Do bathroom humidistat extractor fans work ok

[TanDiy]

Hi, just looking to have an extractor fan installed as part of bathroom refurb. Electrician advises fan at opposite end of bath from the shower, either through the wall or in the ceiling with vent via soffit under roof. He says that a straight fan on the light circuit will do the job and that there has to be an isolation switch outside of the room (in our case this can be sited in the airing cupboard?). I explained that I do not really want the fan coming on with the light and enquired about a humidistat fan that would come on and off as required when we showered. He advised that, in his opinion, these were not very good so I just wondered whether this is really the case - do humidistat fans work ok? If not then is it ok to just have the fan so that we can turn it on and off when we shower? At present we just open the window but this does not work well in the winter when the walls end up running with water. Thanks for your help

 

[bernardgreen]

My experience is they might work but not well enough.

What I have done is to fit a thermostatic switch on the hot water feed to the shower. Thus the fan comes when ever hot water is being used in the shower and after the shower has been turned off the fan continues to run until the pipe has cooled down. This over run is most likely to be long enough clear the water vapour from the room.

Remember that the air removed from the room by the fan has to be replaced so a vent in the door or a gap under the door is needed.

 

[EFLImpudence]

They work ok in the sense that they do what they say.

The problem is the world.
Look on the weather site and see the Relative Humidity today - here it is 94%.

You can just have an on/off switch if you want but from what you say about the walls it would appear that you have a more fundamental problem which the fan probably would not affect.

 

[TanDiy]

Thanks for the advice. Everything is fine in the warmer months - we open the window and there is barely any condensation. However, when it is cold in the winter, opening the window make little difference so I was hoping an extractor might help. Thanks again

 

[Taylortwocities]

94%! Do you live in the Eden Project or something?

RH today in lovely Oxfordshire is a mere 78%

 

[JohnW2]

94%! Do you live in the Eden Project or something?

Maybe it's foggy there - the RH can then presumably approach 100%?

Kind Regards, John

 

[EFLImpudence]

I was a bit surprised but that's what it says.
It's actually quite pleasant and not raining.

It does seem to be 82% nearby but even that is quite high for a fan setting.

 

[JohnW2]

Thanks for the advice. Everything is fine in the warmer months - we open the window and there is barely any condensation. However, when it is cold in the winter, opening the window make little difference so I was hoping an extractor might help. Thanks again

Opening a window in winter can be a mixed blessing - it depends what happens. If the warm moist air rushes out through the window, that's fine. However, if icy cold air rushes in to the bathroom, that can immediately result in water vapour in the bathroom condensing!

Kind Regards, John

 

[bernardgreen]

99.999% relative humidity is when the air is saturated with invisible water vapour. At 100% relative humidity microscopic droplets of water form and then vapourise again. A sudden drop in air temperature or pressure will reduce the amount of vapour required to saturate the air and the droplets form but do not vapourise and thus mist is formed.

The problem with low cost relative humidity sensors is they do not take account of air temperature and rely on water condensing on the sensor to determine "there is a lot of water vapour in the air" but not an accurate measurement.

 

[JohnW2]

99.999% relative humidity is when the air is saturated with invisible water vapour. At 100% relative humidity microscopic droplets of water form and then vapourise again. A sudden drop in air temperature or pressure will reduce the amount of vapour required to saturate the air and the droplets form but do not vapourise and thus mist is formed.

Indeed - as I said, that's what can happen if icy cold air rushes into a bathroom full of air saturated with water vapour when a window is opened in winter.

The problem with low cost relative humidity sensors is they do not take account of air temperature and rely on water condensing on the sensor to determine "there is a lot of water vapour in the air" but not an accurate measurement.

Indeed - but one assumes that the RH figures published on-line by met office, BBC etc., derive from somewhat more sophisticated measurement technologies than that

Kind Regards, John

 

[bernardgreen]

Indeed - but one assumes that the RH figures published on-line by met office, BBC etc., derive from somewhat more sophisticated measurement technologies than that

They use two thermometers, one wrapped in a wet cloth and a formula that takes the two temperatures to calculate the relative humidity.

 

[JohnW2]

They use two thermometers, one wrapped in a wet cloth and a formula that takes the two temperatures to calculate the relative humidity.

They certainly did when I was at school, about half a century ago, but that was a very crude and imprecise method ('wet cloths' are a bit difficult to standardise!) and I imagined that technology had probably advanced a bit since then! Given that it is possible, these days, to produce electrodes which detect (and quantify) a wide range of gases, I would have expected that something like that would now be available for water vapour.

Kind Regards, John

 

[bernardgreen]

Wet and dry bulb is still the most accurate way to measure relative.

There are sensors that can measure absolute humidity and combine this with the measured temperature and a look up table to obtain the relative humidity but ( I am told by the local amateur weather man ) the results are not the same as his trusted wet and dry set up. ( which uses new fangled PT100 sensors instead of mercury in a glass tube so he can read them remotely )

 

[JohnW2]

Wet and dry bulb is still the most accurate way to measure relative.

That rather amazes me. There are so many factors which are going to affect the cooling experienced by the wet bulb one that I would have doubted that it could ever be very accurate.

There are sensors that can measure absolute humidity and combine this with the measured temperature and a look up table to obtain the relative humidity ...

As I implied, that's where I assumed 'we were' in 2014!

Kind Regards, John

 

[flameport]

Electrician advises fan at opposite end of bath from the shower, either through the wall or in the ceiling with vent via soffit under roof.

Of those choices, get the one in the ceiling, but have the actual fan located in the roof space rather than fixed into the ceiling itself. Much quieter and far more powerful types can be fitted if required.

He says that a straight fan on the light circuit will do the job and that there has to be an isolation switch outside of the room (in our case this can be sited in the airing cupboard?).

That is the usual method, mainly to ensure the fan is actually used - separate switches tend to mean people never switch the thing on.

Humidity sensors can work, but are asking for trouble - some better types are adjustable but even they can still trigger when not wanted.
Other options are available such as the temperature arrangement suggested by bernardgreen, or PIR motion sensor or a water flow switch, but in the end the easiest, cheapest and therefore most commonly used option is to connect via the light switch.