Design of ventilation using two extractor fans

[ais]

Hello friends,

 

I live in a block of flats and our flats can accumulate a bit of condensation, particularly in winter.

 

The bathroom and separate loo do not have outside windows, they are fully enclosed. On the inside of the building.

 

The original design of the block relied on a ventilation shaft that rose to the block’s roof where industrial fans supposedly run at all times. This ventilation system seems to have weakened in recent years and I thought it'd take ages to get fixed. (And I was right.) So upon moving into the flat, I decided get my own ventilation system set up.

 

The diagram below is how the builder designed it and implemented it. This was 2.5 years ago.

 

The extractor fans are axial, but now I learn that that was a poor choice. AirVents 10cm diameter.

 

The ventilation DOES work. A toilet paper square will be pulled in, if you hold it close to either grille. And there have been no condensation issues in the flat. When both extractors are working, you can see the flap at the external wall lifted slightly. But, I haven’t been convinced that the extraction itself was very significant.

 

Both extractors have spring operated non-return flaps, to prevent backdraft.

 

The bathroom fan (see diagram) has now stopped working and this failure has led me to rethink the whole thing.

 

I have an eye now on a longer term solution as opposed to a solution that ticks the boxes but probably isn’t doing the job well.

 

Problem #1: Given the bends, shouldn't I replace the axial extractors with centrifugal ?

 

Problem #2: Given that the WC extractor air will have to pass by the bathroom extractor, it is well nigh impossible to prevent mixing of both airs. True or false?

 

Contamination has probably been happening already, it’s just that it’s only me in the flat and how can I be in two places at the same time? Who am I kidding? The WC air is probably going to feed into the bathroom despite the backdraught flap.

 

 

 

 

[Iceverge]

Not 100% sure what the arrangement is here. Are the green lines ducts ? 

 

Where is the fan located? 

 

A pen sketch would suffice. 

[ais]

The green lines ARE the ducts, yes.

And the two fans are indicated by the black arrows. One in the WC, the other in the bathroom.

[PeterW]

If you’re getting condensation then a small heat recovery fan unit that draws from the two “wet” rooms along with the kitchen and replaces it with outside air into bedroom and living space would be ideal. You would need another duct through the wall but all easily doable. 

[ais]

Thanks for your input, Peter, I am not getting condensation, no.

 

I did provide a decent schematic and fairly lengthy description to explain the rationale behind my questions and what help I need.

 

I don't want to invest in new stuff. The failure of one of the extractor fans (which is axial), opened up the design for me to revisit, with an eye on the final cost -- so I do want to get the most effectiveness out of the existing ducting layout.

 

If no one else here has an opinion, I will just go ahead and buy a centrifugal extractor fan and see if it does a better job at suction than the axial. If it does, will switch the other one from axial to centrifugal too.

[PeterW]

8 hours ago, ais said:

so I do want to get the most effectiveness out of the existing ducting layout.

 

If no one else here has an opinion, I will just go ahead and buy a centrifugal extractor fan and see if it does a better job at suction than the axial

It won’t - it will make it worse as you’ll be applying pressure to the system in the wrong place. If you want to add a centrifugal duct fan then it needs to be after both intakes and preferably in the long section for maximum flow. 

[Mr Punter]

I have a similar setup where 2 fans share the same duct.  I used Airflow Icon 30 mixed flow fans, which have an iris valve and this prevents cross contamination and means I am only extracting from the room where the fan is running.

 

They have lots of options for humidity, delayed start, motion sensor etc.

 

You may need a larger diameter cutout for the fans.

 

Shop around as there are some deals to be had.