hydropower water heater wont work. turned off for extended period. how fix? can i use open flame?

Not familiar with European appliances, except I once worked on a gas fireplace made in Sweden. What I mean is, I’m not familiar with what safety devices are commonly used nor am I familiar with common construction practices.

If I had to troubleshoot this on our side of the pond, this is what I would do.

I’m going to assume this unit doesn’t have a pilot and has some sort of electronic ignition.

Is the gas there? Not just turned on, but in the line with pressure. It could have been shut off further up your service or a pressure reducing valve malfunctioned.

Is there air in the gas line? Try cycling the unit through several failures to purge the air form the gas line. STOP immediately if you smell gas and let your home ventilate.

Do you hear the igniter? It would be a rapid clicking noise… usually followed by combustion if it is getting gas. If I recall, Bosch water heaters in North America use a dry-cell battery to power the igniter.

It could also be broken and we are getting into professional serviceman time here.

If you do get a ignition, but the unit just stops, then a safety device to make sure the burner is lit might be malfunctioning. It makes sure the unit doesn’t dump raw gas into the unit when there isn’t combustion.

Lastly, your venting might be damaged or invaded by some small animal and its nest. Safety devices in the unit should prevent operation if the unit can breath and vent the combustion gases. Hope the vent is on the ground floor!

Good luck!