Repurpose old chimney for vent

[SimonH2]

We currently have a through the wall fan in the bathroom which is ... "rubbish". Apart from performance and noise (to the point where SWMBO will switch it off before showering), it's at the wrong end of the bathroom - as far from the shower as it's possible to get. I'm thinking of putting an inline fan in the attic and venting through the roof - that way it can suck the moist air from where it's being created. I also have in mind possibly swapping it for a MVHR at some point to supply pre-warmed air to a couple of places that get a bit cold and damp.
Then I thought, there's a couple of disused chimneys. Would it be reasonable to take a few bricks out of the stack in the attic, lower a piece of 150mm duct down, use a 90˚ elbow and some more ducting to come through into the attic - then refit some bricks round it and fit a cowl at the top ? I'll do the inside work myself, but get someone in to go up and do the work on top.
What would be the best sort of cowl to use ?
How best to seal between duct and chimney stack ?
Could I leave the existing (round) pot in place ?
Is 150mm likely to fit ?
It's an ex council house, circa 1940s, brick built. I'll get a photo when I'm home during daylight and it's not peeing down.

 

[Koolpc]

What about access for Santa?

 

[Harry Bloomfield]

Then I thought, there's a couple of disused chimneys. Would it be reasonable to take a few bricks out of the stack in the attic, lower a piece of 150mm duct down, use a 90˚ elbow and some more ducting to come through into the attic - then refit some bricks round it and fit a cowl at the top ? I'll do the inside work myself, but get someone in to go up and do the work on top.

If you do, expect moisture to condense inside your duct, which will then run down.

 

[foxhole]

Short a route as possible to avoid condensation issues .

 

[noseall]

We currently have a through the wall fan in the bathroom which is ... "rubbish". Apart from performance and noise (to the point where SWMBO will switch it off before showering), it's at the wrong end of the bathroom - as far from the shower as it's possible to get. I'm thinking of putting an inline fan in the attic and venting through the roof - that way it can suck the moist air from where it's being created. I also have in mind possibly swapping it for a MVHR at some point to supply pre-warmed air to a couple of places that get a bit cold and damp.
Then I thought, there's a couple of disused chimneys. Would it be reasonable to take a few bricks out of the stack in the attic, lower a piece of 150mm duct down, use a 90˚ elbow and some more ducting to come through into the attic - then refit some bricks round it and fit a cowl at the top ? I'll do the inside work myself, but get someone in to go up and do the work on top.
What would be the best sort of cowl to use ?
How best to seal between duct and chimney stack ?
Could I leave the existing (round) pot in place ?
Is 150mm likely to fit ?
It's an ex council house, circa 1940s, brick built. I'll get a photo when I'm home during daylight and it's not peeing down.

As per above post, you don't want the fan to labour and you don't want long (unprotected) runs or dips* in the duct. We always strive to locate a tile vent terminal directly above the fan and then wrap as much insulation around the duct as we can.

* I have seen a dip in a duct completely filled with water, thus creating a trap. It happened when a lazy spark ignored my request that he shorten the duct to suit, once the fan was connected. He just left the excess trailing on the loft floor.