Bathroom fan blowing

[stuartR1963]

Hi all i have a greenwood airvac rf90tw in the bathroom that is blowing instead of sucking, i have tried holding a couple pieces of toilet paper against the grill but it will not hold it up it just blows it away. What could be causing this issue as this fan is completely useless at the moment? The ducting is only about meter or so before it reaches the outside grill under the eaves as you can see.

I originally thought the external grill may be blocked so i disconnected the duct from the fan and poured about a litre of water down the duct and water came pouring out the external grill outside so if anything was blocked i am sure that would of cleared it. I reconnected the duct as you can see but im still getting the Same issue. I also have an en suite which has the exact same greenwood fan module and that has no issues clearing the moisture in the en suite and can easily hold up some toilet paper against its grill, that fan works as it should and is sucking instead of blowing like the bathroom fan is doing, i will also point out that this en suite fan has about 3 metres length of ducting as heads towards the outside grill so its much longer than the bathroom duct.
Ultimately i would just like to know what would be causing the bathroom fan to blow? As you can see the run for this fan is very short and how you see it in the picture is how i found it originally,.interestingly when i disconnected the duct from the fan i went back down to test the fan and was now sucking as it should do and holds up paper without issue like the en suite fan does, but for some reason when i reconnected the duct it starts blowing again.
Any help would be appreciated
Thanks.

 

[KenGMac]

stuartR1963, good evening.

One possible answer is that because the corrugated extract ducting is causing a high "resistance" to the air flow [the interaction of air flow against a "lumpy" serrated internal surface] there could be a blow back effect where because of air turbulence, caused by the "lumpy" serrations on the inside surface of the plastic duct you are getting what I can only imagine is a blow back.

If the above is indeed correct?? fitting a cheap rectangular section rigid duct over as much of the length of the duct run as possible will massively reduce the "resistance" caused by the flexible ducts many serrations.

Ken.

 

[stuartR1963]

Hi I did wonder about backflow replacing the ducting is going to prove difficult due to the tight space. I have noticed a slight dip in the duct near the end where it goes outside you may notice it, unfortunately i don't have access to the external grill and i don't have much play with ducting. Could the ducting be extended and pushed more over to the left? Btw the ensuite has the same flexible ducting if that helps.
Thanks

 

[flameport]

Is there a backdraught shutter on the fan outlet (where the duct connects?)
They can be jammed closed if the duct isn't positioned correctly.

 

[stuartR1963]

Is there a backdraught shutter on the fan outlet (where the duct connects?)
They can be jammed closed if the duct isn't positioned correctly.

Hi, no there is no shutter fitted.
I will try get some more pictures tomorrow
Thanks

 

[KenGMac]

no there is no shutter fitted.

Birds nest perhaps??

 

[conny]

Is this a new fitting?
Has it always blown instead of sucking?
Possible internal motor has been fitted incorrectly or wired up incorrectly internally. Both of these should have been checked for in manufacture process but the odd one can slip through.

 

[KenGMac]

I have seen on a few occasions large fans, serving on one occasion a block of flats that had somehow been wired incorrectly, the persistent complaints of smells in the Bathrooms had been on-going for some time, the "tissue test" proved the fans direction of flow was reversed.

Have also seen very large Air Conditioning units incorporating three phase motors where the motor direction had been reversed?

following on from conny above is it possible to reverse live and neutral on some electric motors to achieve the reverse flow??

 

[JohnW2]

following on from conny above is it possible to reverse live and neutral on some electric motors to achieve the reverse flow??

Not if it is just a simple motor with the two connections. With an AC supply, the only difference between neutral and line/live is that the neutral is connected to earth at the supplier's transformer/substation, and the fan motor would no nothing about that. All the fan will know is that it is being supplied by two wires which have 230V (or whatever) AC between them, the polarity of that voltage (i.e. which is positive and which is negative) reversing 100 times per second.

Kind Regards, John

 

[stuartR1963]

Hi, i have tested that theory by swapping the motors from the en suite fan to the bathroom and the motor from the bathroom is sucking (holding up tissue) as it should in the en suite, but in the bathroom however the motor from the en suite is also blowing, so the motor originally from the bathroom is not running in reverse.
Annoyingly i cannot get access to the external grill at the eaves but it can't be blocked as i poured a full litre of water and it came pouring out the external grill.
Could it be that because the run is so short that the motor is not capable of overcoming the incoming draft? It has been extremely windy lately.
The fan is around 10 years old. We don't really use the bathroom shower that much just the shower in the en suite, so i am not entirely sure if the fan always did that?
Thanks

 

[JohnW2]

Could it be that because the run is so short that the motor is not capable of overcoming the incoming draft? It has been extremely windy lately.

If that were the case, it would surely be an intermittent phenomenon, wouldn't it? (wind is not constant, nor always strong).

Also, if that were the case, the flow of air into the bathroom (demonstrated by the 'tissue test') would be even greater when the fan was switch off than when it was switched on, wouldn't it?

Kind Regards, John

 

[stuartR1963]

If that were the case, it would surely be an intermittent phenomenon, wouldn't it? (wind is not constant, nor always strong).

Also, if that were the case, the flow of air into the bathroom (demonstrated by the 'tissue test') would be even greater when the fan was switch off than when it was switched on, wouldn't it?

Kind Regards, John

I guess so, all i know is the fan is completely ineffective, i don't believe the motor is at fault because like i say i swapped it the ensuite and it worked as it should. If its not blocked then what can be causing it to behave that way? As you can see in the picture the run is reasonable straight and there are no significant traps that could be blocking the air flow, the back pressure must be come somewhere but where? Also why does the en suite fan not have this issue when it ducting is a good 2-3 meters longer?

 

[flameport]

but it can't be blocked as i poured a full litre of water and it came pouring out the external grill.

Water only needs a very small hole to flow - so it could easily be 90% blocked, water would still go through but air would not.

 

[JohnW2]

I guess so, all i know is the fan is completely ineffective, i don't believe the motor is at fault because like i say i swapped it the ensuite and it worked as it should. If its not blocked then what can be causing it to behave that way?

Good question. If there is not a 'blockage', there clearly must be some 'air flow problem'.

When you say that you swapped the motors, what exactly do you mean - did you swap it together with its 'blades'.

Has the bathroom door been open when you've done your 'tissue tests'? Whilst it should not, in itself, result in an actual reversal of air flow through the fan, if the room were poorly ventilated (e.g. if the door were closed and 'tight fitting'), that could certainly stop it sucking air out of the room - so if there was also a little wind trying to do the opposite, I suppose that might possibly 'win'?

Kind Regards, John

 

[stuartR1963]

Good question. If there is not a 'blockage', there clearly must be some 'air flow problem'.

When you say that you swapped the motors, what exactly do you mean - did you swap it together with its 'blades'.

Has the bathroom door been open when you've done your 'tissue tests'? Whilst it should not, in itself, result in an actual reversal of air flow through the fan, if the room were poorly ventilated (e.g. if the door were closed and 'tight fitting'), that could certainly stop it sucking air out of the room - so if there was also a little wind trying to do the opposite, I suppose that might possibly 'win'?

Kind Regards, John

Hi i just mean i took out the motor assembly this and put it in the en suite as that fan is exactly the same module.
Here a few other picture of the fan in the bathroom opened up you can see there is no shutter and you can see the electric connections.

Thanks