Depending on the size of the flue, it would be entirely possible to put a 4"-6" stainless corrugated liner in there with a “T-Body” and “snout” going through the side of the chimney in the attic and close off the damper with an opening for airflow. Then you would have in-line duct fans with a rheostat control at both ends pushing air one way or the other. The duct fans that are the same diameter as the duct don’t have much pressure, so you would probably want a centrifugal fan to spend once and get desired results. You can buy the fan with the speed controller or they do have ones you plug into. The fans are loud, so the Mrs would appreciate it if it were inside of a baffle box to keep the noise down while she works. How low speed the fans can go is limited, so don’t expect to have infinitely varial speeds without spending a lot more.
There also are temperature controllers so you can have the fans cycle to regulate temperature without manual control. You would probably want to put that on only the attic fan if you went that route.
You could use galvanized duct, but it will rot out over time in the flue and it is harder to install vs a stainless corrugated liner.
The fans are $200-400 depending on how much chooch you want. The liner runs $12-16 per foot. The T-Body and snout are about $150-200.
There are bi-directional varial speed duct fans but they carry a premium, typically require custom duct manifold/plenum fabrication, and are two fans put together to achieve the pressure required. Going with one fan at each end is easier, cheaper, and easier to repair should one fan fail.
If it were my project, I would try just having the fan in the attic pulling and control it with a speed control plus temperature controller. If that wasn’t enough is when I would add the fan in the basement and do manual control with variable speed and have both fans push. I don’t really see why you would want to pull air from the attic, but you seem to feel you would need to.
It can be tricky to get the snout on the T-Body, you will want some 1/4" extensions and impact, as well as someone who can help you.
ch00f@lemmy.world 6 months ago
Should have been more precise. I have a funny situation. My house has had four major remodels performed over the past 80 years. One of them involved extending the roof and totally covering a chimney (there is another chimney elsewhere in the house). Rather than remove the chimney, they built around it including adding a closet on the middle floor. The closet is wider than the chimney, but the whole thing is framed out as a rectangle, so I have like 1x2’ of empty space leading from my attic to the basement ceiling.
So not need for liners.
I’m by no means an HVAC expert, but I was thinking that pulling hot air from the hottest point in the house (attic room ceiling) would provide the best circulation. Thinking more about it, I think I’d be better off having it be one-directional if only so I can install a filter to keep it from filling up with dust. I can convince myself that either direction is the better option. Maybe I’ll install the blower somewhere in the middle where it’s easy to access.
Thanks for the advice!
Death_Equity@lemmy.world 6 months ago
If the masonry is gone and there is just a framed out hole then galvanized would totally work with fairly easy install with the help of a second person or some clever use of support. You wouldn’t want to just blow air through the cavity without ducting as it would be horribly inefficient and tank the air velocity across that much distance.
Adding an in-line towards the middle does mean having controls up in the attic more involved and you do want the Mrs to have control over the fan there instead of her having to go downstairs.
If you wanted to circulate the air instead of forcing air with a passive return, then you get into a more complex situation with two ducts and fairly informed placement of intake and return.
Pulling into the attic will help to lower temps and will be the simplest/cheapest option so long as there isn’t a wall you can put a window heat pump unit in.
ch00f@lemmy.world 6 months ago
Do you think I could get away with some flexible ducting? Might be hard to navigate the rigid stuff into these spaces.
Death_Equity@lemmy.world 6 months ago
When you say flexible ducting, are you talking aluminized plastic or the corrugated aluminum? That would be a fair amount of weight for flexi in either case, you would want to have access along the route to secure it, especially if it is insulated. The aluminized plastic would need more support than aluminum.
Without site inspection, I couldn’t say how necessary insulating the duct would be. Using insulated duct would be a good “better safe than sorry” move but will make installation without full access a bit of a bother.