It's been pretty cold lately but even with that in mind, our house is unusually cold, even with the thermostat set to 73-74 degrees. I am wondering if a big reason for this is the type of filter I've been using. A few years ago I started using MERV 13 filters because I suffer from seasonal allergies. Also, it's 16X25X4 so it's pretty thick. I am thinking or trying a much lower MERV rating. But can I also use a thinner 16x25 filter? If so, do I need to have any sort of adapter for it to fit snugly into the space? Just in case it matters, our HVAC unit is less than 10 years old. It's a TRANE XV80 system, but I don't how to tell if it's powerful enough for our house: a 2-story split-level built around 1960.