Hot water at the tap and cold radiators can be a confusing combination. Before reaching for the reset button, identify what the boiler and its separate heating controls are showing. A clear description helps you get useful advice without guessing at the cause.
For the UK Ariston CLAS ONE 24 combi, G.C.N. 47-116-85. Match your paperwork to the manual linked below; this guide does not cover other Ariston models.
First, check for an emergency
A gas smell, carbon monoxide alarm or suspected carbon monoxide needs emergency action, not a heating-mode check.
In Great Britain, call the National Gas Emergency Service on 0800 111 999. If you smell gas, open doors and windows, avoid flames and electrical switches, and do not enter a cellar to reach the meter. For a carbon monoxide concern, move everyone into fresh air and call the same number. Follow SGN’s gas-emergency guidance and carbon monoxide advice; seek urgent medical help for suspected poisoning.
Four useful checks before asking for help
1. Match the boiler to its manual
Compare your installation paperwork with the manual cover. If the model or panel differs, ask which instructions apply. Keep the casing closed.
2. Look at the operating mode
The matched manual’s page 6 describes SUMMER as hot water only and WINTER as heating plus hot water. Page 5 identifies the MODE control.
Summer mode is one possible explanation, not a diagnosis. For mode selection, follow your matched manual and installer’s handover.
3. Identify the separate heating controller
Write down the make and model of your room thermostat or programmer and what its screen says. Use that controller’s own instructions for schedules or settings. A boiler manual does not establish the correct button sequence for every thermostat.
COMFORT concerns domestic hot water, not proof that heating is selected. Ask your installer about connected controls.
4. Describe exactly what is affected
For your enquiry, note whether every radiator is cold or only some, when you first noticed the problem, and whether hot water is still available. Include a clear photo of the boiler display and any code exactly as shown.
These are useful observations, not a reason to remove panels, adjust wiring or try internal repairs.
When to stop checking settings
For a fault message, leakage, repeated pressure loss or unusual behaviour, seek qualified help. Ariston directs persistent shut-offs and frequent pressure loss to a technician. Avoid repeated resets or filling.
If the correct mode and controller instructions do not explain the problem, send the details for assessment. Avoid naming a failed pump, valve or other part from this symptom alone.
Common questions
Does hot water mean the whole boiler is working correctly?
It tells you that hot water is available at that moment. It does not establish why the heating is off. Report both observations so the person helping you has the full picture.
Should I reset the boiler?
This guide gives no reset procedure. Keep a record of any displayed code and seek advice for a persistent or returning fault.
Can I use these instructions for CLAS SYSTEM ONE?
No. Match that appliance to its own operating instructions.
Ask MPH for heating help
Use the MPH Boilers repair page and include your postcode, model, display photo, controller details and a short description. The office confirms coverage and appointment availability; an enquiry is not a confirmed booking.
You can also explore Ariston boilers at MPH.
Official reference
Ariston CLAS ONE and CLAS SYSTEM ONE user manual, 420011124402 Rev. 00 (08/23): model scope p.1, safety p.3, panel p.5, modes p.6, controls p.8 and escalation p.9.