Since 1990, a tankless coil boiler has a minumum AFUE of 80%. Most are rated higher than the minimum. A water heater does not have that efficiency as it is a single-pass vertical heater. A tankless coil boiler with an input of 140,000 btuh, equal to a 1 gallon per hour nozzle, can deliver 3 gallons per minute (180 gallons of hot water per hour.) When mixed with cold water it can produce 5 gallons per minute of shower water every hour of the day or all month if your budget allows. There is nothing else besides an external tank system that can deliver this, although an external tank can only produce this much in the first draw for the tank and far less for the following hours.
If you buy another water heater instead of using the tankless coil system, you now have two heaters, one for the house and one for the hot water – with two burners to service. Now the boiler doesn’t have to stay warm for the summer months to make hot water, the extra water heater does – all year round. So which is better? – To have a tankless coil boiler weighing 360 pounds losing heat from the jacket during the summer (it is doing the heating during the other seasons), or a water heater weighing 350 pounds (including the water content) losing heat from the jacket all year round.
I opt for a high efficiency tankless coil boiler set up with a mixing valve so there is only one heating appliance to service with only heat losses from one appliance instead of two, making more hot water than most people can use.