This is kind of HVAC related although not exactly. I didn't want to put it in AOP due to the nature of it and it's kind of DIY. I hope someone here can help. The master bathroom exhaust vent has finally died. My guess is it was original to the house built in 1987. It's a Fasco unit and I'm guessing it was around 50 CFM. We rarely use the secondary bathroom, (these two bathrooms are side by side upstairs) so I pulled the fan unit out of it and put it in the master. Currently, the exhaust is not done right for either one. They are both connected to what looks like non-insulated dryer venting hose (3 inch diameter) and each hose is run about 30 feet and ends just near a gable vent. I guess they chose the longest run possible because the other direction goes over the vaulted ceiling in the master bedroom. There is no airflow making it to the end of the tubing and even if it did, it's not actually being exhausted outside of the house. I believe I'd like to replace both of them at the same time and actually exhaust them outside the home. There's no way I'm cutting holes in the roof to do this although I know this is the only way I can get a short run of ducting.
The master bath is about 50 square feet (counting shower and tub), but half of it is a vaulted ceiling. I've calculated the difference in volume and feel it should be treated as 60 square feet. The secondary bathroom is a little under 50 square feet. Together, that would be about 110 square feet. I've seen that the rule of thumb is to use 1 to 1.1 cfm per square foot, so 110 to 121 cfm total.
Here's what I have in mind. Get a Broan MP 140 inline fan (150 cfm) and mount it in the attic between the two bathrooms. I could then connect a 4 inch intake vent in each bathroom that connects to the fan and then run about a 15 foot long 6 inch duct hose to the side of the house (over the vaulted master bedroom) and exhaust it there. I figure the extra CFM can help make up the for the relatively long run and help avoid any moisture hanging around in the duct when the fan is off. I would make use of a two way switch set up so that you could turn the fan on/off from each bathroom.
What do you think? Broan also makes a 110 cfm version of the same fan. Anyone have a set up like this? The only downside I see is that whenever the fan is on, you're running it in both bathrooms and you'll be sucking out extra warm air in the winter or cool air in the summer. Also, the fan requires a 6 inch duct for exhaust. I haven't seen any 6 inch exhaust flappers for the side of the house at the hardware store. 4 inch seems to the max. Also, would it be worthwhile to use insulated ducting?
Thanks for any guidance you can provide.