Ariston Boiler No Hot Water: Causes, Codes and Checks

Is it an Ariston boiler or an Ariston water heater?

Ariston puts its name on gas boilers, electric water heaters and hot water cylinders, and people often call all three the boiler. A wall mounted gas combi such as a Clas, Genus, Alteas or E-Combi has a flue going outside and a pressure gauge or display. A small white tank under the sink or on a utility room wall with a plug or fused spur and no flue is an electric water heater, and our guide to an undersink water heater not working is the one to read.

A larger Ariston cylinder in an airing cupboard is a stored hot water system heated by a boiler, an immersion heater or both. Our guide to Ariston cylinders explained covers those. The rest of this page is about Ariston gas combis, which are what most London flats with an Ariston have.

What the codes usually mean

An Ariston that has stopped itself shows a number on the display, often with a flame or spanner symbol. Note the whole code before pressing anything. It is the first thing an engineer needs and a reset clears it.

501 generally means no flame was detected when the boiler tried to light. Check the gas is on by lighting a hob, check a prepayment meter has credit, and in cold weather check the plastic condensate pipe outside has not frozen solid. 108 generally means the pressure in the heating circuit is too low for the boiler to run safely. Check the gauge: around 1 to 1.5 bar with the boiler cold is typical. If it is low, top it up through the filling loop as your manual shows, then reset.

Codes vary between older and newer models, so check yours against the manual if the number is not one of these. Whatever the code, reset once or twice at most. Our guide on how to reset a boiler explains why. A code that returns straight away is the boiler protecting itself.

Checks to make before booking anyone

These cost nothing and solve a fair share of Ariston no hot water calls.

• Pressure: low pressure stops many Ariston combis firing at all.
• Hot water temperature: check it has not been turned right down on the panel.
• Summer or winter mode: some models have a setting that turns heating off but leaves hot water on. Make sure hot water has not been switched off by mistake.
• Tap flow: open the hot tap fully. A trickle from a mixer tap set halfway may not be enough to make the combi fire.
• Power: a combi needs electricity to fire. Check the fused spur switch and that nothing has tripped.
• More than one tap: if one tap is hot and another is not, the fault is in the tap or its valve, not the boiler.

Heating works but no hot water from an Ariston combi

With no code on the display and hot radiators, the boiler is lighting and heating. The hot water side has failed in one of three places.

The diverter valve moves the boiler's heat from the radiators to the tap water whenever you open a hot tap. If it sticks on heating you get hot radiators and cold taps, and if it sticks partway you get lukewarm water and radiators that warm up in summer. Our guide to diverter valve replacement cost explains what that repair involves. The flow sensor tells the boiler a tap has been opened. When it fails, the boiler stays silent as you run the hot tap but fires as normal for the heating.

The plate heat exchanger is where the heat is passed to your tap water, and in London it scales up steadily. Hot water that starts warm and fades, or a boiler that lights and cuts out while the tap runs, usually points here. Our guide to a blocked plate heat exchanger explains how it is confirmed. The same parts fail on most makes, and our page on no hot water from a combi boiler covers them in more depth.

Hot water goes hot then cold, or takes ages

Many Ariston combis have a preheat setting, often called comfort, that keeps the boiler's hot water side warm so the first hot water arrives quickly. If it has been switched off, you wait longer for the tap to run hot. That is normal behaviour, not a fault.

Water that runs hot and then cold while the tap stays open is different. First open the tap further, because a low flow can drop below what the boiler needs to keep firing. If a full flow still swings hot and cold, the usual causes are scale in the plate heat exchanger or a flow sensor dropping in and out, and both are confirmed with temperature readings rather than a guess.

Repair or replace an Ariston?

Most Ariston hot water faults are ordinary repairs to a valve, sensor or heat exchanger, and the parts for current models are readily available. On an older boiler, particularly one where the same fault keeps returning because scale has not been dealt with, replacement becomes a fair question. That is a judgement for after a boiler diagnosis, once the actual fault and the condition of the rest of the boiler are known. Older Italian combis from makers with a thinner UK parts supply raise the question sooner, as our guide to a Biasi boiler no hot water explains.

Removing the case of a gas boiler is gas work and must be done by a Gas Safe registered engineer. That includes the diverter valve, the flow sensor and the plate heat exchanger. If you smell gas, turn it off at the meter, open the windows and call the gas emergency line on 0800 111 999.

When to call us

Call us on 020 4652 5111 or send an enquiry if a code returns after a reset, the pressure keeps falling after you top it up, there is water dripping from the boiler or the pipes below it, or the hot taps still run cold after the checks above. Tell us the model name on the front and any code on the display so we can bring the right parts. We have 20 engineers across London, and our rate is on the pricing page, agreed before anyone attends.