What you are currently trying to do is extremely difficult as you have discovered. I wanted to get your comments on the issue of home inspections vs. home warranties. A couple of times I've walked away from counter offers which offer the warranty in exchange for giving up an inspection contingency. The sellers agent always seem to believe that the warranty is adequate protection. A couple of weeks ago there was a, excellent article in the Post about this same topic.
Warranties are no substitute for an inspection.
Warranties are very limited in coverage and some are almost worthless. Stay with an inspection from a qualified home inspector. Look for these and try them. Otherwise, you may have to have a reputable heating contractor install them. Good home builders have them installed when the home is built.