Do you mean tankless, or an indirect system?
Tankless systems are just a coil that is located inside the furnace/boiler. Domestic hot water for the faucets and showers runs through the coil and is heated as it passes through the furnace. This works well for homes in a cold climate (especially during the winter) because the boiler is always hot to keep the house warm. Not as efficient during the summer because you are running the furnace all the time just to make hot water for showers, even though you don't need the hot water for the baseboard heat.
Indirect systems are basically a 30 gallon or larger thermos bottle of stored hot water that has a coil inside that is heated by the furnace as a separate "zone". The storage tanks are super-insulated and once heated up, the stored water stays hot for a long time. As the temp gets too low, the furnace fires up and heats it back up. In the summer, you will be running the furnace a lot less because of how long the water stays hot in the storage tank.
When our house was built almost 20 years ago, they installed a tankless coil in the furnace, which worked "OK". Since we have hot water/baseboard heat, this was the easy thing to do (didn't need to install a separate hot water heater). With 3 teenage kids in the house, we often ran out of hot water and sometimes had to wait between showers. We recently upgraded to an indirect system and the difference is quite noticeable. We have no issue with 3 showers running at the same time, there is plenty of hot water. In our region of the country (New England) an indirect system is reported to be at least 10% more efficient than the tankless coil. I can't speak to how more/less efficient this system is to a regular hot water heater (basically a storage tank that has its own built-in heater), because we never had one.
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