Boiler continuously turning on and off when asking for warm water

Question

I just started renting an apartment and everytime I turn on the warm water (in the kitchen, bathroom or shower) the boiler turns on as expected, to then after some time (seconds) turns off. The water gets a bit colder, and then the boiler kicks in again. This is done continuously until I finish using the water, creating a very uncomfortable warm-cold-warm-cold cycle.

I have seen that if I set the boiler to maximum temperature, and I open the water to full scorching temperature, it doesn't turn off. If I reduce the temperature at the boiler, or I turn the tap to less than scorching water, it starts turning off or on again.

My landlady says a plumber has already checked on it before I enter and that all was ok, that the behaviour was ok. Is it? Or is there anything possibly faulty? Any consequences mid/longterm?

The model of the boiler is a ThemaClassic from Saunier Duval.

Thanks! Alvaro

Answer 1

The ThemaClassic from Saunier Duval appears to be a gas-fired device. It comes in either a chimney-requiring or room-sealed version.

Gas-fired devices need certain conditions to burn correctly, and they have sensors that can test for the desired conditions. Air flow, flame blow-back, overheating, etc, are all monitored. If one of these conditions are out of range, the unit may turn off, and when the condition comes back in range, the unit can turn on again, resulting on off/on/off/on running. Or a sensor could be bad.

Having the water go hot-cold seems not-normal operation. I would contact Saunier Duval with the exact model number of the boiler, and ask them if this is normal for the unit to toggle off and on. I'd be surprised if they said Yes. Also, check with your fellow renters and ask how their boilers perform, and if they also have ThemaClassics.