If the gauge reads above 2.5 bar, the pressure is too high and the relief valve should open. If it reads below 0.5 bar, the system may not have enough water and you may need to repressurise it — your boiler manual will show how, or an engineer can do it.
Can I turn off my boiler in summer?
Yes. If you do not need heating, you can switch the boiler to off or to hot water only (if it heats your tap water as well). Turning it off completely saves fuel and reduces wear. However, if you have a combi boiler that provides both heating and hot water, turning it off means no hot water from the tap either. Some people leave the boiler on a low setting year-round to prevent the system from cooling too much, but this is a personal choice.
How long do gas boilers last?
A well-maintained gas boiler typically lasts 15 to 20 years. Annual servicing, prompt repairs, and keeping the system clean help it reach the upper end of that range. After 15 years, parts become harder to find and repair costs rise, so many people replace boilers around that age even if they still work.
What is the difference between a combi boiler and a system boiler?
A combi (combination) boiler heats water on demand for both your radiators and your hot tap water, so it does not need a separate hot water tank. A system boiler heats water that is stored in a cylinder, so you have a supply of hot water ready to use. Combi boilers are more compact and efficient for smaller homes; system boilers suit larger homes with high hot water demand.