I think every case in particular must be studied. Particulary where I work, in Argentina, the minimum area that must be left to natural ventilation is 5% of the area of the office. Although, there is no maximum and, in my opinion, depends on so many factors that it would be hard to determine a single number for it.

Hi Alan,

Thanks for your thoughts. I was also thinking what is the minimum ‘volume’ exchange rare (for instance 5 Air Change per Hour) that is permissible. There comes a point where too much air flow from outside is practically impractical.

But I think you are right in saying that there is a degree of individual discretion that each case under study might merit. For the record I didn’t find any max allowable values either …

Thanks a lot

Mohammad

I´m glad it helped. You are welcome

While thinking about thermal discomfort - take a look at ISO 7730 standart which determines this factor.

Alowed air movement velocity looks more important and more limited than air change rate (maybe some times extreme rates could couse moisture problems in some climates)

Maybe limiting value of air change is when thermal comfort criteria are not met.

By the way - there is profesor whos papers might be interesting for you from comfort point of view : https://www.researchgate.net/profile/BW_Olesen/

Thanks a lot Karolis for your contribution, I have not found any particular figure on the air change rates, but I think both you and Erin are right that the guiding star (and the index of measurement) is actually air velocity.

Thank you both + best wishes for the festive season.

Thanks John,

Quite a comprehensive publication but yet I couldn’t find any guideline on maximum allowable. To be perfectly honest I don’t think anybody has specified maximum natural ventilation rate as it is such a building and climate specific problem…

Thanks once more

Mohammad - I am not entirely clear whether you are asking this question  from the perspective of thermal comfort or danger from flying objects? As far as I know the rate of natural ventilation is not regulated by Building Regulations or any health and safety laws with respect to flying objects. People generally close windows pretty quickly in my experience when their thermal comfort is negatively affected and likewise when papers start to blow around.

On average humans tend to feel moving air on exposed skin at much lower velocities (circa 0.1-1.0m/s), than those required to move papers (circa 5-7m/s). Thus thermal comfort or sensation is likely to be a limiting factor (in most cases) except perhaps in warmer climates. Note that a small desktop fan can easily generate up to 5m/s of air speed locally and the acceptability of this will depend very much on a number of thermal comfort criteria. Namely the air temperature, turbulence intensity and met and clo values of the occupants.

In terms of thermal comfort the draught rating (DR) was proposed by Fanger et al as an index for correlating the % dissatisfied relative to combinations of mean airspeed, air temperature and air turbulence intensity. Accordingly an upper limit of acceptable air speed can be derived for a given air temperature and turbulence intensity. Where the turbulence intensity(%) is defined as the ratio of the standard deviation of the air speed to the mean air speed.  As such you can back calculate 'acceptable' upper limits for air speed in relation to whatever % dissatisfied you feel is acceptable (15% is commonly used). For example at 26C and with (theoretically) zero turbulence intensity the mean air speed cannot exceed 0.5m/s if you want to achieve within a 15% dissatisfied vote.  However if the internal air temperature rises above this level then higher air speeds can be derived.

According to Fanger's DR equation as ambient air temperatures increase it is the air turbulence intensity that is likely to have an increasingly dominant influence on the % dissatisfied rather than the absolute air speed. Thus turbulence may be the limiting factor even before high internal air speeds start to move paper and other objects about.

ASHRAE 55 comments on the use of elevated air speeds in relation to reducing the discomfort caused by higher temperatures, that "elevated air speed may be used to offset an increase in the air temperature and the mean radiant temperature by not more than 3.0°C above the values for the comfort zone without elevated air speed. The required air speed may not be higher than 0.8 m/s." Since large individual differences

exist between people in regard  to their preferred air speed the elevated air speed must be under the direct control of the affected occupants. 

Thus combining the guidance above it is probably fair to say that exceeding mean internal office air speeds much above 0.5 - 0.8m/s even in warm climates is likely to result in an unacceptable increase in the mean dissatisfied vote, unless there is immediate local control available. Under winter design conditions far lower air speeds (circa 0.15m/s) are likely to represent the upper limit for an acceptable mean dissatisfied vote.

In practice the nature of natural ventilation may make such limits very difficult to control for. If you want to read more on this have a look at CIBSE Guide A (2006), EN7730, ASHRAE 55 and also Fanger, P.O., et al (1988) Air turbulence and sensation of draught. Energy and Buildings 12.

Thank you so much Robert for your insightful answer.

My initial inquiry really was to learn what maximum levels of ACH I could set in my simulation efforts to naturally cool a building in summer. I thought perhaps there is a point where excessive ACH would mean excessive occupied zone air speeds (although we both know ACH is itself limited by wind velocities and fenestration design and therefore cannot be infinitely boosted).

Only one quick final inquiry… is turbulence intensity a function of varying air speed at different parts of a zone over the occupied hours? That (to me) would be infinitely difficult to measure unless one trusts CFD simulations of the target zone and picks several simulated speeds and calculates the standard deviation to then arrive at a turbulence intensity?

Thanks

Thanks Mohammad, I can see where you are coming from now. Regarding turbulence intensity it is indeed difficult to measure accurately. CIBSE Guide A states that "in the main body of most rooms, away from supply air jets, the turbulence intensity is usually between 30 and 50%."  That is of course quite a generic range (though useful for rule of thumb estimation) measuring and calculating turbulence intensity in practice is more complex. Typically either the averaged value of the instantaneous velocity is determined over a specific time period (Eulerian velocity) or the air velocity can be averaged spatially over a line segment as a fixed time point (Lagrangian velocity). Experimentally the measurements are typically obtained by a hot sphere anemometer or some form of thermal anemometer. Due to the chaotic nature of turbulence, and depending on the level of accuracy required a large number (e.g 5000) of short time sample measurements (e.g 4 seconds) are taken (rather than the mean of a long time period) and the ensemble average value is used from N repeats of the measurement. The derivation of the optimum number of time samples (N) and sampling interval (dt) is a function of the average air velocity and turbulence intensity and also the confidence level and inaccuracy which are considered acceptable. Quite a bit has been written and published on experimental in-situ studies (e.g Thorsauge (1982), Hanzawa et al. (1987) and Melikov et al. (1988 and 1990), Loomans (1996 and 1998) and also numerous CFD simulation studies investigating air speeds and turbulence in a variety of building types.  To answer your question most of these studies show that mean velocities and turbulence varied widely over the investigated spaces, as you might imagine. Typically, mechanically ventilated rooms revealed higher velocities (0.05 - 0.40 m/s) compared to the Nat vent rooms (0-0.20 m/s) , whereas the turbulence intensities were of a similar order of magnitude 10 - 60%.

Coming back to your initial context of using high air change rates for cooling - I would be less concerned by the limitations imposed by max air velocities and more concerned about what air change rates can be realistically achieved in practice accounting for real world constraints. Considering limitations such as window opening restrictors (see ROSPA for domestic guidance) cross ventilation obstructions, and also acoustic issues from surrounding noise (urban environments) as well as security issues and also the issue of whether the openings are controlled by actuators or manually. I would err on the side of caution (or model discrete scenarios) as I have seen a number of studies which have predicated summer cooling possibilities on rather unrealistically high levels of ACH.  Good luck.

Yes you are right ACH predictions by models are hardly achievable in reality for reasons you outlined. Doesn't stop our architect colleagues thinking Nat Vent is a cure for all ills so that they can create cathedrals of glass!

I appreciate your input and wish you very well with your work.

Mohammad

In some web pages I found ACH would be min. 5 for naturally ventilated offices. but ASHRAE does not suggest any value.