Methane - Radon - Off Gassing Oh My

I fixed the issue I had with methane gas a year or so ago having a sponge / foam in the sewer vent.
Here is a link to that.

For years I have had issues with something when the furnace goes off. When the air intake goes off before the furnace starts blowing, I will have heart palpitations. I have been talking with many different people recently about how air tight my home is and the things that it could be, and how to fix the issue. 

When I had the energy audit years ago, they found my house to be the tightest they had tested ever as long as any of them had been doing that task. I talked to my brother and for some reason, when I thought of methane coming into the house, I always thought about the bathrooms that we use as if you have basement bathrooms that don't get used, methane can get into the house. 

For some reason, I never thought about the larger bathtub and two sinks that we NEVER use. The water is turned off to the two sinks and the big tub gets used once a year or so. I make sure to run water into the regular bathroom drains but can't believe I never thought to run water in the big bathroom.

I think that all those drains being dry all the time, gas can escape into the basement and when the furnace intake comes on, it draws air from those drains as the house is such a tight home and when I turn on the swamp cooler in the summer, the bathroom fans in across the house blow out and I can't open the door without much strength as there is pressure in the home. 

I believe that the furnace is causing a vacuum / negative pressure / suction effect. I contacted the national Radon site and talked to them asking if I did have radon gas, what would happen with my air tight house and the man that is in charge said that if they put a Radon Mitigation System on my air tight home, that it can create even more of a negative pressure situation and pull more radon into the home. 

I talked to him about "off gassing" as well and he didn't think that was what I was dealing with, but we talked of other ways to fix the problem. 

Currently I am using box fans to air out the house daily. My brother is an expert in sensors and data loggers, and is rigging a logger to detect the air in the house at regular intervals. He will be sending it soon so I can see if the gas in the house is thicker than air and if the house oxygen levels drop, that means that the negative pressure is suctioning either methane or radon into the house. 

If the situation is off gassing, I don't think anything will show, however, this situation has been happening since before I put the new memory foam carpet padding down, so hopefully this sensor would be able to detect anything going on. 

I will start running water down those large bathroom drains every few weeks to keep water in the trap. It would be nice to figure out why I am so tired and what is causing my health problems! 

Have a blessed day!