Extractor fan advice - do humidistat fans work well?

[JohnW2]

I suspect anyone with any sense, will have gone off to make a cup of tea long ago

Whilst waiting to see if anyone has made and drunk their cup of tea and is prepared to contribute to this discussion, it has occurred to me that there is another way of looking at some of this issue which may help you to understand ...

.... we are agreed that, in the real world, there will always be some 'dilution' due to mixing of incoming ('replacement') cold air with the (heated) air within a house, the primary difference between us being in relation to the degree and importance/desirability of that dilution in various situations.

One has to realise that this dilution (due to mixing) cannot be selective - it applies as much to mositure/smells as it does to heat.

If (probably not practical, but convenient for discussion) the heated air were diluted by incoming cold air by, say, 50% , then, in any given period of extraction, one would only lose half as much heated air than would have been the case without dilution. However, in that situation, the moist/smelly air that one wanted to extract would also be diluted by 50% (by mixing with incoming non-moist/non-smelly air) by 50%- so, in any given period of extraction, one would only extract half of the amount of moist/smelly air than would have been the case without dilution.

If, with the dilution, one wants to get back to the amount of extraction of moisture/smells as would have been the case without dilution, then one has to either double the extractor flow rate or double the duration of extraction - either of which would also return the amount of heat loss to what it would have been without dilution.

"50% dilution" is obviously more than one is likely to see in practice, but the above argument applies for any degree of dilution.

Kind Regards, John

 

[Harry Bloomfield]

One has to realise that this dilution (due to mixing) cannot be selective - it applies as much to mositure/smells as it does to heat.

Not quite - Your extract's location is deliberately designed, to be as close to the source of moisture/smells a possible, so there will be minimal dilution of that air. Most of the dilution will be will be to the air after the smells have been sucked out. So concentration levels will gradually fall, as time progresses.

If (probably not practical, but convenient for discussion) the heated air were diluted by incoming cold air by, say, 50% , then, in any given period of extraction, one would only lose half as much heated air than would have been the case without dilution. However, in that situation, the moist/smelly air that one wanted to extract would also be diluted by 50% (by mixing with incoming non-moist/non-smelly air) by 50%- so, in any given period of extraction, one would only extract half of the amount of moist/smelly air than would have been the case without dilution.

The replacement air will enter from numerous points of air leakage, in the same room, from corridors other rooms, doors and windows. A normal home is a very leaky building indeed.

 

[opps]

My downstairs bathroom (in the extension) has an Icon 60 extractor fan (6"). If the bathroom door is slightly, but not fully closed, the fan will suck so much air out that it will fully close the door- yeah, I appreciate that it means that we don't have enough free airflow entering the room...

We have a very poorly insulated extension roof. Our previous 5" fan didn't cut the mustard. The ceiling was painted with waterbased eggshell but the condensation would pool on the ceiling and yellow in time.

I regret buying the PIR and humidity module for the machine. The humidity part I am happy with, the PIR is a PITA if you just want to have a quick pee. And don't get me started on the PIR lighting in the room... after a decent (read: hot) curry, you find yourself waving at it to turn the lights back on.

Great fan, but noisy.

 

[Harry Bloomfield]

My downstairs bathroom (in the extension) has an Icon 60 extractor fan (6"). If the bathroom door is slightly, but not fully closed, the fan will suck so much air out that it will fully close the door- yeah, I appreciate that it means that we don't have enough free airflow entering the room...

Yep, the air in source needs to be a little larger than the extract.

I regret buying the PIR and humidity module for the machine. The humidity part I am happy with, the PIR is a PITA if you just want to have a quick pee. And don't get me started on the PIR lighting in the room... after a decent (read: hot) curry, you find yourself waving at it to turn the lights back on.

Mine - built in PIR + humidistat works well enough. Though the humidistat could usefully be a be more sensitive. I have it's once triggered timer set to 20 minutes and it is over the bath/shower. It picks you up as soon as you attempt to walk in, but it misses you once in the bath, so it's a handy indication that I have spent enough time soaking in the bath -20 minutes and time to get out

On getting out, the fan retriggers for another 20 minutes, but I also put the window on it's vent position, and close the bathroom door behind me. That very successfully sucks all the moist air out. I then aim to go back in an hour or so, to close the window. I don't get a problem with mould in the bathroom, or in fact anywhere in the house, apart from a tiny bit appearing along the corner between roof and wall in the bathroom. A quick wipe bleach soaked cloth, clears that in the spring.

I also have a PIR as a replacement for a wall switch, in the utility room - so the light goes on as you walk in with both hands full. Last week, fixing the washer, the PIR couldn't see me hiding behind the washer, so it was turning the light off leaving me in the dark, so I linked the switch out. Since finishing the job, the PIR has constantly been switching the light on and off, even with no one present, so I have ordered a new one.

 

[JohnW2]

Not quite - Your extract's location is deliberately designed, to be as close to the source of moisture/smells a possible, so there will be minimal dilution of that air. Most of the dilution will be will be to the air after the smells have been sucked out. So concentration levels will gradually fall, as time progresses.

Exactly.

It sounds as if you have at last come to agree with me! As I have said repeated, despite your 'majoring' on the importance of (having) 'dilution', if one wants to extract moist or smelly air as rapidly and efficiently as possible, then one should do all one can to eliminate/minimise dilution - as you say by siting the extraction point as close as possible to the source of the moisture/smells (with the 'air inlet' points more remote from the extractor than the source of the moisture/smells).

If one does that (which is usually roughly the case with extractors near baths/showers and cooker hoods etc.) then one is right back to where I started - in that nearly all the extracted (moist or smelly) air will be ('undiluted') heated air,

As agreed, in practice there will often be at least some dilution (although very little for quite a long time if the extraction point is fairly remote from the main locations of air ingress into the building. The more dilution there is, the slower will be the extraction of moisture/smells, but, to achieve extraction of a certain proportion of the moisture/smells (50%, 75%, 'almost all;' or whatever) will be associated with extraction of the same amount heat, regardless of the amount of dilution.

The only difference dilution really makes is that the more dilution there is, the longer it takes to extract a certain amount of moisture/smells (and heat) - but if one leaves the extractor on for long enough to achieve the desired amount of moisture/smell extraction, the total heat loss will always be roughly the same.

Kind Regards, John

 

[Harry Bloomfield]

As agreed, in practice there will often be at least some dilution (although very little for quite a long time if the extraction point is fairly remote from the main locations of air ingress into the building. The more dilution there is, the slower will be the extraction of moisture/smells, but, to achieve extraction of a certain proportion of the moisture/smells (50%, 75%, 'almost all;' or whatever) will be associated with extraction of the same amount heat, regardless of the amount of dilution.

My point was that there will be a tremendous amount of dilution almost immediately, a fan is turned on, by air entering from so many points. Even without the fan running, there is massive amount of dilution of air within the home - how do you think cooking smells permeate the entire house? In my own case I can usually tell what next door is having for dinner, due to a sensitive nose and air leakage. How do you think my hydrometer, which constantly records, records the rise in humidity when I take a bath? Air moves constantly an dilutes very easily.

 

[Harry Bloomfield]

I've done some thinking Even with 100% humidity, water vapour is only a relatively small component of the mixture of gases we call 'air'.

You have to take into account the word 'Relative' - it's called 'relative humidity', relative to the maximum which the air can hold, taking into account the temperature of the air and it varies considerably with temperature.

 

[JohnW2]

My point was that there will be a tremendous amount of dilution almost immediately, a fan is turned on, by air entering from so many points.

You keep reversing your argument , given that you have just agreed that ...

.... Your extract's location is deliberately designed, to be as close to the source of moisture/smells a possible, so there will be minimal dilution of that air.

As I said, I agree with that second quote, and that 'minimal dilution' scenario is the most efficient (rapid) method of removing moisture or smells.

In any event, as I keep trying to get you to understand, the degree of dilution is essentially irrelevant to heat loss, for a given degree of extraction of moisture/smells. If you had, say, a degree of dilution which halved the rate at which heat was extracted, you would also have halved the rate of removal of the moisture/smells - so, to restore the 'full' extraction of moisture/smells, you would have to extract twice as much air (i.e. run the extractor for twice as long), thereby doubling the heat loss back to what it would have been without dilution.

Kind Regards, John

 

[JohnW2]

You have to take into account the word 'Relative' - it's called 'relative humidity', relative to the maximum which the air can hold, taking into account the temperature of the air and it varies considerably with temperature.

You're 'wriggling'

I've said absolutely nothing about relative humidity - throughout I've been talking the amount of moisture (water vapour) extracted, regardless of what the relative humidity may be.

Yes, if you extract so much heat that the temp inside the house falls substantially then the relative humidity (for a given amount of moisture in the air) will rise - but that is irrelevant to anything I've said.

In any event, you couldn't attempt to confuse matters in the same way in relation to 'smells', because we don't have a concept of 'relative smell' analogous to relative humidity!

Kind Regards, John.

 

[Harry Bloomfield]

You're 'wriggling'

I could say pot kettle

 

[JohnW2]

I could say pot kettle

Not at all. I have been totally consistent throughput in what I've been saying - it's you who seems to keep contradicting yourself and/or reversing your arguments.

There's actually another example of that it your previous message that I didn't bother to comment on ...

.... this whole exchange started because it was your view that using an extractor fan did not result in appreciably loss of heat, hence no appreciable reduction in indoor temp (in the absence of heating). If there were no appreciable fall in temperature, then the issue of relative humidity being temperature-dependent would obviously be irrelevant.

Kind Regards, John

 

[opps]

I also have a PIR as a replacement for a wall switch, in the utility room - so the light goes on as you walk in with both hands full. Last week, fixing the washer, the PIR couldn't see me hiding behind the washer, so it was turning the light off leaving me in the dark, so I linked the switch out. Since finishing the job, the PIR has constantly been switching the light on and off, even with no one present, so I have ordered a new one.

I have a stamp sized senor behind a wall tile in my upstairs bathroom. Wave your hand over the tile and the light comes on. Unfortunately I think the maker (Sensorbility) went bust

I once fitted a similar product for a customer (Tap Tile), they too seem to have gone bust.

 

[JohnW2]

..... It picks you up as soon as you attempt to walk in, but it misses you once in the bath, so it's a handy indication that I have spent enough time soaking in the bath -20 minutes and time to get out ..... I also have a PIR as a replacement for a wall switch, in the utility room - so the light goes on as you walk in with both hands full. Last week, fixing the washer, the PIR couldn't see me hiding behind the washer, so it was turning the light off leaving me in the dark, so I linked the switch out. Since finishing the job, the PIR has constantly been switching the light on and off, even with no one present, so I have ordered a new one.

One of the problems is that even those things sold as "occupancy sensors" are always (or nearly always) actually "moving occupant sensors" - and I've always thought that, with current-day technological possibilities, it ought not to be beyond the wit of man to invent something which truly was an "occupancy sensor" (even if the occupant was not moving (a number of theoretically possible approaches come to mind).

Kind Regards, John

 

[Harry Bloomfield]

(even if the occupant was not moving (a number of theoretically possible approaches come to mind).

Not cheap and cheerful solutions. The only cheap and cheerful solution is a warm body detector, but difficult to discriminate between a warm body, a radiator, or the sun through a window.

 

[JohnW2]

Not cheap and cheerful solutions. The only cheap and cheerful solution is a warm body detector, but difficult to discriminate between a warm body, a radiator, or the sun through a window.

I said nothing about cheap and/or cheerful, only about what the 'wit of man' ought to be able to achieve!

Any approach based on thermal/IR sensing would be subject to the sort of problems you mention. However, as one alternative, I think there might be more mileage to be had in RF (probably 'microwave') approaches, which could probably be persuaded to detect when the fields within the room were disturbed by the presence of the mass of a human body ... but that's just an initial thought 'off the top of my head'.

An ultrasonic or 'radar' system which had 'learned' what reflections were normal in the room when unoccupied might also be worth considering.

... and I'm sure there are plenty of other possibilities.

Kind Regards, John