I have 10 year old polaris water heater that Im having problems with. its a 100,00 btu unit that does both domestic and heats a floor area of about 800 sqft. initially it kept shutting down, and after having the plumber replace everything from the igniter, gas valve, flow switch, check the control board, check venting in and out, we got it running but only with a hard start which gave of a huge bang.In trying to fix the hard start polaris suggested turning the mixture screw down in the gas valve, now we cant keep it on. Our gas values are perfect before and after the gas valve, what else is there to check? would much appreciate answers.

Something is definitely wrong with the polaris installation. you still need to insulate that 1st floor and a 60 degree basement (warmer than your first floor) would seem to indicate you have a more effective radiant ceiling than a radiant floor right now. But you've got issues with this polaris a gas tech with a combustion analyzer should be reviewing at least.