This is my second season now with the wood fired boiler. I built the manifolds and all plumbing myself which was easy as the unit came with detailed plans. It works pretty good but I question the H where the supply and return lines branch off. Basicly it allows the incoming and outgoing to mix and equal out psi. System is low psi type but still not sure why they require this H in the system. Any thoughts?

It's hard to tell for sure from your description, but I'm guessing it's a "boiler-bypass",  schematically similar to this:







("System piping" refers to all radiation & plumbing beyond the near-boiler plumbing)

Yours may/may not include a  balance valve for adjusting the mix to the return water.

The purpose of this is to keep the water entering the boiler to be so cold as to...

A: damage it from thermal shock or mechanical stresses from too large a temperature difference between return/output water.

...and/or...

B: cause the heat exchanger to be so cold as chill the exhaust gases to the point that corrosive condensation or creosote formation occurs (either in the flue or the heat exchanger itself.)

There isn't significant heat/efficiency lost from this configuration as long as the plumbing is insulated.  It doesn't offer perfect protection for the boiler, but it isn't bad- way better than allowing extended cold starts to a high mass heating system.

http://www.centralboiler.com/Tech/C190.pdf Here is a link to the diagram