This paragraph:
Check valve (2) prevents the solar pump from pushing water through the boiler in the wrong direction when in boiler mode.  The check built into pump (3) prevents the boiler from pushing water back through pump (3) and into the solar tank in the wrong direction when in solar mode.

Should read like this:
Check valve (2) prevents the solar pump from pushing water through the boiler in the wrong direction when in SOLAR mode.  The check built into pump (3) prevents the boiler from pushing water back through pump (3) and into the solar tank in the wrong direction when in BOILER mode.