The air filter is part of any Forced air system. Its primary purpose is to keep the equipment clean. Of course a major benefit is it also cleans the air as the system runs. With a high quality air filter you can remove mold spores, pet dander, pollen, and all types of small air born particles. Combined this with a Ultra violet air purifier and you have clean sterile air recirculating from the vents.
We've got a pretty complicated hydronic/solar/propane water heater and Will knew exactly what was wrong and how to fix everything. He's very professional and a perfectionist which is what you want when it comes to plumbing.