How Does a Combi Boiler Work? Heating and Hot Water

The two circuits inside a combi

It helps to think of a combi as two separate water circuits that meet inside the boiler but never mix.

The first is the central heating circuit. It is a sealed loop of the same water going round and round: from the boiler, through the radiators, and back. It holds a small amount of water, is pressurised to around 1 to 1.5 bar when cold, and has an expansion vessel inside the boiler to absorb the swelling as it heats. This is the pressure shown on the boiler's gauge.

The second is your domestic hot water. It comes straight in from the cold mains, passes once through the plate heat exchanger, and goes out to the taps and shower. It never touches the heating water, which is why your tap water stays clean even when the radiators are full of black sludge.

What happens when you turn the heating on

When the programmer or room thermostat calls for heat, the boiler runs its fan, lights the burner and heats the water in its main heat exchanger. The pump pushes that water out to the radiators. The diverter valve is in its heating position, so the hot water goes round the radiator circuit and back.

Most modern combis are condensing boilers. They are designed to take so much heat out of the flue gases that water vapour condenses, which is where their efficiency comes from. They condense best when the water coming back from the radiators is cool, which is why lowering the flow temperature can save gas. Our guide on what temperature to set your boiler goes into the dials, and what is a condensing boiler explains how to tell if yours condenses and why its condensate pipe matters in winter.

What happens when you open a hot tap

This is the clever part, and it happens in a second or two.

• Cold mains water starts moving through the boiler's inlet, and a flow sensor, usually a small turbine or a magnetic sensor, reports it to the control board.
• The board fires the burner if it is not already running, and moves the diverter valve to the hot water position.
• Hot water from the main heat exchanger is diverted away from the radiators and through one side of the plate heat exchanger, a compact block of thin stainless steel plates.
• The cold mains water passes through the other side of the plates, picks up heat, and goes out to your tap.
• The board adjusts the burner to hold the temperature you set on the hot water dial.
• When you close the tap, the flow stops, the burner turns down or off, and the diverter valve springs back to heating.

Why the heating goes off while you shower

Because there is only one burner and one set of pipes through it, a combi cannot heat radiators and tap water at full rate at the same time. Hot water takes priority. While someone is in the shower, the radiators get nothing, which on a short shower nobody notices. On a long bath fill on a cold morning, the house can cool slightly. That is normal behaviour, not a fault.

The same logic explains the classic combi faults. If the diverter valve sticks in the heating position, the radiators carry on perfectly and the taps run cold. If it sticks in the hot water position, you can have hot taps and cold radiators. Our guide on no hot water from a combi boiler goes through how to tell which part has failed.

How much hot water can a combi give?

A combi never runs out in the way a cylinder does, but it can only heat so much water per minute. The limit is its hot water output in kilowatts. As a rough guide, each litre per minute of flow needs about 2.5 kW to raise it by 35 degrees, so dividing the boiler's hot water output by 2.5 gives an approximate flow rate at a comfortable temperature. The boiler's manual or data plate gives the real figure.

Mains pressure and flow matter just as much. A combi cannot deliver more water than the incoming main supplies. In a top floor flat, a house with an old narrow supply pipe or a street with low pressure, the boiler may be fine and the flow still disappointing. Running two outlets at once splits the same flow between them, which is why a combi suits one bathroom much better than two. Our guides on low hot water pressure and on a mixer shower with a combi boiler cover the limits.

Why a combi's hot water is cooler in winter

If your shower feels less hot from October onwards, it is probably not your imagination. The cold mains water coming into the house is noticeably colder in winter than in summer. The boiler has the same output all year, so if it has to lift the water through a bigger temperature rise, it can only do that for fewer litres per minute. At the same flow, the water comes out cooler.

The fix is usually simple: turn the tap or shower down slightly to slow the flow, and the temperature goes up. Check that the hot water dial on the boiler has not been turned down over the summer too. If the water is lukewarm even at a trickle, or the temperature swings up and down, that points to a scaled plate heat exchanger or a sticking diverter valve rather than the weather, and our guide on water not getting hot helps separate the two. The signs of a blocked plate heat exchanger, and how it is descaled or replaced, have their own page.

Hot water that is fine at a low flow and goes cold when the tap is fully open is a useful clue. Mention it when you call an engineer, as it often points straight to the plate heat exchanger or the flow.

Combi or a boiler with a cylinder?

A combi is compact, needs no tanks, and gives hot water at mains pressure without any wait for a tank to heat. It suits most flats and smaller houses with one bathroom. A system or regular boiler with a hot water cylinder stores a tank of hot water, so it can serve two or three bathrooms at once, and an unvented cylinder can do that at mains pressure too. The cost is space for the cylinder and a wait for it to reheat once used.

Neither is better in every home. Our guides on combi or system boiler with a cylinder, and on converting to a combi boiler, go through the decision. If you are not sure which type you already have, a combi has no hot water cylinder in the airing cupboard and no tanks in the loft, and usually has four or five pipes coming out of the bottom of the boiler.

When a combi stops making hot water

Before calling anyone, check the obvious: the pressure gauge is above 1 bar, the boiler display shows no fault code, the hot water dial has not been turned down, and the problem affects every hot tap rather than one. If the heating works but the taps run cold, the fault is almost always in the hot water side: the flow sensor, the diverter valve or the plate heat exchanger. In London, the hard water means scale in those parts is common.

If you need an engineer, call us on 020 4652 5111 or send an enquiry with the boiler make and model and what it is doing. We have 20 engineers across London and our rate is on the pricing page, agreed before we attend. Gas work on the boiler itself is always carried out by a Gas Safe registered engineer.