Repurposing an old central AC system
So this is ultimately a very oddball situation. Some background:
I live in a rancher in southern NJ. Fully electrified home.
I had recently installed minisplits to replace some electric baseboard heaters, covering about 2/3 of my home. This was fortunate, as I believe my blower fan in my central AC unit blew out. The minisplits + 1 window unit have actually been cheaper to operate than the old AC unit, so now I have a vestiegal high-velocity central AC system in my attic.
Namely, this means a lot of unused flexible, insulated ductwork and some ferro-fluids in my attic that should probably be blocked off and drained, respectively.
I've been contemplating on how to possibly repurpose some of this stuff to fix one of the biggest blind spots in my home: ventilation and filtering
My one bathroom exhaust fan vents directly into the attic, which is a moisture hell that needs solved.
There's no other ductwork in my home, and pretty much the only time fresh air gets in the house is if we crack windows or open doors.
So the theory is:
I route the bathroom exhausts into the old air handler coils to help capture the moisture and drain it out, then have it mix with some outside air and recirculate it into the house again.
Alternatively, routing some of the air between the attic/crawlspace/attached garage for preconditioning outside air as as well.
Is this insanity, or a remotely plausible idea? I'm fairly handy, and since its sbeing made with vestigial bits in spare time labor cost is much less of an issue than parts.
What you're trying to accomplish as you have envisioned it would not work. Your bathroom fan isn't going to move nearly enough air to stop the coil from freezing in your attic. I mean even a 1.5 ton AC needs 1.5 tons worth of air to stop it from freezing.
If you didn't run the AC you'd just be pumping humidity into your air handler and then dropping it back into your house.
Why don't you just vent the bathroom fan out of the roof? That's where it's supposed to go.
Pretty much insanity. Basically, you're looking to do two things:
Dehumidify your bathroom air
This is the closest to working. But basically, to do this right, you need the two sides of the heat pump to be right next to each other. Unfortunately, the outdoor unit for a traditional system is huge and is a PITA shape that will not help you here. You'll also need a special license and equipment to be working with refrigerant gas.
Build a energy recovery ventilator
These use a very special kind of heat exchanger that isn't found in any other kind of HVAC equipment.
This post has been living rent free in my head for a few days now, and I do have a couple of ideas for things you could do, instead of just saying what you can't do:
If you repaired / replaced the blower motor, you could install a 5 inch filter cabinet to the air handler and put a MERV 16 filter in there, run the blower all the time, and that would give your house a psuedo whole house air cleaner.
https://atomicfilters.com/collections/whole-house-air-filters/merv-16?srsltid=AfmBOoouUg3Muh4J1dbbLt89jZC-3sbVs-Q1tee8gWrfcpOxxnsx2AzW
You could even go for an electronic cabinet with UV bulbs if you wanted. I wouldn't recommend an EAC that uses a reuseable pad and electricity, those get gross. Keep in mind the fancier stuff has some pretty expensive annual maintenance in terms of bulb and filter replacement.
https://atomicfilters.com/products/lennox-x8796-healthy-climate-pureair-system-annual-maintenance-kit?srsltid=AfmBOoqSrAoSflAAQ9bkw8c8QnxcArBFx_Z3mvkHb5QwmNB4-R38l38q&variant=40797758412
Take it a step further, install a whole house dehumidifier into the duct work. It does what it sounds like. Mind you when I say whole house I mean whole house, not just your bathroom. But then you could run your bathroom duct into the return feeding the dehumidifer, and it wouldn't hurt anything. No AC required.
https://www.aprilaire.com/whole-house-products/dehumidifiers
I don't know what the climate is in Southern NJ, but if you did the dehumidifier, then you could route a small outdoor duct into the return as well, take out excess humidity so you're not pulling it in from outside. They make fresh air damper kits with controllers that check the outdoor temperature so they're not pulling in super cold or super hot air.
https://www.fieldcontrols.com/fresh-air-ventilation-systems/
And if it's just your blower that's dead, honestly replacing them is pretty easy, Just need an adjustable wrench and a couple of nut drivers, typically 5/16 & 3/8. You can find nut drivers at most hardware stores that have swappable sizes that all fit on the shaft.
https://www.homedepot.com/p/Klein-Tools-7-in-1-Impact-Flip-Socket-with-Handle-Hex-Impact-Socket-Set-32900R-32900R/318280920