You might try
a thermostatically operated exhaust fan for forced air or
a thermostatically operated heat source for convection, like a small bulb.
The t'stat would sense outside temps.
With the bulb you can experiment as to size but if the bulb is small enough it may not pay to control it with a 'stat.
I though CO was the main danger, not CO2, but CO2 is considerably heavier than air.
You might also have an exhaust fan somewhere in the house that is pulling the LP exhaust back into the house.