# Multiple PPD values?

@chris
Hello Chris,

I think you are an expert on thermal comfort so I am writing to ask a question.

So the comfort module outputs the PPD value, which is calculated based on the thermal conditions from E+. However, the `draftComf` and `ankleDraftComf` models also give the PPD values based solely on the discomfort from downdraft. I am wondering if you know the approach to combine these two PPD values (e.g. taking the average, or the max of the two, or some other magic equations)?

Thanks

Hi @sz

Iâ€™ll jump in off of the bench for @chris in the time before he provides his response.

The first thing to understand is the differences in Percent People Dissatisfied (PPD) among the different analyses. The PMVComfortCalculator that you have snapped an image of is the most basic, and most widely used thermal comfort model that represents a populationâ€™s mean vote (from a seven point thermal comfort survey) and satisfaction based on perception of comfort across the whole body. In otherwords, what percent of the population will feel uncomfortable given the ambient temperature, humidity, air speed, and radiant temperature. This differs from the draftComf and ankleComf because those are discrete, targeted thermal comfort models that maintain their own PPD thresholds. For example, the image below outlines the acceptable radiant temperature differences given non-uniform radiation field within an indoor environment, taken from ASHRAE 55-2013 Table H1. The acceptable limit for radiant asymmetry is 10% so if you draw a line across at 10%, you can determine acceptable differences in radiant temperature.

Radiant asymmetry, along with other types of unique comfort models, are covered in ASHRAE 55 and the draft and ankle discomforts are categorized as types of â€ślocal discomfortâ€ť, along with radiant asymmetry and stratification.

To answer your question, you may find this example script that @chris developed useful for your work. In this example, Chris takes the max value between the differences between the PMV-PPD and the local discomfort PPD to determine the worst case.

Hopefully this helps in the meantime!

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Hello @KitElsworth,

Thank you so much for your valuable response, and it helped a lot!

So if I understand correctly, the `ankleDraftComf` corrects the PPD values (whole-body comfort), and the final PPD value for displaying is the max of either whole body PPD or local PPD (i.e. the corrected PPD based on ankle downdraft). Am I right?

My other question is that why `draftComf` (as shown on my screenshot in my initial post) is not implemented in this example case?

Again, thank you very much.

S

@sz, sorry for the late response, and thank you, @KitElsworth , for giving an initial answer.

@KitElsworth essentially hit it on the nail when he highlighted that the draft discomfort models are intended to estimate localized discomfort while the PMV model estimates full-body thermal sensation. So these two models are not mutually exclusive and are intended to complement one another. For example, you can have a full-body thermal sensation that is neutral but your feet can still feel cold if there is a high-speed draft running past them. So, if you have 7% PPD out of the PMV model but 25% PD out of the Ankle Draft Discomfort model, you have 7% of people with a full body sensation of cold (or hot) but 25% of people have cold feet.

For clarification, I will highlight that the following models are intended to reflect full-body thermal sensation:

• Predicted Mean Vote (PMV)
• Universal Thermal Climate Index (UTCI)
• Physiological Equivalent Temperature (PET)
• Any of the comfort models in the Thermal Comfort Indices Component

The following models are meant to model localized discomfort:

• Ankle Draft Discomfort

The general Draft Discomfort component is also meant to look at local discomfort but it is a very old model that has since proved unreliable. For this reason, it has been taken out of recent thermal comfort standards like ASHRAE-55 I donâ€™t recommend using it.

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Hello @chris,

Thank you very much for your valuable response!

Just want to clarify: For the example that you illustrated where 7% PPD for the whole-body discomfort while 25% PPD for the local discomfort, you are essentially taking the max of these two as final results.

I understand that they are not essentially mutually exclusive, so there is certainly an overlap between the two; for example, there is a certain percentage of the population who experiences discomfort at BOTH whole-body level AND local level. If this is true, is taking the max still holds true? Also, you cannot add them up because otherwise, the resultant PPD may go over 100%.

Thank you again for your time.

S

Thanks, @chris. To follow-up on your questions, @sz,

The ankleDraftComf produces a new PPD value based on the whole body PPD/PMV and the air velocity at ankle height (fed from the downdraft component). The reason for using whole body PPD/PMV is because oneâ€™s general thermal sensation influences oneâ€™s ability to be affected by draft at the ankles.

The maximum value, which produces the PPD Combined, is the worst case (maximum) value between the draft and the radiant discomfort. I think this is acceptable as long as you can retain, as a designer, the understanding of why each cell is experiencing discomfort.

Draft discomfort was not included, i am guessing, because it was not considered in the study question. The local discomfort factors do not necessarily need to be studied collectively in all cases so there may be situations where you can omit draft or radiant asymmetry.

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I think this makes sense. Thank you very much for your valuable responses! @KitElsworth @chris