Extractor fan advice - do humidistat fans work well?

[JohnW2]

10ml a minute?

I have no idea, but I rather doubt that. If I put a small drop of water (a small fraction of 1 mL) on a table, I reckon it would probably be 'hours' (certainly 'very many minutes') before it completely evaporated.

More to the point ... if I take your ratio figure of 1,600 then 10 mL/min evaporation would correspond to 16 L/min or 960 L/h of water vapour. However, given that air saturates at not much over 5% of water vapour, one would (with simplistic calculation) have to 'extract' something like 19,000 L/h of air in order to allow evaporation to occur at a rate of 10 mL/min even if the air was initially totally dry. If the air were already 'humid', one would need a far greater rate of extraction than that to allow 10 mL/min to evaporate.

Kind Regards, John

 

[Harry Bloomfield]

I have no idea, but I rather doubt that. If I put a small drop of water (a small fraction of 1 mL) on a table, I reckon it would probably be 'hours' (certainly 'very many minutes') before it completely evaporated.

Rate of evaporation depends on several factors - Temperature, air humidity and airflow, plus the surface itself. The warmer, the drier, the more air flow and irregular the surface - the more rapid it is.

When I put the car in the garage wet, I leave the big, front west facing roller door open 6" and open the back door. It is usually dry in an hour or two.

Rather than use the dryer to dry washing, I have washing lines along the utility, a wall mounted dehumidifier a fan, but little, if any heat. A room full of washing can be dried over night.

 

[JohnD]

I have no idea, but I rather doubt that. If I put a small drop of water (a small fraction of 1 mL) on a table, I reckon it would probably be 'hours' (certainly 'very many minutes') before it completely evaporated.

But in a bathroom after a shower, there are thousands of such drops, and a wet towel and mat.

 

[JohnW2]

Rate of evaporation depends on several factors - Temperature, air humidity and airflow, plus the surface itself. The warmer, the drier, the more air flow and irregular the surface - the more rapid it is.

Exactly. However, as I illustrated with my figures, the fact that a small volume of water turns into a large volume of water vapour, coupled with the fact that air saturates with water vapour very easily, means that one has to have a very high extraction rate (c.f. the 'air flow' in your garage and washing examples) in order to facilitate that evaporation, even if the air is dry - and, as you imply, if the air is already quite humid (as it would be, at least initially, in a bathroom), one needs even greater extraction rates.

Once the air has become saturated with water vapour (which will happen rapidly if evaporation of water is happening), evaporation will stop unless/until the 'saturated air' is extracted (or 'blown away', or whatever).

Kind Regards, John

 

[JohnW2]

But in a bathroom after a shower, there are thousands of such drops, and a wet towel and mat.

Indeed so, but see what I previously wrote, as summarised in what I have just posted in response to Harry. In essence, evaporation will stop happening very quickly unless the saturated air is removed ('extracted', blown away or whatever).

Kind Regards, John

 

[Harry Bloomfield]

But in a bathroom after a shower, there are thousands of such drops, and a wet towel and mat.

I get mostly dry, from a shower whilst still standing in the bath, use a facecloth first, then a towel for the finals, so only the bath gets wet, and most of that will drain down the plug hole.

 

[Harry Bloomfield]

Once the air has become saturated with water vapour (which will happen rapidly if evaporation of water is happening), evaporation will stop unless/until the 'saturated air' is extracted (or 'blown away', or whatever).

Yep, which is why I put the bathroom window on vent for an hour after a shower or bath. The fan, plus the window, plus natural convection, as it is upstairs and on the windward side - will quickly extract the moisture and enable any sitting moisture to evaporate.

 

[JohnW2]

Yep, which is why I put the bathroom window on vent for an hour after a shower or bath. The fan, plus the window, plus natural convection, as it is upstairs and on the windward side - will quickly extract the moisture and enable any sitting moisture to evaporate.

Indeed - and it would be even better if you implemented that ventilation whilst having the shower/bath, rather than 'after'. Whether you need the extractor fan at all if the room itself has adequate ventilation is, of course, a different question.

However, whether the ventilation is 'natural' (convection, wind etc.) or assisted by an extractor fan, all of the (heated) moist air which is removed from the room will have to be replaced (within the room or somewhere else in the house) with unheated air from outside - you can't get 'replacement air' from anywhere else

Kind Regards, John

 

[Harry Bloomfield]

However, whether the ventilation is 'natural' (convection, wind etc.) or assisted by an extractor fan, all of the (heated) moist air which is removed from the room will have to be replaced (within the room or somewhere else in the house) with unheated air from outside - you can't get 'replacement air' from anywhere else

Indeed, but there will be some element of dilution to that, so 100L of extracted air, will never contain 100% of the air you have paid to heat

 

[JohnW2]

Indeed, but there will be some element of dilution to that, so 100L of extracted air, will never contain 100% of the air you have paid to heat

You mean that some of the air extracted with be that which has just come in from outside through the window? If so, I agree, but that part of the 'extraction' will have served no useful purpose whatsoever.

The only relevant/useful part of the extraction will be that of moisture-laden are, which will inevitable be 'heated' air (unless one gets to the point at which the entire house has been filled with ('replacement') cold outside air!

There are only two places where extracted air can have come - from the (heated) air within the house or 'from outside'. As above, extraction of the latter achieves nothing, whereas extraction of the former inevitably results in cold outside air being drawn into the house.

Kind Regards, John

 

[Harry Bloomfield]

You mean that some of the air extracted with be that which has just come in from outside through the window? If so, I agree, but that part of the 'extraction' will have served no useful purpose whatsoever.

You are of course wrong, because of dilution of the air in the bathroom, moist air will be drawn out along with it. You seem to have an problem understanding that air does mix, and mix very easily.

 

[JohnW2]

You are of course wrong, because of dilution of the air in the bathroom, moist air will be drawn out along with it. You seem to have an problem understanding that air does mix, and mix very easily.

I'm afraid I still don't understand your argument.

The issue is not about what air is moist and which is not but, rather, about what air is heated and what air is not heated.

Start with the situation in which the bathroom (in practice, probably the whole house) is full of 'heated' air. It doesn't matter whether you say that some is moist and some is not or that (because of mixing) it all is moist (with an 'average' level of moisture, after the mixing) - the important point is that, moist or not, it is all heated air.

Now turn on the extractor fan. Since the only air in the room (probably in the house) is heated air (whether moist or not), all of the air extracted from the room is necessarily heated air - and that can only be replaced by' unheated' air entering the house from outside (whether entering into the bathroom or other places in the house).

Hence initially every X litres of air extracted will be 'heated air' and will necessarily be replaced by X litres of unheated air entering the house from outside.

Even subsequently, when that unheated 'replacement air' mixes with the heated air in the house (thereby reducing temp within the room/house) when that 'partially unheated' mixture is extracted, it will be replaced with 'totally unheated' air, thereby reducing the temp in the house even further. In the absence of a heating system (see below **) if the extractor continued to run that process would theoretically continue indefinitely until, eventually, the entirety of the house's heated air had been replaced with unheated air (i.e. inside temp became equal to outside temp).

[ ** I am, of course, ignoring the fact that there will usually be a heating system in operation, which will try (and probably succeed) prevent the fall in temp within the room/house - but that is the very 'wasted energy' which this whole discussion is about. In other words, what one would normally expect to see as a result of turning on an extractor would not be a fall in room/house temp but, rather, an increase in energy usage to maintain the desired temp ]

What is so difficult to understand about that?

Kind Regards, John

 

[Harry Bloomfield]

I'm afraid I still don't understand your argument.

Oh dear - this is hard work..

Start with the situation in which the bathroom (in practice, probably the whole house) is full of 'heated' air. It doesn't matter whether you say that some is moist and some is not or that (because of mixing) it all is moist (with an 'average' level of moisture, after the mixing) - the important point is that, moist or not, it is all heated air.

Now turn on the extractor fan. Since the only air in the room (probably in the house) is heated air (whether moist or not), all of the air extracted from the room is necessarily heated air - and that can only be replaced by' unheated' air entering the house from outside (whether entering into the bathroom or other places in the house).

We do both agree that 100L of extracted air, will be replaced with 100L of outside air, but the bit you are missing is that very quickly the warm air will be diluted by the incoming air - so not all of the extracted air will include the heated air, the heated air extracted, will be diluted by the incoming fresh air to quite a large extent. A house is not a sealed environment, with only one ingress and egress point for air, air comes in through leaks in brickwork, windows and doors.

Now do you understand?

 

[JohnW2]

Oh dear - this is hard work..

It is, from both our perspectives

We do both agree that 100L of extracted air, will be replaced with 100L of outside air ...

Glad you agree. So you presumably must also agree that there will initially (when all of the extracted air is heated) be a loss of heat in the house (hence fall in temp in at least some parts of the house) ?

, but the bit you are missing is that very quickly the warm air will be diluted by the incoming air - so not all of the extracted air will include the heated air ...

Yes, eventually the amount of heat lost in each 100 L of extracted air will become less - but only because the air inside the house has already got colder (due to mixing with incoming air). However, unless/until one reaches the theoretical end-point of the temp of the air in the house being the same as the outside temp, the extracted air (a mixture of heated and unheated air) will always still be warmer than the replacement air, so there will continue to be a net loss of heat (until one reaches that theoretical point when inside and outside temps are equal)

, the heated air extracted, will be diluted by the incoming fresh air to quite a large extent. A house is not a sealed environment, with only one ingress and egress point for air, air comes in through leaks in brickwork, windows and doors.

In the most common real-world situation, there is relatively little opportunity for air ingress from the outside world in the bathroom itself (usually no explicit ventilator, so usually only leaks around a window), the main points of ingress into the house being relatively 'remote' (explicit ventilators, letterboxes, leaks around doors/windows etc.), quite possibly with closed doors in between.

In such a situation (and particularly if there are 'closed doors') the initial mixing of incoming air with heated air (*hence falls in temp) will primarily be in 'remote' parts of the house, and it could well be quite a long time before significant amounts of that 'mixed' (reduced temp) air get anywhere near the bathroom.

Hence, in such a case, something approaching the 'initial situation' (in which all air being extracted is 'heated air') is likely to persist for quite a long time - i.e. the temp in the bathroom will remain fairly high, whilst that in other parts of the house (closer to the points of air ingress) will become progressively colder, as a result of (largely 'heated') air extracted from the bathroom having been replaced, in other parts of the house, by cold air.

Now do you understand?

I always have understood my view of the situation

Kind Regards, John

 

[Harry Bloomfield]

Glad you agree. So you presumably must also agree that there will initially (when all of the extracted air is heated) be a loss of heat in the house (hence fall in temp in at least some parts of the house) ?

Yes,

Yes, eventually the amount of heat lost in each 100 L of extracted air will become less - but only because the air inside the house has already got colder (due to mixing with incoming air). However, unless/until one reaches the theoretical end-point of the temp of the air in the house being the same as the outside temp, the extracted air (a mixture of heated and unheated air) will always still be warmer than the replacement air, so there will continue to be a net loss of heat (until one reaches that theoretical point when inside and outside temps are equal)

So we agree on the principle of dilution.

Hence, in such a case, something approaching the 'initial situation' (in which all air being extracted is 'heated air') is likely to persist for quite a long time - i.e. the temp in the bathroom will remain fairly high, whilst that in other parts of the house (closer to the points of air ingress) will become progressively colder, as a result of (largely 'heated') air extracted from the bathroom having been replaced, in other parts of the house, by cold air.

With that I do not agree. First of all - the heating system will more than easily make up for any loss and secondly dilution is a very major factor, in not extracting as much heat as you would seem to assume. I would not expect any noticeable difference in the cost of running my heating system, or the indoor temperature whether my bathroom fan ran continuously or not. However, leave a outside door open for longer than a few minutes and I certainly notice that no matter where in the house I am and if near the boiler, I hear it working overtime to cope.

Extract fans are relatively tiny, compared to the air volume in a building, which is why the need to be located close to what ever they are intended to extract.

I always have understood my view of the situation