[BikerDoc5968]

HD, by now you may have resolved your heating situation. I live in Michigan with night time temps between -5 and 20 deg. I have a standalone twin wall acrylic GH (8 x 15) and use a vent-free NG 20,000 Btu heater I purchased at a big box store for about $200.00 I ran a 1/2" gas line inside a PVC sleeve from my home to the GH (underground) and I've been toastie warm all winter. I have had NO problem with CO. Of course as would be expected there is lots of additional water vapor from the combustion of the NG along with CO2, but the plants eat that up and no GH is so tightly constructed that there isn't some air leaks...just my 2 cents worth. The red box to the right of the heater is the 15,000 Btu 240v electric heater that came with the GH and used as a back-up heat source.