Why is it so hard to pass ASHRAE 55 (PMV ±0.5)?

If you've opened this article you've probably tried to benchmark ASHRAE 55 performance on your building in Sefaira and found that it's very difficult to pass. This article tries to explain what's going on.

Why do I care?

You're trying to comply with ASHRAE 55 by using thermal analysis so that you proceed in confidence with a design that is considered universally comfortable. You're struggling to get your design to comply.

Zones are regularly too cold

If your model is constantly failing because zones are too cold, then it is mainly because the standard is pretty unforgiving when it comes to cold conditions.

To explain this, we have attached below a screen grab from the CBE calculator with the inputs Sefaira uses. (this is an interactive tool you can access at comfort.cbe.berkeley.edu)


This shows that if your building is conditioned to 70-75°F (21-24°C) and is maintaining 40% relative humidity, the mean radiant temperature needs to be 68°F (~20°C) to comply with ASHRAE 55. 

In most zones with any glazing or thermal mass, a relatively short warm-up cycle and typical overnight setbacks this will be extremely difficult during the winter and colder months. Here's why:

  • The glazing temperature will almost certainly be substantially lower than 68F. During the first hour it's likely to be somewhere between the overnight setback temperature temperature and the ambient temperature outside, both of which are probably below 68°F (20°C)
  • The other surface temperatures will most likely be somewhere between the overnight setback temperature and the heating setpoint, meaning they are also very likely to be below 68°F

In addition when we apply the dynamic clothing calculator, the clothing rate is less than 1 if outdoor temperatures are above 23°F (-5°C) at 6am. This makes the required mean radiant temperature even higher.

Workarounds for cold results

Although reducing glazing and adding lots of insulation helps, for the above reason it is not the only solution to improving comfort. The best work-arounds include:

  • Maintaining high minimum setback temperatures overnight
  • Adding long warm-up times
  • Setting the heating setpoint to be substantially higher than the default (eg 72.5°F)

Considering a wider comfort range for compliance

You may want to consider a wider comfort range as a target for your project. The ASHRAE 55 "wider acceptability" band would be equivalent to a PMV score of ±0.8. GreenStar give credit for a PMV score ±1. 

Alternatively you could strive to meet the PMV upper range target of 0.5 but set the lower range at -1.

Zones are regularly too hot

If your zone is air-conditioned but regularly fails because it is too hot, you probably need to control solar gain. This is the most common cause of PMV results being too hot. Consider the following:

  • External Shading 
  • Less glazing
  • Lower SHGC values for the glazing

Buildings with full external shading and a cooling setpoint of 75°F / 24°C should be able to pass a PMV target of <0.5.



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