General: Energy efficient strategies for manual ventilation

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- CommentAuthorlineweight
- CommentTimeApr 16th 2022
I saw this in a list of suggested energy-saving measures from an electricity supplier:

"When you ventilate your apartment by opening windows, you should do it for a short time with a transverse draft, so that the air is replaced quickly without unnecessary heat leaking out."

Obviously, this is advice for "manually" ventilated buildings without any heat recovery systems.

I'm not sure that makes sense. It seems to be suggesting you should rapidly dump a large volume of air outside.

Firstly, isn't the volume of air you shift more important than the speed at which you remove it?

Secondly, my understanding is that ventilation is mainly aimed at reducing levels of CO2 and humidity, and that to some extent, these things can diffuse without the air actually "moving", as such.

Is it not better to strategically open some windows a little, for example, opening a window near the source of humidity, than to open windows on each side of the room?

Of course, in summer it's a different matter because you are not worried about losing heat when you ventilate, so then it does make sense to get a transverse draft going.
- CommentAuthorEd Davies
- CommentTimeApr 16th 2022 edited
Hmm, this sounds like an interesting strategy. At first glance it seems to make some sense:

If you replace the whole volume of air in one swell foop then you immediately get the COâ‚‚ and (absolute or specific) humidity down to the level of the outside air. If you slowly transfer the same volume of air through the building then, on average, you replace half the air so halve the difference between the indoor and outdoor air-quality attributes.

Letting a gale blow through for a few minutes will probably replace a lot more than the volume, of course, but since much of the air leaving will be outdoor air that's just arrived and hasn't had time to warm up there's no extra energy lost.

The actual amount of energy in the air is not great, its specific heat capacity is similar to most other materials except for water but it's not very dense so its volumetric heat capacity is low (hence the difficulties with heating with warm air unless there's a lot blowing around or the heating requirement is very low). More significant is the heat stored in the fabric of the building, furniture, etc. Doing a one-off quick air change ought to cool those less.
- CommentAuthorlineweight
- CommentTimeApr 16th 2022
Is my understanding that CO2 and humidity can diffuse somewhat, without the volume of air moving, wrong?

Some time ago I did some experiments with a CO2 monitor in my flat and found that the CO2 level could be reduced quite substantially even with a single window just a little bit open.

I didn't have any way of measuring how much air moved in or out though.
- CommentAuthordjh
- CommentTimeApr 16th 2022 edited
Posted By: lineweightIs my understanding that CO2 and humidity can diffuse somewhat, without the volume of air moving, wrong?
No, that's correct. It's worth remembering that humidity and to some extent CO2 are just easy to measure proxies for other things (VOCs etc) that need to be cleared too.

I think Ed's right that a wholesale exchange of air in a short period is likely to lose less heat than a slow exchange over a period of time. At worst it will be the same as the alternative.
- CommentAuthorbhommels
- CommentTimeApr 16th 2022 edited
Posted By: djh
I think Ed's right that a wholesale exchange of air in a short period is likely to lose less heat than a slow exchange over a period of time. At worst it will be the same as the alternative.

What the fell swoop avoids wrt a trickle ventilation approach is local cold spots due to continuous cold air ingress, leading to condensation and potentially mould growth.

At normal ventilation rates, diffusion wins the race over the distances at play: when humans, or other CO2 producers, are sitting in a corner of an unventilated room (which I believe is your question), the air in the whole room will be at the same CO2 concentration unless you are talking very large halls.

BTW: CO2 is a more reliable proxy for VOCs than humidity, since the outdoor concentration of CO2 and VOCs are both constant, contrary to humidity. Not saying that humidity should be ignored for indoor ventilation controls, mind.
- CommentAuthordjh
- CommentTimeApr 16th 2022
Posted By: bhommelsBTW: CO2 is a more reliable proxy for VOCs than humidity, since the outdoor concentration of CO2 and VOCs are both constant, contrary to humidity. Not saying that humidity should be ignored for indoor ventilation controls, mind.
Yes, agreed. Humidity is cheaper to measure so gets used more. What I meant originally was that CO2 is a useful proxy but is also useful to control for its own sake. Humidity tends to be pretty obvious to our bodies when there's too much or too little of it.High CO2 isn't immediately obvious at levels that can cause problems.
- CommentAuthorlineweight
- CommentTimeApr 16th 2022
Posted By: bhommels
At normal ventilation rates, diffusion wins the race over the distances at play.

Can you recommend any reading / links where this is quantified in any way? It's something I've wanted to understand better but have never found any great information on it, especially as applied to building ventilation.
- CommentAuthorbhommels
- CommentTimeApr 16th 2022
Posted By: lineweight
Posted By: bhommels
At normal ventilation rates, diffusion wins the race over the distances at play.

Can you recommend any reading / links where this is quantified in any way? It's something I've wanted to understand better but have never found any great information on it, especially as applied to building ventilation.

When looking at CO2 and building ventilation, you can safely ignore diffusion and assume that gases are perfectly mixed, at least at room level. If you want to know more about the physics behind it and which laws apply, Wikipedia is hard to beat:
https://en.wikipedia.org/wiki/Diffusion
and for the case of normal diffusion of CO2 in air, FIck's law applies:
https://en.wikipedia.org/wiki/Fick%27s_laws_of_diffusion
This & googling the diffusion rates, should give you the handles to calculate simple cases.

For humidity in buildings, it gets more complicated very quickly!
- CommentAuthortony
- CommentTimeApr 17th 2022
I donâ€™t particularly like using the term energy efficiency for this, I am gradually working on HMG and wearing them down on this. In energy terms it wastes heat to do purge ventilation but it should be done as a short sharp shock to minimise the amount of energy lost. I call it energy use reduction.

Old wisdom was to open windows in the morning in winter to change the air, 20 mins will do. If you can get cross ventilation then the length of time depends on how windy it is, a few seconds in a gale! A minute if it is very windy, longer if it is only blustery, if calm then 30mins.

One full air change will do.

To more than that increases energy use unnecessarily. Collect condensation from window glass before opening windows. Minimise the time open. You only want to change the air not cool down the fabric, there is not a lot of heat stored in the air and the new air will soon warm up only slightly cooling the fabric.
- CommentAuthorlineweight
- CommentTimeApr 17th 2022
Posted By: bhommels
Posted By: lineweight
Posted By: bhommels
At normal ventilation rates, diffusion wins the race over the distances at play.

Can you recommend any reading / links where this is quantified in any way? It's something I've wanted to understand better but have never found any great information on it, especially as applied to building ventilation.

When looking at CO2 and building ventilation, you can safely ignore diffusion and assume that gases are perfectly mixed, at least at room level. If you want to know more about the physics behind it and which laws apply, Wikipedia is hard to beat:
https://en.wikipedia.org/wiki/Diffusion
and for the case of normal diffusion of CO2 in air, FIck's law applies:
https://en.wikipedia.org/wiki/Fick%27s_laws_of_diffusion
This & googling the diffusion rates, should give you the handles to calculate simple cases.

For humidity in buildings, it gets more complicated very quickly!

What I'd like to understand is not so much what happens within the building but what happens when the air inside a building is connected to the outdoor air via an open window.
Basically, how long it takes for the levels inside to equalise with the levels outside, and how this compares with the amount of air that gets exchanged during the same time period and same opening size.
Because if it's the case that CO2 (and other gases) and humidity equalise with the outdoor air through diffusion, before the entire air volume gets exchanged, then isn't that an argument against purge ventilation.
- CommentAuthorWillInAberdeen
- CommentTimeApr 17th 2022 edited
Here's a visualisation:
-create a smell in a room (ideally a nice smell of coffee in the kitchen, but could be a bad smell in the bathroom!)
- time how long it takes to permeate into the next room, and to dissipate
- do it again next day with the windows open a crack
- do it again with the windows wide open and the extractor fan running.

Chances are that bulk air movement makes a big difference to removal of smells (and therefore CO2, humidity, etc), so it's not all about diffusion (which is independent of air movements).

Most movement of gasses within and out of rooms is actually by convection (bulk air flow and mixing) - diffusion is over rated. Diffusion is more important on millimetre scales. Same with air's thermal conductivity, and its viscosity.

Here's another visualisation: you have a sink full of dirty water. You want it full of clean water. You can
a) drain out the dirty water and refill with clean water
b) dribble clean water into the sink and let it overflow, until the sink is full of clean water.

a) will use much less water than b) to get the same result, because the dirt is removed in concentrated form rather than in progressively more diluted form.
Purge ventilation needs fewer air changes than trickle ventilation. In chemistry this effect is called "plug flow".

Posted by Tony: "there is not a lot of heat stored in the air and the new air will soon warm up only slightly cooling the fabric."
This is the key to it. Completely replacing the air in a house will only remove a small fraction (~0.01kWh/mÂ²) of the heat in the house, but will remove all of the excess CO2 and a fair amount of the excess humidity.

Posted by Tony:"open windows in the morning"
This is the other key to it. In the morning the outside air is warmer than if you had ventilated overnight, so you lose less heat, and you are active and tolerant of cooler conditions than you will be in the evening.
- CommentAuthorlineweight
- CommentTimeApr 17th 2022 edited
It's not really analagous to dishwater though, because dishwater is made dirty in a short period of time as an isolated event. If you completely replace it after doing the dishes it stays like that until the next time someone does the washing up.

Air inside a building is being continually "polluted" so it only achieves that "clean" state momentarily. Then it's only a matter of time until the next purge is required, presumably at some kind of threshold of acceptable levels... Which would follow a kind of sawtooth pattern.
- CommentAuthorWillInAberdeen
- CommentTimeApr 17th 2022
That's right. Mathematically, the purge is a step function and you can add those up to make any pattern you like, but sawtooth is easy to visualise.

Practically, let's imagine a busy 24h restaurant with a constant rate of dirty dishes arriving back in the kitchen to be washed. You start your shift with a sink of clean water, it gets steadily dirtier, at some point you drain the water and start again with fresh, so the pollution follows a sawtooth profile. If there are more/dirtier dishes at some times of day than at other times, then it's a kind of wonky sawtooth profile.

You could decide to change the water only when it gets unacceptably dirty, or you could decide to do it after a fixed time interval, which would be less efficient but you might have other reasons to prefer that.

Traditionally people purge-ventilate houses on a time interval, once a day each morning, for reasons mentioned above. They have enough air leakage to control CO2 the rest of the day, and they can afford to let the humidity buffer up over 24h in most rooms. The humidity buffering 'blunts' the sawtooth.

Very airtight modern houses might get stuffy before that, but probably not a good idea to start an automatic purge cycle at 3am! But those airtight houses might have MHRV which changes the game, because the capital cost is too great to make the MHRV big enough to do purge ventilation, and you are less worried about heat losses anyway.
- CommentAuthordjh
- CommentTimeApr 18th 2022
Posted By: WillInAberdeenbecause the capital cost is too great to make the MHRV big enough to do purge ventilation
Well, it's not just the capital cost of MVHR. People like opening windows and doors for other reasons, and are required to have some of them by law. So opening doors and windows is always available as a means of purge ventilation and there's no sense in compromising the design of the MVHR to deal with it. But an MVHR deals with most situations through its normal continuous ventilation, so there's hardly ever a need for purge ventilation anyway. Certainly not on a daily or weekly or perhaps even monthly basis; it's only required for emergencies.
- CommentAuthorMike1
- CommentTimeApr 18th 2022
Posted By: tonyIn energy terms it wastes heat to do purge ventilation but it should be done as a short sharp shock to minimise the amount of energy lost.
Yes, 'shock ventilation' is the correct solution (in the absence of MVHR). Cracking windows open for extended periods of time is a recognised impediment to building energy efficiency in Germany, as summarised in this article:
https://www.politico.eu/article/germanys-energy-efficiency-open-windows-ventilation/
- CommentAuthorPlHadfield
- CommentTimeApr 18th 2022
Posted By: lineweightBasically, how long it takes for the levels inside to equalise with the levels outside, and how this compares with the amount of air that gets exchanged during the same time period and same opening size.

Not entirely relevant to manual, whole-building ventilation but some (to me) interesting graphs and explanation of the physics of vapour pressures are in an ASHRAE paper on crawl space ventilation, from the AECB knowledgebase website : "Crawlspace Moisture Control - A Fundamental Misunderstanding" at https://aecb.net/knowledgebase-archive/
- CommentAuthorSimonD
- CommentTimeApr 19th 2022 edited
Posted By: PlHadfield
Posted By: lineweightBasically, how long it takes for the levels inside to equalise with the levels outside, and how this compares with the amount of air that gets exchanged during the same time period and same opening size.

Not entirely relevant to manual, whole-building ventilation but some (to me) interesting graphs and explanation of the physics of vapour pressures are in an ASHRAE paper on crawl space ventilation, from the AECB knowledgebase website : "Crawlspace Moisture Control - A Fundamental Misunderstanding" at https://aecb.net/knowledgebase-archive/

Any chance of a high level summary of key points as I'm not a member of AECB and login appears to be required to access the paper.

Ta
- CommentAuthorlineweight
- CommentTimeApr 19th 2022
Posted By: SimonD
Posted By: PlHadfield
Posted By: lineweightBasically, how long it takes for the levels inside to equalise with the levels outside, and how this compares with the amount of air that gets exchanged during the same time period and same opening size.

Not entirely relevant to manual, whole-building ventilation but some (to me) interesting graphs and explanation of the physics of vapour pressures are in an ASHRAE paper on crawl space ventilation, from the AECB knowledgebase website : "Crawlspace Moisture Control - A Fundamental Misunderstanding" at https://aecb.net/knowledgebase-archive/

Any chance of a high level summary of key points as I'm not a member of AECB and login appears to be required to access the paper.

Tahttp:///newforum/extensions/Vanillacons/smilies/standard/smile.gif" alt="" title="" >

Same here!
- CommentAuthorlineweight
- CommentTimeApr 20th 2022 edited
Posted By: WillInAberdeen

Traditionally people purge-ventilate houses on a time interval, once a day each morning, for reasons mentioned above. They have enough air leakage to control CO2 the rest of the day, and they can afford to let the humidity buffer up over 24h in most rooms. The humidity buffering 'blunts' the sawtooth.

Here is my attempt to visualise different approaches in the "traditional" scenario - that is, a leaky building, with no MHVR or suchlike, where that background leakage goes some way to equalising humidity & CO2 whether you like it or not.

That background leakage goes some of the way but needs to be supplemented by some form of deliberate leakage, either in the form of periodic purges or in the form of a carefully chosen amount of "trickle".

In the "purge ventilation" scenario then under normal use, the CO2 & humidity will climb at a steady rate determined by the amount of background leakage. The shaded blocks of time are supposed to donate some kind of event such as cooking, which increases that rate temporarily. The occupants open the windows for a purge once they perceive the air to be stuffy.

The blue line represents the heating energy needed, beyond whatever would be necessary to keep things at a steady state with all the windows closed. So it's at zero for much of the time but there is a spike each time all the air is removed.

In the "trickle" scenario, there is some deliberate additional leakage, set to happen at a rate that means CO2 & humidity can always return to a low level, within a typical use cycle period. The energy input (over baseline) is constant, but at a low level.

These graphs are massively over simplified and have no quantities. But that's why I'd be interested to see some kind of "real" version of this. Because, let's say the area under the blue line is similar in each scenario - then one of my questions would be whether a disadvantage of the "purge" strategy is that the occupants spend a greater amount of time in higher levels of C02 and humidity.

Maybe it can be demonstrated that the area under the blue line is going to be significantly more in the "trickle" scenario.

I realise that occupant behaviour has to be considered also. Maybe the windows would get frequently opened anyway in the "trickle" version. But then, maybe they would get opened more than necessary in the "purge" version too.
- CommentAuthortony
- CommentTimeApr 20th 2022
Trickle ventilation varies greatly depending on wind speed.
- CommentAuthordjh
- CommentTimeApr 20th 2022
As does open window purge ventilation
- CommentAuthorWillInAberdeen
- CommentTimeApr 20th 2022 edited
Well, isn't the idea that if you consciously open a window, you open it wider if it's calm than if it's windy? I do.

Nice pics! Key things for me are:

- the top one shows the ventilation happening when the pollutants are at their most concentrated, that's how less volume of air is required to shift the same amount of pollution. Compared to the bottom pic where the pollutants are ~3x more dilute on average when the ventilation is happening, so ~3x more ventilation is required. (Edit: The area under the blue line)

- the purge could be done immediately after the cooking if desired, (or during!) which would reduce the time spent at high humidity. Or could be done only in the daytime to reduce the amount of heat loss.

- the trickle ventilation rate could be reduced by half without letting the air get stuffy during cooking.
- CommentAuthordjh
- CommentTimeApr 20th 2022
Posted By: WillInAberdeenWell, isn't the idea that if you consciously open a window, you open it wider if it's calm than if it's windy? I do.
AIUI you always open the windows fully and vary the length of time they're open depending on the wind strength. But I was just responding to Tony.
- CommentAuthorlineweight
- CommentTimeApr 20th 2022
Posted By: WillInAberdeen
- the trickle ventilation rate could be reduced by half without letting the air get stuffy during cooking.

Would it? - my notion is that it is set at the minimum possible rate such that levels don't build up over multiple cycles. Therefore if the ventilation rate was reduced by half it would look something like the purple line here compared to the red original. And eventually you'd have to resort to a purge.
- CommentAuthorWillInAberdeen
- CommentTimeApr 20th 2022 edited
Ah ok! Understood.

(Edit: if the ventilation were reduced, the pollutants concentration would increase as shown, until they found a new balance. If the ventilation was halved, the pollutants level would double, that would still be within the acceptable level in the lower pic)
- CommentAuthorlineweight
- CommentTimeApr 21st 2022
Posted By: WillInAberdeenAh ok! Understood.

(Edit: if the ventilation were reduced, the pollutants concentration would increase as shown, until they found a new balance. If the ventilation was halved, the pollutants level would double, that would still be within the acceptable level in the lower pic)

I don't think the pollutants would double. They would increase over each cycle. At some point they would double but they would continue going up. I've rethought the graph slightly. Red is with the "ideal" rate of trickle ventilation (plus background leakage). Green is no trickle ventilation - just background leakage. Halving the trickle rate would mean the gradient of the downward slopes would be halfway between these two lines - so the purple line represents what would happen with trickle ventilation halved.
- CommentAuthorlineweight
- CommentTimeApr 21st 2022
Posted By: tonyTrickle ventilation varies greatly depending on wind speed.

I guess in my hypothetical scenario, you would set the trickle rate so that on average, over time, it (plus background leakage) would equal the rate of production of pollutants, again averaged over time.
- CommentAuthordjh
- CommentTimeApr 21st 2022
I think Will's point is that your model isn't right, in at least one important way. You show the pollution reducing linearly in the zags between the zigs where it increases. But pollution doesn't reduce linearly. It reduces faster when its concentration is greater. Will is saying/predicting that the effect of that will be that an new higher equilibrium will be reached rather than that pollution will increase without limit as you show.
- CommentAuthorlineweight
- CommentTimeApr 21st 2022
Posted By: djhI think Will's point is that your model isn't right, in at least one important way. You show the pollution reducing linearly in the zags between the zigs where it increases. But pollution doesn't reduce linearly. It reduces faster when its concentration is greater. Will is saying/predicting that the effect of that will be that an new higher equilibrium will be reached rather than that pollution will increase without limit as you show.

Ok - so the downwards zags should be curved (and never quite reach zero?). But I should (in theory) still be able to find a rate of trickle ventilation that ensures the overall pollution level doesn't increase over a cycle, no?
- CommentAuthordjh
- CommentTimeApr 21st 2022
Posted By: lineweightBut I should (in theory) still be able to find a rate of trickle ventilation that ensures the overall pollution level doesn't increase over a cycle, no?
That obviously depends on the maximum ventilation rate you can achieve versus the rate of pollution generation. I don't expect anybody would expect trickle ventilators to keep the air clear of smoke in the event of a major fire!

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General: Energy efficient strategies for manual ventilation