how did you clean the heat exchanger? I've heard anything up to and
including muriatic acid. How old is you exchanger? A new exchanger for
about $150 may be an investment, at least so you can keep your old one
as a spare, but first try to see if you can get good water flow through
it.

Email me directly about the bypass,p lease, and I'll try to send you
pics.

cheers
mickey