Checking the flue size
If you are considering adding a second water heater you will need to be sure your flue is large enough to support additional BTUs. Here is an example of a home in which the homeowner wanted to add a second water heater to help fill up a large tub. The water heater flue was a 5" Type B double wall pipe serving two appliances with a total distance of 30'.
The first step in determining if a flue is large enough to support an additional water heater is to add up the BTUs for the existing water heater and furnace. This home had a furnace that was sized at 150,000 BTU and a water heater that was 40,000 BTU for a total of 190,000 BTU.
The maximum BTUs allowed for this situation was 193,000 BTU so we were able to determine that adding a second water heater would not be safe. If this house had a 6" flue the maximum BTU would have been 300 BTU.
In most cases you will be using a single wall connector to connect the second water heater. In most cases a 4' Y is required when combining the second water heater. If you currently have a 5x3 Y off your main flue you will most likely need to replace the boot with a 5x4 Y.
Series vs Parallel
When installing a second water heater you can setup it up in one of two basic methods. The first method is shown in this picture is referred to as Parallel. In a parallel piping system both water heaters work independently to heat there share of the water.
A Series piping method is setup for the water to be heated in the first unit and then pumped into the second unit. The series installation is a little easier to install and has a few other advantages. The main reason we like to install a second water heater in Series and so that you can turn the first unit down to a low temperature to preheat the water. This will still give you an increased supply of hot water but will run more efficiently than two water heaters both running at operating temperature.