Green Building Forum - Does Co2 and humidity "even out" far more quickly than heat?

Does Co2 and humidity "even out" far more quickly than heat?

RobinB — Thu, 06 Oct 2011 10:30:20 +0100

I mean in air, just pondering this partly since reading aobut Viking House's intruguing FiWi and also since I'm a bit disappointed that our stove heats the rest of the house less than expected. And assuming heat evens out more slowly just how does that happen?

Hoping I can understand any answers!

RobinB

Does Co2 and humidity "even out" far more quickly than heat?

JSHarris — Thu, 06 Oct 2011 10:48:40 +0100

It will depend very much on air movement. CO2 is denser than air, so in a sealed room will eventually end up in greater concentrations at the floor than at the ceiling. Water vapour is less dense than air, so under the same conditions will end up more towards the ceiling. Add in some heat from a source in the room and things start to change though. Warm air will tend to rise, drawing in cooler air lower down. If that warm air then contacts cooler surfaces it will transfer heat to them, cool and start to descend. The result will be air circulation and movement that will, to some extent, tend to mix the water vapour and CO2 with the air, a bit like stirring a cup of tea.

Getting heat around a house from a single heat source is challenging, as the house layout has to be arranged to promote convection through every room. Bedrooms can be a problem, because they tend to be closed rooms, with no separate inlet and outlet for convected air flow. This is one reason why MVHR systems tend to markedly improve air quality in these rooms. My own bedroom is a good example, it has very poor air quality, even with the door and window left open. The position of the house, and the local air flow seem to ensure that this room stays relatively stagnant, in terms of air flow.

Does Co2 and humidity "even out" far more quickly than heat?

CWatters — Thu, 06 Oct 2011 10:59:49 +0100

Does Co2 and humidity "even out" far more quickly than heat?

You can have quite strong convection currents with relativly small differences in temperature - enough to hold up a glider for example. So I'd say yes, you can have water vapour transported around a house by convection without that convection also transporting much heat.

Does Co2 and humidity "even out" far more quickly than heat?

beelbeebub — Thu, 06 Oct 2011 11:00:09 +0100

The big driver for distribution of heat/CO2/H2O will be bulk air movement from place to place. Diffusion does happen but much slower than straight forward air movement.

To boost the distribution of heat from the stove to other parts of the house you need to move the air around.

The house I used to live in had a fan (ordinary bathroom type fan) that pulled air (via a duct) from the further end of the house and discharged it into the space around the stove. It made a noticeable difference.

Does Co2 and humidity "even out" far more quickly than heat?

fostertom — Thu, 06 Oct 2011 12:59:10 +0100

CO2 and H2O diffuse quite fast, even thro still air, in addition to any spread by bulk air movement. Whereas heat spreads only by air movement and conduction (except where you have line-of-sight to the heat emitter, when radiation is instantaneous).

'Extraction' of CO2, H2O vapour and other pongs and toxic gases by ventilation (actually not extraction, but dilution) is a crude old fashioned method, based on misunderstanding of principles. The BauBiologie 'movement' has the answers - a whole new field of study - google it!

All heat distribution in buildings is a mixture of convection and radiation. The higher the radiant %age, the more comfortable humans feel, and at lower actual air temp (which directly reduces heat losses both thro the fabric and by airchange). The higher the convective %age, the stuffier it feels, the greater the perception of draughts, and the higher the air temp has to be.

Examples of high-radiant are underfloor heating, and/or (even better) wall heating. So-called 'radiator' heating (actually primarily convectors) is in the middle. Warm air heating is high-convective. And, getting to the point, distributing heat around the house from a stove, by air movement, is also as high-convective as it gets. You'll be burning a lot of fuel, overheating the room it's in, and under-heating elsewhere. 'Elsewhere's' windows will also attract all the house's H2O vapour to them, to condense there.

Does Co2 and humidity "even out" far more quickly than heat?

JSHarris — Thu, 06 Oct 2011 13:11:50 +0100

Posted By: fostertomCO2 and H2O diffuse quite fast, even thro still air, in addition to any spread by bulk air movement.

I can, I'm afraid, say with absolute certainty (having been a caver for a few years) that CO2 doesn't diffuse in still air at all, it sinks to the lowest point. It's a potential problem in any sealed cave system or even old mine workings that have no ventilation. If you have a room where the air isn't moving at all the CO2 will eventually settle out at floor level.

Water vapour similarly always rises, even in air that's moving (clouds being a pretty god example). Diffusion doesn't appreciably help the equalise water vapour percentage in a volume of still air (due to the density difference). Water vapour is around 0.8 kg/m³, dry air is around 1.26 kg/m³, so there is a density difference between the two of around 40%.

Does Co2 and humidity "even out" far more quickly than heat?

RobinB — Thu, 06 Oct 2011 14:04:26 +0100

Thanks for the responses. The existence of Death Valley implies CO2 sinks and stays, so I was just struggling a bit to understand FiWi. I really like the idea of energy efficient ventilation with no ducts.

From a personal point of view we do have underfloor heating as well as the stove and I absolutely agree with FosterTom that radiant heat feels better.

Off to Google BauBiologie

Robin

Does Co2 and humidity "even out" far more quickly than heat?

djh — Thu, 06 Oct 2011 14:13:49 +0100

Posted By: RobinBThe existence of Death Valley implies CO2 sinks and stays

Eh?

Does Co2 and humidity "even out" far more quickly than heat?

SteamyTea — Thu, 06 Oct 2011 14:14:24 +0100

Isn't is a bit hard to compare heat (kW or kWh), CO2, that is measured in parts per billion and water vapour that is measured in kg/m^3 in a chaotic system. Through in air pressure, radiation changes and thermal inertia and trying to model it would lead to such large errors that it would become pointless.

What you could do is set lower and upper parameters and statistically model what should be happening, then test it (going to see some people that have the kit for this on Monday) so that you can create a unit less number that describes the system.

As for CO2 being more dense, yes it is, but I think that it will only separate out at very high concentrations (such as in a mine). One of the arguments against anthropogenic climate change was that CO2 levels at ground level would be higher than at the altitude that planes fly at. This was soon dismissed. What is true though, and well studied and understood is that water vapour at high altitudes is lower, making the effect of CO2 greater (water vapour is the largest 'green house gas').

Heat loss in a system can be modelled from the temperature differences, thermal inertia, SHC's and convection currents. But I think that you will find that bulk air movement (opening a door or window) will have the biggest impact on all three.

Does Co2 and humidity "even out" far more quickly than heat?

tony — Thu, 06 Oct 2011 14:15:08 +0100

CO2 gas may sink but CO2 mixed into air will stay mixed and tend towards an even distribution through mixing, diffusion and thermal interactions.

The same is true of water vapour, it will migrate towards places of lower relative humidity (generally the cooler places) unless there is a vapour barrier in its way.

Does Co2 and humidity "even out" far more quickly than heat?

JSHarris — Thu, 06 Oct 2011 14:35:57 +0100

Posted By: tonyCO2 gas may sink but CO2 mixed into air will stay mixed and tend towards an even distribution through mixing, diffusion and thermal interactions.

The same is true of water vapour, it will migrate towards places of lower relative humidity (generally the cooler places) unless there is a vapour barrier in its way.

I agree. We were specifically referring to still air though, where CO2 will eventually settle out and sink (in any concentration) and water vapour will rise.

As soon as you consider any practical house environment there will be air movement, convective or otherwise, that will ensure reasonably good mixing of CO2. The same isn't true for water vapour, as this is so much lighter than dry air that it will always rise, hence the formation of clouds. You can sometimes see this in a bathroom, where condensation will often start at the top of a uniformly cold wall. It is quite noticeable on my tall chrome towel rail (when its not heated), with the top often being covered in condensation whilst the bottom remains dry.

Does Co2 and humidity "even out" far more quickly than heat?

SteamyTea — Thu, 06 Oct 2011 14:58:24 +0100

C=PV/T

Does Co2 and humidity "even out" far more quickly than heat?

tony — Thu, 06 Oct 2011 14:59:24 +0100

I do not agree that CO2 will settle out of still air, it is a gas not a heavy piece of dust, it mixes, if it could be settled out you could get very rich doing it as CO2 is extracted from air at great cost at present

The reason condensation forms higher up in bathrooms is because warm air rises with water vapour in it not that the water vapour rises on its own

Does Co2 and humidity "even out" far more quickly than heat?

fostertom — Thu, 06 Oct 2011 15:00:11 +0100

Posted By: JSHarrisspecifically referring to still air though, where CO2 will eventually settle out and sink (in any concentration) and water vapour will rise

Don't know - as ST says that may happen (temporarily) at high concentrations - until in due course Partial Vapour Pressure drives the migration towards equalised concentration, of the the molecules of ea individual chem compound. This as I understand it is due to the collisions between molecules (Brownian motion?) regardless of bulk air flow.

Experience shows that in a house, if lots of water vapour is being generated say in bathroom or laundry, it's very soon felt as a clamminess all around the house, and increased condensation on cold surfaces such as single glazing in cool bedrooms. This effect far exceeds what can be explained by bulk air flow from A to B - it's a vigorous migration thro still air - a dispersal outward from areas of high concentration of that particular molecule - a mutual replulsion of the molecules of ea chem compound (which are 'blind' to all the other surrounding molecules). 'Volatility' is the measure of that dispersal force (or speed?), for that compound.

The only way to really prevent such dispersal is to set up a current of air that exceeds the speed of dispersal - and that's a tall order! Catering kitchen hoods, or lab fume cupboards, have to create an absolute hurricane, in order to beat that speed of dispersal.

Does Co2 and humidity "even out" far more quickly than heat?

JSHarris — Thu, 06 Oct 2011 15:19:28 +0100

Posted By: tonyI do not agree that CO2 will settle out of still air, it is a gas not a heavy piece of dust, it mixes, if it could be settled out you could get very rich doing it as CO2 is extracted from air at great cot at present

The reason condensation forms higher up in bathrooms is because warm air rises with water vapour in it not that the water vapour rises on its own

You may not agree, but CO2 most certainly does settle out over time. Anyone who has ever had to enter an enclosed, sealed, space, be it a cave, mine or even a sealed room, will confirm that this happens.

Its a slow process and the effect is usually only ever noticeable in very tall enclosed spaces (because the concentration of CO2 in air is only around 0.04%) but nevertheless the CO2 content will eventually settle to the lowest point.

Air is just a mixture of gasses, not a compound, so there are no bonds or charges holding it in a uniform mix. Gravity being what it is it will always separate out the heavier from the lighter components over time.

Practically this is of no significance in a house, as there will always be more than enough air disturbance to keep the constituent gasses well mixed.

With regard to water vapour, it is significantly less dense that dry air, even far less dense than hot dry air, hence the reason it rises. Air has a density of around 1.26 kg/³ at STP, water vapour has a density of around 0.8 kg/m³, also at STP, so a given volume of water vapour is around 40% lighter than the same volume of dry air at the same temperature.

For comparison, a hot air balloon works with a typical air density differential between the hot air inside and cold air outside of around 15 to 20%, about half that of the density difference between dry air and water vapour.

Does Co2 and humidity "even out" far more quickly than heat?

fostertom — Thu, 06 Oct 2011 15:25:35 +0100

Nice little scientific controversy here! Most evidence so far anecdotal - which as our boffin members keep annoyingly insisting, personal experience counts for nothing!

Does Co2 and humidity "even out" far more quickly than heat?

SteamyTea — Thu, 06 Oct 2011 15:48:59 +0100

Tom you tease.

If you observe something, think up some reasons why (the hypothesis) and then go and test it. If necessary, rethink the hypothesis and test again.
So if I think that water vapour is less dense than dry air and it rises in an undisturbed column, would it be fair to compare this with the condensation on the bathroom window. No, as it is not testing the same thing.
Are clouds and water vapour the same thing, not always. What we see when we look at clouds tends to be ice, or water in a solid state (though low clouds or mists can be in the liquid state). Water as a gas (not liquid or solid) is invisible to the naked eye (within normal bounds) and must be treated differently than the other two states.
So by building on what is known, different models can be created for different conditions. These can then be tested and personal experience can be quantified and verified. This has to be a better way than just assuming that what is seen a few times holds true all the time. We tend to remember the exceptions (think of the weather over the last 2 weeks, in a year or twos time we will be referring to it as a hot summer).

Does Co2 and humidity "even out" far more quickly than heat?

JSHarris — Thu, 06 Oct 2011 15:50:28 +0100

Posted By: fostertomNice little scientific controversy here! Most evidence so far anecdotal - which as our boffin members keep annoyingly insisting, personal experience counts for nothing!

Well, parachutists will tell you that their descent rate is faster in cloud (if they've been foolhardy enough to jump through it!) than it is in clear air, which counts for personal anecdotal evidence that water vapour is less dense than dry air (yes, I know clouds aren't pure water vapour, but they will have a higher percentage of it than the surrounding clear air).

Old miners will also confirm that CO2 concentrates at the lowest levels of old mines ("blackdamp" as it was referred to years ago), so more personal anecdotal evidence.

Practically the tendency for CO2 to settle out and the tendency of water vapour to rise isn't that significant when compared to the convective air movement in a house. This is particularly true for water vapour movement, because the highest concentration of water vapour in a house is inevitably going to be associated with a high temperature source, be it cooking or bathing. The result will be a lot of energy to convect air (and water vapour) from the warmer region into cooler parts of the house (the cooler air moving in towards the heat source as warm air/water vapour moves up and away).

Does Co2 and humidity "even out" far more quickly than heat?

joe90 — Thu, 06 Oct 2011 16:36:50 +0100

SJHarris,

I am curious, I have been a skydiver for many years and jumping in cloud is officially a big no no but occasionally it may happen for a short distance (a bit of low broken cloud over the airfield). We dont have a way of measuring our speed but terminal velocity is about 120MPH and we would only judge our decent rate against others we were jumping with, but as they are also in the same air we would not notice a difference!!!. I suppose we could time our decent but this is not normally done.?

Does Co2 and humidity "even out" far more quickly than heat?

fostertom — Thu, 06 Oct 2011 16:53:40 +0100

Amazing account in http://www.amazon.co.uk/Cloudspotters-Guide-Gavin-Pretor-Pinney/dp/034089590X/ref=sr_1_1?ie=UTF8&qid=1317916316&sr=8-1 of the only person who accidentally parachuted into a cumulo whatsit storm cloud and lived to tell the tale. The violence of what goes on inside those pretty fluffy things!

Does Co2 and humidity "even out" far more quickly than heat?

JSHarris — Thu, 06 Oct 2011 17:00:58 +0100

Posted By: joe90SJHarris,

I am curious, I have been a skydiver for many years and jumping in cloud is officially a big no no but occasionally it may happen for a short distance (a bit of low broken cloud over the airfield). We dont have a way of measuring our speed but terminal velocity is about 120MPH and we would only judge our decent rate against others we were jumping with, but as they are also in the same air we would not notice a difference!!!. I suppose we could time our decent but this is not normally done.?

A little background. Around 30 odd years ago I worked at a place (near Steamy Tea's hideaway) that designed and developed parachutes for air launched anti-submarine stuff. One involved dropping a thing (with its parachute deployed for the whole descent) from heights up to 35,000 ft into the oggin, which inevitably meant dropping it through cloud. We wanted to predict, reasonably accurately, where it landed (in the ocean) so had to take account of wind speed variation with height (measured by other sensors), release velocity and descent rate (at TV). The descent rate would vary with air density and air density varies with altitude, temperature and water vapour content. The hardest bit to do with the splash point prediction algorithm was model the descent rate increase in clouds. It was marked, but measuring cloud depth and position was very tricky and threw the whole calculation out by a fair bit if we got it wrong.

Does Co2 and humidity "even out" far more quickly than heat?

RobinB — Thu, 06 Oct 2011 18:55:56 +0100

Just actually looked up "Death Valley" in Wikipedia. For years I'd thought it was CO2 collecting in the valley as well as the heat that did you in. Wrong! Just very very hot.

No clear answer to my question really yet, but some interesting tales.

thanks

RobinB

Does Co2 and humidity "even out" far more quickly than heat?

TimSmall — Thu, 06 Oct 2011 20:12:12 +0100

I'm not that sure about real-world water vapour and CO2 distribution rates in dwellings (although opening my bathroom door does up the humidity in the hallway pretty quickly after a shower/bath), but you do need to shift relatively large volumes of air around to move much heat. This is possible to do using natural convection within some house designs, but is difficult or impossible in many.

Forced convection (i.e. a fan) will do some more (and also certainly shift the CO2 and water vapour about along with the air), but you'll need to be moving the hotest air around (as one of the previous posters did) if you want to achieve significant heating to other parts of the building, and the system will need to be smoke and fire proofed.

You'll also need a relatively large fan at low fan speeds with large diameter ducts (unless you don't mind it sounding like a vacuum cleaner). This may or may not be practical in your building!

If you want to check how much heat you can push about like this, then there's a calculation method in this thread:

http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=7836

Tim.

Does Co2 and humidity "even out" far more quickly than heat?

wookey — Fri, 07 Oct 2011 01:13:33 +0100

I believe that both JSHarris and Tony are correct on aspects of CO2 settling. It's a well-known (and potentially dangerous) phenomenon in caves and mines, that CO2 forms high concentrations in hollows, holes and the lower portions of pitches. If you don't notice soon enough when you abseil/climb in, you may not have time to prusik/climb back out before passing out. If you don't beleive this phenomenon is due to settlement/stratification then you'll have to think up something else to explain the phenomenon. (I'm a potholer too).

This paper: http://wasg.iinet.net.au/Co2paper.html says that although higher concentration can be stratified (with very sharp divisions between 'enough/not enough O2 to burn lighter fuel' - maybe 30mm height difference), diffusion is sufficient to keep CO2 at normal concentration mixed in, even in very still air. However if there is excess CO2 for any reason (such as plant decay) then high concentrations can be formed in closed depressions.
It happens rarely in practice as you need very still air, a closed 'bucket' and a source of excess CO2, but it does happen, and occaisionally it kills people.

Does Co2 and humidity "even out" far more quickly than heat?

fostertom — Fri, 07 Oct 2011 11:49:22 +0100

Posted By: wookeysource of excess CO2

must mean a high sustained rate of CO2 production, which exceeds the rate at which it diffuses away. So something in the mine, pothole etc must be releasing CO2 continuously. If not continuously topped up, then diffusion wd soon dissipate the stratification, even if the air is still.

Does Co2 and humidity "even out" far more quickly than heat?

JSHarris — Fri, 07 Oct 2011 12:07:50 +0100

Posted By: fostertom
Posted By: wookeysource of excess CO2
must mean a high sustained rate of CO2 production, which exceeds the rate at which it diffuses away. So something in the mine, pothole etc must be releasing CO2 continuously. If not continuously topped up, then diffusion wd soon dissipate the stratification, even if the air is still.

Sometimes this is the case, but often its just the depth of the hole and the stillness of the air. A deep hole, like a mine shaft, tends to allow heavier gasses to settle out at the bottom, and as the temperature will be near constant the air won't have any significant convection to help mix it up. Diffusion is a weak process and isn't that effective under those conditions at keeping the gasses evenly mixed.

Does Co2 and humidity "even out" far more quickly than heat?

joe90 — Fri, 07 Oct 2011 12:54:26 +0100

SJHarris, point taken, when I dropped through cloud I was too busy looking around for my chums so as not to bump into them rather than notice a change in the decent rate!!!!

Wookey, great name, nothing to do with Wookey hole by any chance??

Back to the thread:- I plan to have a wood stove in a new build and put duct to direct heat to other rooms (with fire proof protection) so all this info greatfully recieved.

Does Co2 and humidity "even out" far more quickly than heat?

djh — Fri, 07 Oct 2011 13:58:42 +0100

Posted By: tonyThe same is true of water vapour, it will migrate towards places of lower relative humidity (generally the cooler places)

No, it will migrate towards places of lower partial vapour pressure. The cooler places typically have HIGHER relative humidity, that's why condensation occurs there.

Typically the vapour pressure is pretty constant across any open area, regardless of any temperature variations in the area, whilst the RH does change inversely with the temperature. It is barriers such as walls that cause differences, or large bulk motions of air (subsequently experienced as wind).

Does Co2 and humidity "even out" far more quickly than heat?

fostertom — Fri, 07 Oct 2011 14:17:09 +0100

Posted By: djhNo, it will migrate towards places of lower partial vapour pressure. The cooler places typically have HIGHER relative humidity, that's why condensation occurs there.

But as condensation occurs, and vapour drops out of the air locally, then local PVP decreases and that's when more vapour is indeed drawn to the spot to fill the hole.

I'm not clear whether PVP is
a) proportional to absolute concentration of (water) vapour molecules, regardless of other vapours (air) - or
b) proportional to RH, which wd include the other vapours (air) in the calc.

If a), then

Posted By: djhNo, it will migrate towards places of lower partial vapour pressure.

remains true, but

Posted By: djhThe cooler places typically have HIGHER relative humidity ...

has no bearing on the migration.

Does Co2 and humidity "even out" far more quickly than heat?

djh — Fri, 07 Oct 2011 14:44:35 +0100

Hmm, several opinions in favour of CO2 settling. I'm always confused about how gas mixtures behave. On the one hand there's the gas laws telling us that we can treat the gases as if they separately fill the empty space and that the gases just 'ignore' each other. On the other hand, buoyancy is real and hot air balloons work. So what's the real story?

I went googling. First to find out about blackdamp. Wikipedia says that blackdamp is the absence of oxygen rather than the presence of anything specific. It also describes the mechanism by which the oxygen is depleted in coal mines - namely some coal is oxidised to carbon dioxide, which also accounts for the greater pressure of CO2 found since diffusion in a still mine is so very slow. Decay of timber props etc in other mines also causes similar depletion. There are also bacteria and other organisms growing on the surfaces in caves and other mines and they consume oxygen and produce carbon dioxide.

The paper [1] also points out that as the atmospheric pressure changes, large volumes of air are pumped in and out of caves and mines, so they're never truly still. And as the caving page wookey linked to points out, carbon dioxide travels into out out of caves dissolved in ground water and reacts with limestone and other substances, to further complicate matters. The CO2 in ground water can often be at a greater concentration than in the cave air, so some evaporates from solution.

FWIW, Wikipedia is quite discouraging about diffusion - http://en.wikipedia.org/wiki/Diffusion - "Under normal conditions, molecular diffusion is relevant only on length scales between nanometer and millimeter". It does point out that heat diffusion is a far more important macro scale phenomenon.

So what have I learned? Well, I think there are many plausible explanations for the presence of excess CO2 down mines and caves that don't rely entirely on large scale buoyancy or diffusion. So I still don't know what happens! Does anybody have a reference for an authoritative statement of whether and how fast buoyancy acts in gas mixtures? I'll do some more digging.

[1] http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1882904/pdf/bmj00089-0033.pdf
"Breathing" coal mines and surface asphyxiation from stythe (black damp).
D. J. Hendrick and K. E. Sizer BMJ. 1992 August 29; 305(6852): 509–510.