Bathroom extractor fan sizing pipework routing

6 Oct 2009
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United Kingdom

I have a bathroom (14.7 cubic metres) which also includes a walk in shower. The only internal extractor grill (100mm) is installed above the shower and is currently connected to a cheap inline fan (currently broken) via flexible ducting in the loft. The exhaust of the inline fan (if it actually worked) vents into the loft space.

I have checked that the electrical supply and switched live (independent of the room light) to the fan and this still works so, I plan to do the following.
  • Replace duct work with insulated plastic pipe (assuming it exists);
  • Replace the inline fan with one more appropriate for the volume for the room;
  • Cut a hole in the gable wall of the loft for the external vent.
My questions are:-
  1. I used the TLC calculator and it says I need to have a fan performance of 295 cubic metres /hour for "In Shower Area", do I need to also account for the bath? Or do I use the small "Bathroom with Shower" at 265 cubic metres /hour!?
  2. Given that my internal grill is currently 100mm, I guess I'm going to have to get a bigger grill as it seems fans of the performance above, I need larger pipework, or is there a 100mm fan that is also quiet?
  3. What fans would be recommended for this use case? I was looking at:
  4. Given that the bath is located in the opposite corner of the room from the shower, would it be beneficial to have another grill above the bath and then have a Y junction in the loft to join with the shower duct before the fan? Could I then also use two 100mm grills before the Y junction and then use 125mm duct into the fan and out through the wall?
  5. If so, I have read in other posts on this forum that I should keep pipe work vertical before the fan. How would I achieve this if I use a Y junction, especially since the bath grill would be closer to the gable?

Thanks in advance,

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I personally would stick with the single in-line fan, the choice is yours which you use providing it dispels the moisture, there is no need to alter the grille vent size within the bathroom. The TLC calculator will have taking into account that the bathroom includes both bath and shower providing you in-putted that information. The best location generally for Ex-fans is the opposite side of the room to the opening door.
PS the fan most be vented to the outside (I understand this is your intention), I do not know what the original installer was thinking when dispersing condensation in the loft space.
Regarding the TLC calculator I was confused with the difference between the room type choices "In Shower Area" and "Bathroom with Shower" as I can understand why "Bathroom" has a smaller requirement than "Bathroom with Shower". But shouldn't "In Shower Area" also be smaller than "Bathroom with Shower"? What does "In Shower Area" actually mean, because my internal vent/grill/valve is above the shower head.

Showers cause more condensation than baths do, as the water droplets/spray is airborne and comes in contact with colder air, thus produces condensation.
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Showers cause more condensation
So because my vent is above the shower head, I guess I'll get the best results with my inline extractor.

I'm just having a hard time deciding on a make/model as I want good performance, but I want to have low running costs and low noise. :)
The grille above the shower will be a bonus and hopefully allow clean air in as well. But you are extracting in to the loft space then out via duct through gable wall. If the loft is insulated well and the in-line unit is within the loft, that will help greatly with noise reduction.
So I'm interested in using this fan: becasue it has the increased performance 280-380 cu.m/hr and has a noise value of 19 dB(A) @ 3m. The downside is the duct diameter is 125mm.

Can I use a 100mm duct and then use a 125mm reducer just before and after the inline fan, or is this defeating the point of having a 125mm based fan?

Also, do I need a telescopic wall sleeve for the wall ( or can I just use rigid duct?
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necking down the duct will restrict flow and increase noise. I get the impression that is the opposite of what you want.

You can use a rigid duct through the wall.

Running rigid duct (sloping slightly towards the outside so condensation runs out) will trap less fluff and condensation than convoluted hose, and is much smoother inside, and far more resistant to damage. If the duct runs in your loft (you don't say) it will not be difficult to run 125mm
Okay back to the 100mm version then and hope that the performance is high enough and that the noise is low enough.

What is the fall rate for the duct work, is it the same for waste water pipes?
If condensate water is going to 'run' down this pipe to the outside does this limit my external grill choice, or do I need an additional condensate trap with an additional exit out of my loft?

there will only be a few drips, and it will be minimised if you flop loft insulation over the duct to keep it warm. But having it drip outside is much better than risking it pooling in the duct or dripping onto the fan or ceiling.

I doubt you will need a condensate trap.

I would go for a more powerful 125mm fan, you may be able to switch it to high or low speed.
Okay fair enough. I'm actually looking to leave the current insulation where it is and use Polypipes Rigid Duct Insulation (just so it gives a nice neat look in the loft for all to see and admire!) plus there is about a vertical meter distance from the bathroom ceiling to 'horizontal' duct run. Worst comes to the worse, I'll by some separate insulation.

The fan does have an adjustable range.

I would like to use the more powerful fan (125mm), but unfortunately I only have 110mm cutter for the wall. Although I don't want to, if I use a reducer/enlarger immediately after my internal 125mm grill, then use 125mm pipework and fan, before a second reducer/enlarger just before the pipe enters the first skin of the external wall. Will this reduce the the extra load on the fan?
I think it will be OK, but not as good as 125mm all the way.

you might try a tool hire shop.

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