Extractor fan advice - do humidistat fans work well?

[JohnW2]

It's quite normal for houses to be ventilated.

It obviously is (and necessarily to some extent) - and, indeed, it's necessary for an extractor fan to work as desired.

However, as I said, one cannot get away from the fact that every litre of air pumped out of the house by an extractor has to be replaced by a litre of air entering the house (somewhere) from outside.

One could theoretically restrict the 'replacement of of heated air with outside air' to just the bathroom/kitchen. if one 'sealed' that room from the rest of the house and provided that room with adequate ventilation (to the outside world). However, certainly in the case of bathrooms, there is commonly little or no ventilation from the room, the air replacing that extracted coming 'under the door' from the rest of the house (unltimates from the outside world, through some 'ventilation' somewhere in the house).

It's really just "Physics 101", and I don't really understand why Harry is resting it.

Kind Regards, John

 

[JohnW2]

I didn't say we did, rather I think we had to agree to disagree.

One could put it like that. We certainly had no choice (since it was apparent!) that we "agreed that we were disagreeing"

The only way your theory would work, would be if the house were an entirely sealed tunnel, only openable at the ends, with the fan the diameter of the tunnel. Then has much fresh cold air would be drawn in at the far end, as is extracted warm air. Other than that very limited scenario, dilution of cold air with warm air becomes a major factor, so not as great a heat loss as you suggest.

As I've just written to JohnD, what I've said is not "theory" but, rather, fact. As I've said, one simply cannot get away from the fact that if one pumps X litres of ('heated') air out of the house, it must be replaced by X litres of (potentially very cold) outside air entering the house somewhere 9through 'ventilation', even if 'accidental') - and that remains the case regardless of the layout of the house and the location of the ventilation relative to the fan.

As I also recently wrote, the best one can do is to restrict the replacement of heated air with cold outside air to one part of the house (e.g. just the bathroom), by having local ventilation (not common in bathrooms) and sealing the room from the rest of the house (obviously not usually done). In that situation, the temp of the bathroom air would fairly rapidly fall until it was at the same temp of the outside air, but (since the room was 'sealed') the rest of the house would not be affected.

There is one way that one could largely avoid the heat loss, but it would be silly, because it would render the fan essentially useless. As above, "X litres out, X litres in" is inevitable, and I have been assuming that the "X litres out" are X litres of heated (and 'water-laden) air (which is what one wants to 'extract'). If one located a substantial ventilator very close to the extraction fan, then most of the air being pumped out of the room/house would be the ('cold') air drawn in from the outside world, leaving the heated (and 'water-laden') air within the room largely unaffected. In other words, the extractor would merely be 'recirculating' cold outside air, from the outside and then immediately back to the outside.

However, since the whole point of the fan is to remove the 'water-laden' (heated) air, such a situation would mean that the extractor would not be achieving anything! Put another way, if one wants to extract heated water-laden air from the room, then like it or not, one has to extract heated water-laden air from the room - and the only thing it can be replaced by is cold air which has entered the house from outside.

To what extent that heat loss affects different parts of the house will obviously depend upon the relative locations of the fan and the ventilation - as I've said, it could theoretically be restricted to just one room or, at the other extreme (if the ventilation was very distant from the extractor) might affect most of the house. Is that perhaps what you mean by 'dilution'?

Kind Regards, John

 

[JohnD]

Bear in mind that water vapour occupies an enormous volume

So a lot of the extraction will not be sucking in the same amount of air

 

[Harry Bloomfield]

It's really just "Physics 101", and I don't really understand why Harry is resting it.

'Resisting it'??? I am not, I am simply pointing out the obvious, that some of the air being extracted, a proportion, will not have been heated up at all- so 100L extracted does not mean 100L of heated air extracted. A home is not a sealed environment, nor is it a wind tunnel, so a large proportion of the warm extracted air will have been diluted by the incoming cooler air.

Allow me try to explain it a different way. A mug full of tea, the actual tea colouring is the warmth in your home, the mug your home. If you put the mug under the tap and slowly run water into it, you have to have an awful lot of water run into the mug, many times the volume of the mug, over spilling, before the tea colour is completely washed out of the mug contents. The water going in, the equivalent of your extract fan. It works so poorly, because of dilution of the tea in the mug 100% of the water entering, does not mean 100% of the tea flowing out, because of the effects of dilution. Dilution being the principle I have been struggling to get across to you.

The more sensible way is obviously to pour the tea out of the mug then rinse, using a much lower volume of water to achieve the same effect.

I don't understand why you are resisting that obvious fact.

 

[JohnW2]

Bear in mind that water vapour occupies an enormous volume .... So a lot of the extraction will not be sucking in the same amount of air

I'm not sure I get that - but I guess I need to do bit of thinking about vapour densities etc.

Does the 'volume in" not have to equal to "volume out", regardless of what it consists of?

Kind Regards, John

 

[JohnW2]

'Resisting it'??? I am not, I am simply pointing out the obvious, that some of the air being extracted, a proportion, will not have been heated up at all- so 100L extracted does not mean 100L of heated air extracted.

I have, at least implicitly (and with an extreme example, as repeated below) acknowledged that.

However, extraction of any air which has not been heated is essentially wasted effort on the part of the fan - what one wants and needs to extract is the heated air (which is what contains the water vapour) - and, as the house as a whole is concerned, that can only be replaced (within the house as a whole) by cold air from outside.

As I said in my previous post, one could contrive a situation in which virtually all of the extracted air is unheated, by siting a large ventilator close to the extractor - but the extractor will then achieve very little, because it will not extract a significant amount of the (heated and moist) air that one wants/needs to extract.

In essence, one can't have it both ways. If one wants to extract the moist heated air, then it will be replaced by cold air. If one wants to avoid the extraction of heated air, then one won't extract the moist heated air which one wants/needs to extract.

Kind Regards, John

 

[Harry Bloomfield]

As I said in my previous post, one could contrive a situation in which virtually all of the extracted air is unheated, by siting a large ventilator close to the extractor - but the extractor will then achieve very little, because it will not extract a significant amount of the (heated and moist) air that one wants/needs to extract.

That would very much depend up where in the room the intake and extract are located. If at opposite ends/corners of the the room, that is as good as it gets so far as efficient extraction goes. In the more normal environment of a leaky home, intake air will be via numerous sources, most not far from the extract and so warm air dilution will be quite minimal.

 

[JohnW2]

That would very much depend up where in the room the intake and extract are located.

Exactly - that's what I said. In my 'extreme' (contrived) example (which you quoted) the 'intake' would be located very close to the extractor. The extractor would then achieve virtually nothing, merely endlessly sending outside air in circles through the wall in both directions.

If at opposite ends/corners of the the room, that is as good as it gets so far as efficient extraction goes. In the more normal environment of a leaky home, intake air will be via numerous sources, most not far from the extract and so warm air dilution will be quite minimal.

In the most common situation, air 'intake' into the bathroom will be 'under the door' from other parts of the house. When the extractor is first turned on, the air extracted will obviously be 'heated' air, since that is all that will then be present in the room. That will be replaced by incoming air 'under the door' which (if the whole house is heated) will also be heated air, so the extractor will continue to extract heated air (now a mixture of the heated air in the bathroom and heated air from other parts of the house).

That heated air which has gone under the door from other parts of the house into the bathroom has to be replaced, and that has to be replaced with unheated air from outside of the house, through explicit 'ventilators of just 'leaks'. The extractor will continue to extract heated air (much of it now coming under the door from the rest of the house) but, if it remains on for long enough, some of the air coming under the door will eventually be air from the rest of the house which has been mixed with cold air entering the house to replace the extracted air.

So, in terms of the bathroom itself, you're right - for quite a long time there will be little/no dilution of its warm air, since the 'replacement air' coming under the door will be heated air from other parts of the house. However, what will inevitably have happened is that that warm air which has moved from the rest of the house into the bathroom will have been replaced (through 'ventilation') with cold air for outside, therefore (possibly considerably) 'diluting' the warm air in the rest of the house with that cold air which has necessarily entered.

Kind Regards, John

 

[JohnD]

I'm not sure I get that - but I guess I need to do bit of thinking about vapour densities etc.

Does the 'volume in" not have to equal to "volume out", regardless of what it consists of?

Kind Regards, John

While you are extracting, and reducing the humidity in the room, the towels and tiles will be drying out. The dryer the air, the faster they will lose water as vapour.

I will see if I can look up the volume of water vapour than 1ml of water turns into (it is a lot)

 

[JohnD]

About 1600

Some of the online answers are for steam at 100C which is about 1700

And water vapour is lighter than air, so if the door and window are shut, it will tend to rise towards the ceiling (and the extractor) with the dry air, entering under the door, stratifying beneath it. Which is ideal.

 

[JohnW2]

Bear in mind that water vapour occupies an enormous volume So a lot of the extraction will not be sucking in the same amount of air

I'm not sure I get that - but I guess I need to do bit of thinking about vapour densities etc. Does the 'volume in" not have to equal to "volume out", regardless of what it consists of?

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'.

Totally dry air has a composition of about 78% nitrogen, 21% oxygen and 1% 'the rest' (primarily argon, but also other 'trace' components such as CO2). Even with 100% humidity at 'high' temperature (e.g. 35°C) water vapour only amounts for about 7%, and I think one would rarely see anything above 5%, if that, within a UK house.

I therefore don't think that the presence of water vapour really makes any appreciable difference to what has been discussed. Indeed, the water vapour will be at essentially the same temp as the other components (and so the temp of the 'air' as a whole). so I don't think will change anything significantly.

Kind Regards, John

 

[JohnD]

I wonder how many ml of water will evaporate off the tray, tiles and towels after a shower.

More than that many in litres of water vapour.

 

[JohnW2]

I will see if I can look up the volume of water vapour than 1ml of water turns into (it is a lot)

About 1600

Yes, the ability of air to contain water (as vapour) is very limited. As I've just written, even totally saturated air contains only about 7% water vapour, and that's unlikely to be seen within a house. However, as I've just written, the ('heated') water vapour is just a fairly minor one of the components of the 'air' we are discussing,

And water vapour is lighter than air, so if the door and window are shut, it will tend to rise towards the ceiling (and the extractor) with the dry air, entering under the door, stratifying beneath it. Which is ideal.

All true. However, as I've said, when that water-containing air is extracted, it will be replaced (in the house as a whole) with cold air drawn from the outside world.

In passing .... as for ('lighter than air') water vapour 'tending to rise', that will undoubtedly be true to some extent, but gas mixtures seem to remain fairly 'well mixed' (due to 'diffusion', perhaps?) - nitrogen is about 14% lighter than oxygen, but it does not all 'rise to the top of the atmosphere', since it's present at about 78% at ground level.

Kind Regards, John

 

[JohnW2]

I wonder how many ml of water will evaporate off the tray, tiles and towels after a shower.

As you know, it's a pretty slow process, particularly at lower temperatures - because, as I've said, the ability of air to hold water vapour (before becoming 'saturated') is pretty limited.

Kind Regards, John

 

[JohnD]

10ml a minute?