Which is better indirect or tankless hotwater heater
Tankless units are limited in the amount of hot water they can produce. It is rated at a certain number of gallons per minute. Think of it in terms of continuous versus multiple use. With a tankless unit rated at 7 gallons per minute you could shower with a 3 gallon per minute shower head until you ran out of fuel. If three people were multiple showering at the same time your use would use 9 gallons per minute and you would run out of hot water. The plus of tankless is you only heat water when needed. However if the tankless unit is part of your furnace it will cycle to maintain a minimum water temperature. With an indirect unit the water in the furnace will heat tank water via a coil in the storage tank. The furnace will still cycle to maintain the internal temperature. You are keeping approx. 40 gallons of water hot for when you need it. The key to an efficient indirect heater is a well insulated tank so the furnace doesn't have to run very often to heat the stored water. This is usually specified as degree drop per hour. Your decision needs to be based on a calculated maximum hot water use per minute versus the capacity of the tankless. Then compare the added cost and how long it will take to recoup the cost based on utility prices.