Of course he is going to use a quality mixing valve to lower the water temperature from the tankless coil. He wouldn’t think for one second to find the cheapest valve there was making hot water for his family. He wouldn
‘t want a scald for anyone. That’s why he wouldn’t select an instantaneous water heater with its varying water temperatures that can lead to scalds.
If he doesn’t want to settle for the greatest amount of hot water instantaneously as from a tankless coil, he can go to the external tank system which has proven to be more than satisfactory for homeowners. you would need a larger boiler than most to use a tankless coil boiler because the heat loss of a home would be that large in a house of 5000 square feet. An instantaneous water heater does not have the choice of a burner matched to the heatloss of the house, it must be large enough an input to qualify for heating a 4000 square foot house just to make some hot water.
With an external water heating tank, the water temperature could be set down to 120F with a recirculating system for instant hot water at the tap. The boiler could be sized for the heat loss of the house, so a small house could have a small boiler input to match the maximum efficiency while still getting all the hot water he needs from an external tank. He would be able to draw 2.4 gallons per minute for the shower or 1/2 a cup of water without turning on the burner for quite a while if the water had been sitting there warm already. If he needs more hot water, he can install another or a larger tank. Once he exceeds the amount of water he needs with the instantaneous unit, he would have to install another unit with its large input burner to provide more hot water.
Every heating system needs service and examination every year by a skilled, experienced plumber, so the frequency of service may be less for gas, but the frequency of examination must be the same – at least once per year for all these appliances.