On Tue, 15 Mar 2011 17:45:55 -0400, clare wrote:
The most common application of series connected water heaters is a 5
gallon unit at point of use to provide hot water instantly, connected
after the 40 gallon or whatever "main" heater which might be at the
other end of the house.

I should have mentioned both water heaters are gas fired and 50 gallons
and they're both right next to each other (practically touching).

The hot output of one heater goes into the cold input of the other.

There is a hot-water recirculating pump on the output also that allows
any of the five bathrooms in the house to have hot water within ten or
fifteen seconds.

I will read all that is written here until I understand what the design
goal was in the first place (and what then to set each temperature at).