it's trying to kill us!

Good morning.
Well, it happened again! I was walking through the dining room when I noticed the co2 detector showing a 135! I opened the door to the water heater and sure enough, it was venting into the room again. So I opened the windows, and immediately called the company to have another tech come out. While I waited, I decided to try and find out why all of a sudden we are getting so much co2 venting inside. I tried a couple different scenarios. First I turned the water heater up with out the air on. It vented perfectly. Then I turned on the air, it still vented perfectly. Then I first turned on the air, and then the water heater. It vented into the room!
It appears the air running sometimes causes a downdraft on the hot water vent pipe, sucking air in from outside! If the hot water heater is already venting when the air comes on, the exhaust out the pipe is stronger than the pull from the air. So I recreated these scenarios several more times , and 3 out of 4 times it vented into the room. I felt I had some good info for the tech. When he finally arrived at 6:30pm, I explained my finding. He new exactly the problem. The air conditioner fan is pulling in more air than is sometimes available. He said usually, there's enough air leaks in a house to compensate for it, but our house must be fairly air tight. Good for insulating, but bad for air flow. Also, the air coming into the unit is from a small area. Since our house is on a slab, the 2 cold air returns are in the living room and hall. There's none in the bedrooms or TV room.
He said I'd have to call the furnace people and get a fresh air make-up air intake with a damper to allow enough air flow. In the mean time we can just keep the water heater on vacation until we need it and then turn it up when the air isn't running.
So now I at least know why only sometimes we are getting venting into the house. The water heater has to come on while the air is running. And we've run the air a lot this past weekend, and this is also the time Craig does a lot of laundry.
So while we waited for the tech to show up, we did get the lawn mowed. I cleaned out the burning hole and bagged up 7 bags of ash, and did some more spot weeding of the lawn. It's been a mini heat wave this week and today is suppose to hit 95 before cooling back down into the 70's this weekend.
I also managed to get the recipe's for the chicken and pork dishes we had this past weekend posted on Cooking with Craig.
Now, time for another work week....