The volume of your system is fixed, so adding more water increases the pressure. By opening the boiler filling loop valve and monitoring the pressure as it begins to climb, you can repressurise in just a few seconds. Just make sure the valve is firmly shut before firing the boiler. Boiler filling loops are used when you install and service a

Description of water heater 1)Hot water outlet 2)Temperature and pressure relief valve (30 litre only) 3)Cold water inlet 4)Control cover 5)Regulation knob Ariston Thermo S.p.A. Viale Aristide Merloni, 45 60044 Fabriano (AN) Italy Tel. + 39 0732 6011 Telefax. + 39 0732 602331 ….

Sorry also if the boiler is not hanging straight (leaning slightly to the left) it causes condensate water to build up against the sensor causing it to short across the points. Levelling out boiler and removing restrictor should do the trick. Lee January 2008
Try putting it in service mode. I would only do it to test the heating works, dont do it multiple times. I would not do this. The service mode is not designed for that purpose. If it is the old style Drayton RF stat there will be 2 x little square ish buttons on it. Try pressing the top one and see if the heating comes on.
If your boiler is losing pressure slowly and falls below 1 bar on the pressure gauge, then it will need to be topped up using the filling loop. To do this, follow this simple step-by-step: Turn the heating system off and allow it to cool. Slowly add water (which will also increase pressure) via the filling loop, by turning the screws or lever
Ensure it is set to the appropriate temperature. Gradually increase the temperature setting and listen for a clicking sound, which indicates the thermostat is triggering the boiler. If the boiler doesn't respond or there is a significant temperature difference between the setting and room temperature, it might be a faulty thermostat.
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how to turn on ariston boiler