Gas Multipoint Water Heater Not Working? What to Check

How a gas multipoint works

A multipoint is a gas water heater that heats water as it flows, with no storage tank. It is usually a box on a kitchen wall, and it supplies several hot taps, which is where the name comes from. Ascot is the name most people know, and plenty of London flats built or converted between the 1960s and the 1980s still have one, or had one until recently.

Inside, water flowing to an open hot tap pushes on a rubber diaphragm in a water valve. That movement opens the gas valve, and a small pilot flame, which burns all the time on older models, lights the main burner. The burner heats a copper heat exchanger that the water passes through on its way to the tap. Close the tap, the flow stops, the diaphragm relaxes and the burner goes out. Almost every fault is a problem in that chain.

Stop first if you see any of these

Some symptoms mean the heater may be producing carbon monoxide or spilling fumes into the room. Turn it off at its gas tap if you can, open a window, and do not use it again until it has been checked.

• A lazy yellow or orange flame instead of a crisp blue one.
• Black soot or brown staining on the casing, around the flue or on the wall above.
• The heater lights, runs for a few minutes, then shuts off on its own. Many have a device that turns them off if fumes are not going up the flue.
• A smell of burning or fumes when it is running, or a carbon monoxide alarm going off.
• Headaches, dizziness or nausea in the house that ease when you go outside.

If you smell gas, turn off the gas at the meter, open windows, do not use switches, and call the National Gas Emergency number on 0800 111 999. Our guide on what to do if you smell gas walks through each step.

The pilot light will not stay lit

The pilot is watched by a thermocouple, a small probe that sits in the pilot flame and holds the gas valve open while it is hot. If the pilot goes out, the thermocouple cools and the valve shuts the gas off. That is a safety feature working.

The common causes are a thermocouple that has worn out, a partly blocked pilot jet giving a weak flame that does not heat it properly, or a draught blowing the pilot out. If you can relight it following the instructions on the heater and it stays lit, keep an eye on it. If it goes out again within hours or days, it needs an engineer. Do not keep holding the button in and relighting it over and over, and do not wedge anything against the controls.

The burner does not light when you open a tap

If the pilot is lit but nothing happens when you run the hot tap, the heater is not seeing enough flow to open the gas valve. Open the tap fully, because many multipoints need a decent flow before they light, and a tap cracked open for a trickle will never start them.

If it still does not light at full flow, the usual cause is the diaphragm in the water valve. It is a rubber part that hardens and perishes with age, and a split or stiff diaphragm will not push hard enough to open the gas. A blocked filter on the water inlet, scale in the water valve, or low mains pressure in the building can do the same. In some older mansion blocks the cold supply is shared and pressure drops at busy times of day, which makes the heater work in the evening and not in the morning, or the other way round.

The water is only warm

An instantaneous heater can only raise the temperature of the water by a fixed amount at a given flow. The more water you draw, the cooler it comes out. In winter, when the cold main is colder, the same heater gives cooler water than it did in summer. Turning the tap down a little so less water passes through often brings the temperature up, and if your model has a temperature control, check it has not been turned down.

If it has got steadily worse over months, scale is the likely cause. London water is hard, and a heat exchanger that has been heating it for decades can be heavily scaled inside, so heat does not get into the water. Our guide to hard water and hot water cylinders in London explains what scale does to any water heating appliance. A scaled heat exchanger can sometimes be cleaned, but on an old heater it is often the point at which replacement makes more sense.

Is it worth repairing an old multipoint?

Sometimes. A thermocouple or a diaphragm is a small part, and a heater that is otherwise safe and working well can carry on after either is replaced. The question is whether parts are still made for your model, and whether the heater passes the safety checks once it has been opened up.

Many of the multipoints still in London homes are very old. Parts for some are no longer made, and a heater that fails a flue test or shows signs of spillage cannot be put back into use until the problem is fixed. Where the heater is in a bathroom and is open flued, meaning it draws its air from the room, it would not be allowed as a new installation today, and an engineer will look at it closely. Any engineer who works on it must be Gas Safe registered for that type of appliance, and our guide on how to check a plumber on the Gas Safe Register shows you how to confirm that in a couple of minutes.

If it needs replacing: your options

When a multipoint reaches the end, most people replace it with one of three things. A combi boiler gives instant hot water in the same way and heats the flat too, which is why it is the usual swap where the flat already has gas central heating from a separate boiler. Our guide to converting to a combi boiler covers the trade offs.

Where you would rather not have gas on the wall, or the flue is a problem, an electric heater or cylinder is the alternative. A small undersink heater can serve a single sink, and our guide to an undersink water heater not working explains how those behave. For a whole flat, an electric hot water cylinder, often unvented so it runs at mains pressure, is the more common choice, and the unvented version must be installed by a G3 qualified plumber. We fit those under our unvented cylinder installation service.

When to call us

Call us on 020 4652 5111 or send an enquiry. Tell us the make and model if you can see it, whether the pilot is lit, what happens when you open a hot tap, and whether you have noticed soot or a yellow flame. A photo of the heater and its flue helps. We have 20 engineers across London, and our rate is on the pricing page and agreed before anyone attends. If you are in any doubt about whether the heater is safe, turn it off and leave it off until it has been checked.