Bathroom & kitchen extractor question

[bullethead123]

I have a 1970's ground floor flat which has a condensation problem. The condensate forms mainly in the bedroom ( two large o/s Walls).
The bathroom has 4" extractor fan pulling air through an 8" duct of length 6 metres running through the kitchen and bedroom.
It has been suggested to me (by a builder) that I install a 4" kitchen extractor with it's own ducting 4"x2" length 4 metres, within the original 8"x8" ducting.
I appreciate that the kitchen extractor is a good idea but Do I need to install this interior ducting at all & wouldn't it be counter-productive anyway (I've read somewhere that 4 metres of ducting may cause problems)?
Wouldn't it be better to just install the kitchen extractor in the original 8"x8" ducting with an elbow duct to send the extracted air in the right direction and leave it at that?

 

[stem]

Have you considered a dehumidifier? That will remove the moisture from the air, unlike a fan which will suck out the heat along with the moisture.

If you do want to add a fan, I think the builders idea is feasible. Then you could operate the fans separately. You would have to completely seal the points where the small duct enters and leaves the main duct.

Balancing extraction with several inlets can be difficult especially with existing ductwork, as the main air flow will always take the easiest route, often leaving the other(s) starved. There is also the possibility of transmitting sounds and smells (when the fan isn't running) between the two rooms.

For axial fans (the type with fan blades) performance generally starts to drop off due to the duct resistance in runs over 3 metres, especially if there are several bends. However I've installed fans with 4 & 5 metres of duct that work well. If you're worried, fit a centrifugal fan. They create a higher pressure and can cope with much longer runs, they are often used in flats with centrally located bathrooms and no external walls.

 

[bullethead123]

Thanks for the reply Stem, I'll go with the builder's idea then as it would be difficult to put a cooker hood in and a benefit would be cooking smells extraction. I'd not thought of a dehumidifier, aren't they expensive to run and don't they need constant emptying?
Another idea (mine this time) is to put some more ducting inside the main ducting to facilitate ventilation (no mechanical extraction) from the bedroom. This Would hopefully give some form of "trickle ventilation" particularly from a built in wardrobe which is 3 1/2 m from the outlet. Does this sound reasonable ?

 

[stem]

According to ebac they cost about 2p per hour to run. See link to their FAQ's below.

ebac faq's

You don't have to run them all of the time, you could use a time switch. They do need emptying, how often depends upon how much water they are sucking out of your home. They do switch themselves off if the tank gets full. (Some have the facility to run a drain pipe outside) A fan still costs probably a quarter of much to run, but also during winter, it would be sucking expensive heat out of your home.

You could hire a dehumidifier for a few days and see how you get on. Most tool hire centres have them.

I'm not sure I understand the bit about the wardrobe, but I doubt the trickle suggestion would work very well, probably too much resistance in the duct to get much air movement, unless it was windy then you could have a gale blowing in. Fans with back draught shutters prevent this happening.

There is a type of ventilation that does rely on natural air circulation, Its called "Passive Stack" but it requires careful design and is normally installed whilst a property is being built.