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    Let’s make it two 40 gallons for discussion purposes. No matter how they are connected if it is done correctly, your initial storage would be the same as a 75 gallon heater. Once the burner comes on the gas input will determine how fast the water heats. With the 75 gallon the input will be about 80K btu’s. With the two 40’s in parallel the total input will be about 80K btu, (two 50’s will raise it to 100K btu), so the recovery will be about the same. In a series setup, the first heater is going to start operating when its temperature drops to the thermostat on point, after approximately 10-15 gallons are drawn off. So it will be sending a slightly tempered water to the second tank which will keep it from operating until the demand is such that its temperature also drops to the set point, at that point the combined recovery input is also 80K btu. The major differences come into play down the road when the heaters start to leak. The 75 will be the most expensive to replace. In a parallel system either heater can leak first, but if valved properly, it can be isolated and you will still have hot water. In a series setup, the first heater is the most likely to fail, but you will most likely have to shut off both heaters until it is replaced. Now if you change the two 40’s in the above example to 50’s, you get 20 more gallons of initial drawoff, and about 20K mor btu’s of recovery input. You just have to determine which of the three models fits your lifestyle. The 75 gallon and the parallel may be about the same initial cost, depending on the models you install. The series installation should be the least expensive to install, again depending on which model you select.

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