Hi there! I have a house in a mountainous area in Europe. The most cost-efficient way to heat water is an electric water boiler, as natural gas is more expensive because of the remote location. I have an 80L boiler, which works really well when I am there alone, or with a single guest. When the entire family visits, it's difficult though; The water is really cold to begin with, so the boiler takes 4 hours to heat, and is barely enough for 2 showers, so having 4,5,6 guests... It becomes impossible. Which is why I am considering an instant gas heater, even though the cost is higher. This would allow us to have as much hot water as we need, on demand. I would like to keep the boiler though, for the times I am there alone. I could have them installed paralely, and have a manual switch from one to the other. But it would be a better idea to somehow have a switch that automatically switches over to the gas heater, when there is no more hot water from the boiler. That way, both could work together, saving some costs. I am not sure though, if such a switch exists and how it would work. Any help would be appreciated!