Did you consider series where one preheats the water and then go into the other?

Piping for Parallel is OK but the piping has to be exactly the same to make sure you have equal draw.

I take it your looking for a lot of volume and have peak demand periods your concerned about rather then a steady slow draw situation.

Any how many manufacturers will supply you with a schematic if you request it and telling them what you want series or parallel

Thanks for the reply. Yes, since posting I have found information that suggests in series is better.....especially since they are of different size and of different pipe length. I found an article from the Journal of Light Construction (October 2004) that provides some insight along with diagrams. In series seems to be the way to go for my paricular situation.

Thanks again.

The series installation is a whole lot easier and allows for mistakes.

The parallel is lot more complex but in certain applications I have no choice.

It all has to do with the actual demand and exactly what one is looking for. Glad to have been of help