The earlier post from SRN with links pretty much covered this. You
need to make sure that your supply and return lines are the same
length for both heaters, and they must be of the same capacity. The
equal length supply and return lines are easily accomplished by
putting tees in the right spot of both. This will keep one heater
from running out before the other.
Running is series will likely give you "steps". You run the first one
out, then the water will be pretty much coming out at whatever the
other heater is set at. It would make no sense to have them set at
the same temps in series, as the second one would rarely run.
Also covered in the link was that you need to resist the urge to shut
one heater off when they are in parallel, even at times of lower
demand, since stagnant warm water in the unused tank could grow some
pretty bad microbes.
Thanks for your reply, Rich. I'm leaning towards the series setup if i go
with two tanks.
Lots of answers but not much directly at your question. I would think
series would be the best way. In Parallel for some unknown reason
one might draw out first.
If your water is very cold the first one could be used to raise the
temperture part way, perhaps turned off during the summer.
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