I sat in a review class years ago, prior to my Building Performance Institute (BPI) written exam, and I heard the first explanation as to how a basement would become more humid after replacing a furnace. So, I have to share with as many folks as possible in our HVAC field.
You see, a Category I furnace: naturally vented and 80%+ AFUE, takes the needed air for combustion from the basement. Once burned, said byproducts go out the chimney; the draft (or draft inducer) pulling the needed combustion air, and with it moisture out of the home. What happens when a responsible contractor identifies an opportunity to save fuel and sells the homeowner a sealed combustion 95%+ AFUE furnace? Or even better, a cold-climate heat pump! Now, that combustion powered ventilator was removed from the basement, leaving the high moisture content air to sit there.
The solution? Add ventilation. But, one does not know what one does not test. How do you know if you actually need to add ventilation to a home that has had the natural conditions changed, and how long does that fan need to operate? During the retrofit furnace replacement, the HVAC Contractor more than likely changed the way the home performs as a system. If an air-conditioner was not working correctly you would have to send a technician out to test it, right? This is where a Home Performance Contractor, or Energy Auditor, could identify this ventilation need (or not). You don't know what you don't test! Since you installed a sealed combustion furnace you are more than likely not going to kill anyone, but this does not mean you're not killing the house! There is some simple math that can be completed, coupled with a blower door test and then you know if you must add ventilation to the home.
I remember a lot of local HVAC contractors up in arms a while back about BPI certifying heating professionals. Instead of complaining, I would highly recommend these guys jump on board. As an HVAC professional for all of my working life - granted that is "only" the last 20 years or so, I can contest that is is much easier for an HVAC technician to become a Building Analyst Professional than the other way around. There is some training involved, but believe it or not these guys have some real important information to share that just may save your business one day - or at least help you sleep at night! Check out their standards, easiest way to learn something new/free this week!