it is verry important that the all the pipes are equal length and size for instance
all 4 pipes ( 2 for the cold in and 2 for the hot out ) should be XX inches by XX inches ( 3/4 1/2 ect ect )
and all the lines that twin togher ( go to the tee ) should be the same size
now some people say the water heaters should be the same size in all aspects ( hight width gallon size ) i also agree with that as well
from one heater its cold in and from cold in it
then hot out to cold in on the other heater
then its hot out of the other heater
I am Bud - Another online "Plumbing Consultant"
Your heaters are plumbing in parallel and you are wanting to change to series plumbing, I think. With parallel plumbing, Mario is correct, every pipe, tank, fitting has to be the same length for restriction purposes. With series it doesn't matter about pipe length. The draw back to series plumbing is, the primary heater will do all the work. It is very possible some buildup in the bottom of one of the tanks is causing the lack of adequate hot water.
I hope the above information helps, if it does not please let me know.
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I am Bud - Your personal online "Plumbing Consultant"
Series is much less problematic.
Both will give the same amount of hot water, in theory... I would set both themps the same. If you set the first at a lower temp it will require both heaters to run every time, if you set both the same the secondary heater should only come on when you have a large demand for hot water.
My opinion is series.