[Witchdoctor]

They are right,

Parallel is the best way to maintain volume and temprature.
I pipe my commercial water heaters and chillers this way.
You just have to remeber that the header has to have eaqual lengths and fittings to maintian an even flow through out the manifold, if not one end or the other could suffer.

Theiry is that all blocks are getting the same temp as the manifold as opposed to one then the second getting the first cards gain and the third recieving the first and second cards gain. This is absolutly manditory for chiller and domestic water heating and many times when not done properly it is disasterous

Example:

When at a hotel 6 the hot water for your shower is weird, real hot then cold then hot again
When done right hanging at the Hyatt Regency your shower is solid as a rock with this proceedure being executed properly

Parallel = WIN for a multi card set up