[Closed] Riddle me this - bathroom extractor fan woes

The pipe to my extractor fan sagged dramatically as it went up to the roof tile which resulted in water condensing in there and dripping back down into the fan. The fan failed twice and then the ceiling went saggy before the builder came and straightened out the duct after which it worked perfectly.

Stick your head in the loft and look at the ducting run (and what make and model of extractor you have there). Is it an inline one or just mounted to the surface?

Site manager says that’s normal

It's normal for the site manager to say that it's normal, even when it's not 🙂

Thanks, the ducts are arrow stright right out of the roof. It's quite a long distance between the fan and the roof though. I'll get back onto Mr Site Manager. The fan is mounted on the ceiling (I can see the blades just beyond that.

Has the site manager been and had a look ? If not ask him to come and have a look, if he doesn't show any interest then ask him for his supervisors contact details.
You have just spent a lot of money, don't let someone else try and fob you off, good luck.

Chances are the fan is not man enough to actually provide any flow along such a long run of pipe.
We had a centrifugal fan in our old house that went pop. My father-in-law replaced it with a really cheap axial fan and within weeks there was mould all over the place and drips of water coming from the ducting.
I replaced the fan with the same type as previously fitted (a Manrose centrifugal fan - bit ugly, the box for the main impeller sat on the bathroom ceiling) and it was back to working as it should.
I also lagged the pipe that led to the vent on the roof.

Do you have a backdraught shutter? That should prevent draughts coming in.

Oh, and the site manager is talking rubbish.

The fan is mounted on the ceiling

The fan will presumably be the minimum required to pass building regs for a bathroom installation. It will be too weedy to do a decent job pushing air along that run.

Regardless or not of whether you get any joy with your site manager, upgrading to a decent inline centrifugal fan and lagging the duct will make things a lot more pleasant.

Crap installation?

One of my neighbours had similar issues with their bathroom extractors. Turns out that the builders had installed the fan and the vent hose in the loft but attached it to part of the roof/soffit where there was no vent so no escape for the air from the hose!

Thanks guys. I’ll make sure someone comes back and takes a proper look. Sounds like it’s a weedy little fan without enough oomph. I’ll try and get them to insulate the ducts too

If it is a ceiling mounted one it won't be up to the job of clearing a steamy bathroom (just need to be fitted to meet regs. if its the only source of ventilation - ie, if there are no windows).

You need a Manrose inline fan for the win. Try to get the one I have linked to (not the new model) as they are meant to be more reliable.

Mine does exactly the same, we don't use it in Winter because of this issue.

Effectively warm damp air is being pushed up through the cold loft space. If the gap between the fan and roof is significant then the moisture condenses and comes back down the pipe in to the bathroom.

The easy solution is often to swap to a pipe that is running along and out a soffit that you can insulate instead of it running out of a roof vent and spanning the cold space.

Does sound like a weedy fan (and the solution is an in-line centrifugal one as others have posted), I doubt changing the ducting will have much effect (although if it's really long and can't be re-routed you should also look at fitting a condensate trap).

You bought a new house and haven't been up in the attic yet to check for bodges? Like... wow!

Try opening the bathroom door - does that stop the cold air from coming back in around the housing? Even the weediest fan will create low pressure in the room and air will come in to replace it through window vents, under the door or in any other gap, including cold air from the attic.

Others on here have explained the condensation - insulation is extremely cheap so go and buy a roll of glass wool and wrap it around the pipe.

Thanks all. I’ll take another look myself tonight now I’m STW educated on it. We’re not actually living there yet. Must say the builders and site manager so far have been top notch in sorting all our snags. I was just confused with his response re the extractor fans so I’ll follow this one through again.

@globalti - making assumptions that I’ve not been in the attic to check for bodges yet. Like.... wow!

Recently bought a new build house

Unless you had it built then you can expect any number of issues such as this. A colleague discovered the bathroom extractor simply went into the loft and wasn't vented out anywhere. In built appliances had cables extended just using choc block. Friends are currently in rented accomodation while the entire upstairs floor is completely replaced.

Check absolutely everything, if you are not confident in doing it pay someone to give you a full report it could save a load of grief later down the line. In fact I think that is only way I'd consider a new build. Sounds like the manager is responding toi stuff now but it'll become very difficult to get anything done when you've been in a while and the developers are off site.

We did get an independent snagging inspector in and we have quite a list of things that the builders are sorting this week. You’d think buying a new build is nice and stress free....!

if you have a bit of spare time you may be interested in reading about the extractor fan i put in my bathroom with help from here.

plenty of good condensation/extractor fan advice to digest.......

https://singletrackmag.com/forum/topic/condensation-and-dehumidifiers/

If air is coming back down the vent, you've either got a restriction in the pipe (backdraft flaps freeze for instance) or your vent pipe is full of water/ice (don't laugh, I've actually seen that) or your fan is too powerful, or not enough make up air is allowed in or your builder shoved some rubbish into the end of the duct and forgot to take it out again (yep, seen that too, although it may have been to prevent animals getting into the duct until the roof vent was installed). Insulate the duct, check how much air the fan moves. 10 room changes per hour is a decent rate of exhaust in cfm. Make sure the door isn't blocking flow (sometimes if the hallway is carperted it fills the gap at the bottom.
Ceiling mounted fans are fine, decent ones move more than enough air, sometimes more than the vent pipe can cope with. Plenty of places in the world with way colder temps than the UK, where exhausting air is crucial because of those low temps, use them.

Turns out we’ve got these extractors installed throughout the house at the moment -

https://www.envirovent.com/products/extractor-fans/silent-100/

For anyone that understands the spec of these things, am I safe to assume these are pretty weak? Turns out that the ducting is actually insulated.

I think more power will solve this situation! Assuming there isn’t a blockage somewhere due to dodgy installation which is the next thing to check at least

You’d think buying a new build is nice and stress free….!

No chance but it sounds like you went into it with your eyes open knowing there would be issues. And far better to go to the developer with a list they know has been drawn up by a professional in the industry.

This might sound like a stupid question but if the extractor fan is working well, should there be any steam in the bathroom during/after a shower? I'm wondering if the fan we have fitted is a bit weak as the place fills with steam, droplets run down the walls and the room needs the windows to be opened if it's going to clear relatively quickly.

*Sorry for the hijack!

Alpha - you’d need a ridiculously powerful extractor to have the room 100% steam free (depending on length/heat of shower) but it should clear relatively quickly after. We have identical Manrose inline extractors in our two bathrooms and the one in the smaller one clears really well but the other struggles a bit as it’s quite a big room (approx 10’ x 10’ as a guess).

 

For anyone that understands the spec of these things, am I safe to assume these are pretty weak? Turns out that the ducting is actually insulated.

So it gets a bit complicated. The duct diameter, smooth vs corrugated wall, length and number of the turns of what angle all add up to the static pressure in the duct (resistance if you like). The fan has to move the required amount of air to empty the room 8-10 times in an hour. If the house is really well sealed then the fan creates a vacuum which makes the fan less able to move air after a while.
The graph for that fan shows the amount of air it moves at certain m3/h at certain static pressures. Just to confuse you changing one changes the other as well.

That fan moves a reasonably good amount of air assuming your venting is appropriate. It is an axial fan, which might explain the draft you feel around the edges. "Squirrel cage" types are more efficient.

What size is the bathroom?
How long do you run the fan for after a shower?

Regs state all ducting passing through cold space should be insulated. Tell the site manager that.
Wrighty, proper site manager...

For anyone that understands the spec of these things, am I safe to assume these are pretty weak?

As mentioned above, the static pressure scale (y-axis) on the graph for your extractor is basically how much pressure can the fan push against, while the X-axis shows how the flow is affected by that increase in pressure (resistance).

I bought an inline fan a few years ago for our bathroom, as the tiny ceiling mounted one that our electrician fitted wasn't really doing anything.

Here is a link to the specs for that fan; we have got the 100 (4") version

Comparing this, to your fan, you can see the max flow you would expect from yours is 90m^3/hr, whereas for the Aventa it is about 175m^3/hr on high speed.

But, more telling is that the graph only plots static pressure up to 40Pa for the envirovent.

10 Pa roughly equates to the fan pushing air up 1m with no resistance from other sources. So, if your fan has to shift air 3m vertically in ideal conditions (no losses at all), the flow rate drops to 35m^3/hr. But, as soon as you put in flexible ducting/elbows/fittings/backdraft shutters etc. you are massively increasing the resistance (pressure) in the system that the fan must overcome.

Look again at the spec for the Aventa fan....the high speed graph shows a much larger pressure range.
The envirovent fan trace doesn't go below 20m^3/h at a pressure head of just under 40Pa (4m vertical lift in ideal conditions), so we'll consider that as our comparison point.
From the Aventa chart, you would need a pressure head of ~110Pa (11m vertical lift in ideal conditions) to drop the flow to 20m^3/hr.

Our bathroom is not that big - I would estimate 8x8x8ft.....that Aventa fan is mounted in the ceiling and the ducting runs horizontally from it & out of the soffits - so the only vertical lift that the fan is having to do is perhaps a max of 0.5m from the ceiling to the centreline of the fan. Total pipework is perhaps 3m....
I would say the flow from this extractor is adequate, without being amazing in terms of how it clears steam from the room.
For your set-up, if the pipe cannot be moved to a horizontal outlet position near the soffit, I would say this kind of fan would be the least you'd want in there. If you have the wiring in place and a suitable vent plate on the bathroom ceiling & in the roof, it is a pretty simple job to remove the fan & replace it with something like this.

Also some provisions on max length of ducting for axial vs inline extractors. And the need for condensate traps in some installations...

Not to go too far down the rabbit hole, but it seems like you are suggesting that you are accounting for the "weight of air" in the column, stumpy?
The graph almost certainly ignores that as, unless your duct is running several stories, the effect is negligible. Static pressure on that graph will only account for the duct losses.
OP, the only way to figure it out is to know size of the bathroom, diameter and length of duct, number of turns and radius of each and whether the wall is smooth or corrugated and what type of termination is at the roof. Then you can figure out the resistance of the duct based on it's "equivalent length". Then you can figure out how much fan you need.

cromolyolly Member
Not to go too far down the rabbit hole, but it seems like you are suggesting that you are accounting for the “weight of air” in the column, stumpy?

The graph almost certainly ignores that as, unless your duct is running several stories, the effect is negligible. Static pressure on that graph will only account for the duct losses.

Yeah, I was getting at the weight of air in any height gain that was caused by ducting path between the fan itself & the outlet.

This is interesting. I didn't realise that static pressure is not taken account of for these graphs. I assumed it was included, because whenever I have seen guides for installing extractor fans they often state that the ducting should be routed with minimal height gain.

Have I got my calculations for static pressure incorrect? Does a 1m rise, not equal a 10Pa pressure increase? Am I out by a factor of 10, or something; I could well be.
I would have thought that if you only had 40Pa to play with (as per the OP's fan) 10Pa cause by a 1m height gain would be significant.

I'm not trying to argue against your statement; just trying to understand where I have got it wrong...

Spends on what your calculations are! Pascals fluid theorem isn't used for air fluids. Also I'm assuming it isn't included in the graphs because it isn't generally included in calculating duct sizes in this sort of system. Industrial systems get more complex for sure. Essentially the numbers in the graph the OP linked to seem to point to it. That fan would be so underpowered if you have to include stack height, you'd be better blowing out a window.

My understanding of duct design, which is by no means encyclopedic, is that static pressure refers to that of the duct only. Essentially that casued by the outwards force exerted by the air on the walls of the duct. The effect of the height of the duct isn't a factor because air is bouyant in air. Essentially you treat it like a closed fluid system. For every unit of air you push up the duct, there is a unit of air replacing it at the bottom which is being pressed on by an column of air just as tall as the duct. Plus there will be some helpfuk chimney effect with warm air, offset by any humidity in air, etc. Etc. If you think about how small a temperature difference it takes to create a stack effect this makes sense. The effect isn't nil but isn't of any real consequence. Effective length is a much greater limitation.
The only time the height of the duct starts to become an issue is when the air being pushed up the duct is significantly colder than the ambient outside (like 30°F) or when the duct is several stories tall.
In fact one design that has sometimes been used to overcome the tall duct is to open the bottom to atmosphere.

In general horizontal ducts are a good idea, not simply from an air flow perspective but that is part of it.