Interim adjustments for PHIUS+ 2015 annual cooling demand limits

Graham S. Wright, Senior Scientist & Product Program Manager

The interim cooling demand protocol does not apply to all projects, and can be disregarded for projects that are not challenged on meeting the annual cooling demand target. This temporary protocol was developed specifically to provide an alternative calculation methodology and adjusted criteria for projects that are unable to meet their cooling demand limit, despite all reasonable efforts made to reduce this value. This design limitation typically arises for buildings with high occupant density. The standard-setting optimization studies for PHIUS+ 2018 will incorporate buildings of different size and occupancy, and therefore the interim protocol applies only for PHIUS+ 2015 certifications. This protocol will not apply after the release of PHIUS+ 2018.

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. However, certification staff and some Technical Committee members report that projects are more often limited by the Annual Cooling Demand criterion than the other three. This is the case especially for large buildings or those with higher occupant density.

PHIUS and the Technical Committee are currently working on the PHIUS+2018 standard updates. Remember, 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 close 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® 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 (downloadable Excel-based tool). 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 – downloadable via link in sidebar) 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.

Climate zone AHD difference (% or kBtu/ft2.yr) ACD difference (% or kBtu/ft2.yr)
1 0 +10%
2A -28% +9%
2B -30% +10%
3A -19% +11%
3B -21% +26%
3C -22% +0.23 kBtu/ft2.yr
4A -16% +16%
4B -19% +27%
4C -18% +0.23 kBtu/ft2.yr
5A -16% +28%
5B -14% +0.42 kBtu/ft2.yr
6 -16% +0.42 kBtu/ft2.yr
7 -15% +0.15 kBtu/ft2.yr
8 -11% +0.14 kBtu/ft2.yr

— Graham S. Wright, PHIUS Senior Scientist

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