Thought Leadership

A Load of HVAC TLAs

By Robin Bornoff

As humans we’ve lived in ‘built environments’ for a very long time.  From the cave to the penthouse suite, the necessity to at first stay warm and dry has now evolved into the desire for comfort. The HVAC industry, rather those with the responsibility to design the built environment, have at their disposal a range of metrics used to judge comfort (comfortability? comfortness? comfortitude? comfortosity? stick to the day job Robin, your time as a wordsmith is well and truly not even started) .TLAs (three letter acronyms) are always the symptom of a mature and/or niche industry. HVAC is no different. Here are but a few, plus their description but not their definition or derivation (the math can get a bit hairy)…

  • PMV ‘Predicted Mean Vote’. A scale of 7 thermal sensations from hot, through warm, neutral, cool to cold quantified on a scale from +3 to -3
  • PPD ‘Predicted Percentage Dissatisfied’. Derived from PMV, reflecting the % of people who would be uncomfortable, i.e. hotter or colder than they would like to be
  • LAQI ‘Local Air Quality Index’. A ratio of the contaminant level at a point compared to the average contaminant being exhausted from a room/space. A value of 1 indicating that the contamination in the room (regardless of its absolute level) is equally distributed. A value <<1 indicating that that point is very clean in relation to the amount of contaminant around.
  • CRE ‘Contaminant Removal Effectiveness’. A single value that quantifies how efficiently a contaminant is removed from a source, an integral of the LAQI
  • LMA ‘Local Mean Age’. The time taken for fresh air to reach any given point in space, in other words, how old and musty that point in space is.
  • LACI ‘Local Air change Index’. A non-dimensionalised form of LMA, normalising the LMA to a ‘perfect venting’ configuration where all the air was removed without any air being ‘trapped’ in a recirculation

All the above values can be readily calculated by FloVENT, our CFD tool for the simualtion of air flow, thermal comfort, contaminant dispersion etc. in the built environment. With the above metrics it is easy to compare and contrast a series of proposed room/office/occupied space designs.

Here’s a typical image of the distribution of PPD in an office. 100% = everyone uncomfortable at that point, 0% = everyone chilled, rather thermally neutral. Note that near the windows with the sun streaming in and near the ceiling where hot air gathers and stagnates, are not comfortable places to be:


28th July Ross-on-wye

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This article first appeared on the Siemens Digital Industries Software blog at