[lastlight]

Des the negative pressure somehow affects your furnace and the risk is related to carbon monoxide poisoning I believe. From what I've read it's quite serious which is why high cfm fans for microwaves etc require hrv installed. I guess the hrv allows extra fresh air into the house. The one for our furnaces must only be rated for whatever the furnace is capable of pushing.

My condo is old and there are air leaks all over. Might be more of an issue for you.

At least swap your detector in the basement for a dual fire/carbon monoxide one. We just put one in.

As for rodents I'm going to just put a really fine metal mesh at the mouth of the intake. Both ducts will have to be retro fitted to my window somehow.