I have an indoor pool that is on the basement level of a two story.  It's heated by a Raypak R206A heater with an indoor stack and exhausting through the roof, two stories up.  The heater is in a separate room that has cold and has two 6 in. outside air inlets.   I have a CO detector in the room where the heater is located.  Recently, the CO detector has been going off intermittently.  I thought it might be the detector but now know that CO is getting into the room from the heater.   When the heater fires up I can now see exhaust coming from under the stack bonnet.  After it runs for a few minutes I can no longer see it.  The detector goes off after a short while (indicating CO) but when the heater runs for quite a while the alarm will stop.   I'm in Michigan and right now we're in a cold spell.  Cold air is flowing into the house from heater exhaust when the heater is not running.  I put a lit match under the bonnet to prove this and it will blow the match out.   My conclusion from this is that before the heater is on there is flow into the house.  When the heater initially starts up and the temperature of the exhaust gas goes up it has to fight the flow in the pipe until the air/CO flow reverses and goes out of the house.   I don't recall ever having the problem in the warmer months.  This leads me to believe that the cold outside temps and change in atmospheric pressure may be the root cause. I've also gone through winter's before without an issue.   I know that Raypak makes the D-2 Power Vent which would instantaneously create a positive pressure and flow out the exhaust pipe.  But, I haven't been able to find any criteria for when the power vent is required versus the stack on an indoor application.     Ant idea what's going on?