Adding a Tankless Hot Water Heater after a 40 Gallon DHW Tank to extend Hot Water Supply

If 3 tenants means 3 persons, there may be options to save money and energy without compromising the comfort - without much installation work or none at all.

Assuming the tank water temperature is set to >= 70 degree Celsius, there is plenty water for 3 persons to shower at the same time, if the shower heads are special types with lowered throughput. F.e. by mixing more air into the water to make it feel like high throughput.

If the showers have thermostatic faucets, there might be a problem with failing back - flow preventers. Cold water will be pushed into the hot water net, thus the hot water will be pushed back giving the illusion of non sufficient hot water supply.

Back flow preventers are prone to fail in thermostatic faucets.

Another important maintenance point is the heat exchanger unit inside the boiler. There might be a thick layer of limestone that delays the energy transfer. It should be removed with non-metallic tools in order to prevent pit corrosion.

Another option is to decrease the hysteresis of the burner/heater of the tank and to add more isolation to the tank and lines, if there is enough space. And of course to increase the power of the burner, e.g. by exchanging the oil burner nozzle (more throughput) or the electric heating element.

For hygienic reasons, the temperature should anyway be kept above 45 degree Celsius in the total hot water net - especially in rented out apartments.