A 7 GPM heater is a true wonder. That calculates out to an appliance that uses a 300,000 btuh gas or electric input. Now let’s figure how far the gas line has to go to get to the heater and if the gas line in the house is large enough to carry this input and still supply the other appliances. Same with an electric input of 75 kw to do the same.
Considering that the major draws are 1-1/2 hours in the morning or 2 hours at night, the unit sits there ready to do the duty of a commercial boiler in a factory for the little home. The unit has nice little coils of tubing to become restricted with lime in a short while; then the unit burns up or refuses to work. These units are serviced by rebuilding them every year in Europe and Japan, but not on the North American continent.
A standard 75 gallon water heater can have a 40,000 btuh input to do the same 7 gpm flow that a home needs, with proven neglect.