Interim Adjustments for PHIUS+ 2015 Annual Cooling Demand Limits

As you may know, for building project certification, PHIUS’ standards require that the peak heating and cooling loads and the annual heating and cooling energy demands are limited by design.  That is, there are four criteria imposed on space conditioning energy / power, that must all be met.  Clearly then, for any particular project, one of these four will be the tightest, the most difficult to meet.  

One perspective on this is that whichever criterion is tightest indicates the weak aspect of the design, that is, its greatest “improvement opportunity” as a high-performance passive building. If there was a random pattern across projects as to which criterion was the tightest, that would bolster this perspective. However, it seems the pattern is not random - certification staff and some Technical Committee members report that projects are more often limited by the Annual Cooling Demand criterion than the other three. That bolsters the perspective that there is something off about the criteria themselves.  The issue seems to come up more often for large buildings or those with higher occupant density.  

PHIUS and its Technical Committee are currently working on the PHIUS+ 2018 standard updates. Remember that we committed to updating the standard every 3-5 years, as cost parameters change and as we learn more about how specific building typologies behave. Our models are only as good as the assumptions we make in them and these updates (together with measured data) will help us to assure that our modeled predictions will meet measured results as closely as possible. We are planning to incorporate buildings of different size and occupancy into the cost-optimization standard-setting studies for PHIUS+ 2018, and thereby come up with adjustments for these factors in a more systematic way.  

In the meantime, for certification under PHIUS+ 2015, we introduce the following three adjustments to the criteria and calculation protocol:

1. Lighting seasonality adjustment  

The standard-setting studies for PHIUS+ 2015 used Building America simulation protocol for lighting, which uses lower interior lighting energy in the summer (and therefore lower internal gains). But the monthly-method calculations in Wufi WUFI Passive / PHPP don’t reflect this seasonality. Therefore, for the purpose of calculating the annual cooling demand, the interior lighting energy may be reduced. The lighting energy reduction is latitude-dependent and is calculated using the [PHIUS Residential lighting adjustment calculator]. This measure applies to all residential projects, but only to the interior lighting energy of the dwelling units.  

2. Detailed occupancy sensor credit for multifamily hallway lighting

Until recently, only a 10% reduction in lighting power density due to occupancy sensors was allowed for multifamily common spaces, as in ASHRAE 90.1 Appendix G.  As of March 2017 though, a more detailed calculation can be done that can give larger reductions in many cases, using the [PHIUS MF Interior Corridor Lighting Occupancy Sensor Calculator]. (See Certification Guidebook section 6.6.1.1). It is predicated on a daily pattern in which most of the foot traffic is from people leaving the building in the morning and returning in the evening. Therefore, this measure applies to multifamily residential projects, but only to the lighting energy of interior pass-through common spaces i.e. corridors, lobbies, stairwells. 

3. Annual-demand criteria adjustment for high internal gains

Larger buildings appear to have a tendency toward higher internal gains on a per-unit-of-floor-area basis, which tends to increase the annual cooling demand and decrease the annual heating demand. Adjustments to the certification criteria may be made on a climate zone-by-zone basis, up to the limits shown in the table below. If an increase in the annual cooling demand limit is taken, the project must also take the proportional decrease in the heating demand limit. The adjustment limits were determined based on a corridor lighting calculation, but may be used for high internal gain situations of any cause. They allow for up to about a 1/3 increase in specific internal gain, e.g. from 0.9 to 1.2 Btu/h.ft2. This measure applies to multifamily residential, nonresidential, and mixed-use projects.

Table 1.  Zone-by-zone adjustment limits for annual heating/cooling demand criteria.