Wow - 300 ppm of CO is really high. Many gas fired devices have a powered ventilation. This is a small fan that forces flue gasses to escape. Not hard or expensive to add on.
The other problem in tight houses is ventilation fans (kitchen, bathroom) as well as a household dryer in the conditioned space. A dryer exhausts ~500 ft3 per minute, so running one of those in a conditioned space causes a lot of negative pressure.
Steve
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consulting on geothermal heating/cooling & rational energy use since 1990
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