A lot of people assume a combi boiler keeps a secret tank of hot water somewhere inside it, quietly waiting for a tap to open. It doesn’t. There is no stored reservoir at all. The hot water you get is manufactured in real time, heated in the few seconds it takes to travel from the unit to your tap. Understanding that single fact explains almost everything about how these boilers behave.
The Moment You Turn On the Tap
When you open a hot tap, cold water from the mains begins to move. That movement is the first and most important signal. Nothing has been heated yet, and the boiler itself may have been sitting idle and cool. What happens next is a fast sequence triggered entirely by water flow, not by any schedule or stored supply. Within a second or two of the tap opening, the boiler wakes up and starts producing heat specifically for that stream of water.
How the Flow Sensor Triggers the Burner
Inside the boiler, the incoming mains water passes through a flow sensor. This is a small device that detects water moving and measures roughly how fast. Once the flow crosses a minimum threshold, the control board gets the message that hot water is wanted. It then opens the gas valve and ignites the burner. The flame fires in proportion to demand, so a trickle calls for a gentle flame and a fully open tap calls for maximum output.
This is also why a combi can be fussy about very low flow rates. If you barely crack a tap open, the flow sensor may not register enough movement to fire the burner, and you get cold or lukewarm water. The boiler simply hasn’t been told there is real demand to meet.
What the Heat Exchanger Does to the Water
The heart of the process is the heat exchanger. In a combi, the burner heats a loop of water that runs through a block of thin metal channels, usually stainless steel or an aluminium alloy chosen for how readily it conducts heat. Your cold mains water flows through a separate set of channels packed tightly against the hot ones. The two streams never mix, but heat passes through the metal walls from the hot side to the cold side almost instantly.
Because the metal is thin and the surface area is large, the transfer is rapid and efficient. Cold water entering at around ten degrees can leave heated to a comfortable washing temperature in the short distance it travels through the exchanger. The faster you draw water, the less time each drop spends against the hot metal, which is exactly why output temperature drops when you demand a very high flow. The boiler is trading temperature for volume, because physics allows only so much heat to cross that metal per second.
Why Diverter Valves Prioritise Your Shower Over the Radiators
A combi handles both heating and hot water with one burner, so it needs a way to decide which job comes first. That decision is made by a diverter valve. Under normal running, the valve sends heated water around your radiator circuit. The instant the flow sensor detects a tap or shower in use, the diverter switches position and routes all the boiler’s heat to warming your tap water instead. Your radiators briefly pause while you run the tap, then resume once you close it.
This prioritising is deliberate, since nobody wants a shower that runs cold because the heating kicked in. When a diverter valve starts to stick or wear, you see the symptoms directly: radiators that stay warm during a shower, or hot water that falters. Installers such as WPJ Heating & Plumbing often trace exactly these complaints back to the valve rather than the burner itself, because the mechanism switching dozens of times a day is a natural wear point.
The Limits That Decide How Many Taps You Can Run
The ceiling on a combi’s performance is heat transfer, not storage. Its output is rated in litres per minute at a given temperature rise. Open two taps at once and that capacity is split between them, so both run cooler or weaker. This is the real reason a single combi can struggle to feed a bathroom and a kitchen simultaneously.
Because everything depends on clean, unobstructed flow through narrow channels, the heat exchanger is sensitive to scale and sludge building up inside it. Have the boiler serviced annually and the water quality kept in check, and that instant hot water will keep arriving exactly when you ask for it.