The Influence of Climatic Variables on Building and HVAC System Design

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1 The Influence of Climatic Variables on Building and HVAC System Design Andrew Persily Building and Fire Research Laboratory National Institute of Standards and Technology June 7, 2010 National Academy of Sciences Workshop on Climate Change and Indoor Environment Outline How climate (temperature, humidity, wind, precipitation) impacts building and HVAC system design What designers do with these factors to select and design systems and building envelopes (enclosures, facades) What might occur given significant changes in temperature, relative humidity, frequency/intensity of storms,... How changing climate might influence the operation and performance of existing buildings 1

2 Temperature Design dry-bulb temperature exceeded 99.6 %, 99 % or 98 % of winter or summer hours ASHRAE Fundamentals Handbook; NOAA data plus Based on data over 25 yrs (do not change quickly) Humidity Design wet-bulb temperature exceeded 0.4 %, 1 % and 2 % of winter or summer hours With dry-bulb temperature, used to size cooling equipment; design day for humidity not same as design day for temperature Wind speed Impacts infiltration rate in heating/cooling load calculations Impacts air leakage rates (heating/cooling energy), temperature distribution and moisture transport in walls Design consideration in moisture control ASTM E241 Standard Guide for Limiting Water-Induced Damage to Buildings (wind driven rain) Air retarder specification ASTM E1677 Spec for an Air Retarder Material or System for Low-Rise Framed Building Walls (structural integrity) Precipitation Design consideration in moisture control, ASTM E241 Outdoor air louver design/selection, rated for rain penetration 2

3 What designers do with these factors in system selection and design, envelope design,... Systems Heating/cooling capacity Dehumidification capacity, SHR = sensible load / total (sensible + latent) load Envelope Manage precipitation and condensation of water vapor Positions of thermal insulation, vapor retarder, air barrier and drainage planes Methods to analyze water vapor transport Climate-based guidelines Simpler approaches to system selection & design, and other regional preferences Climatic regions Heating & cooling degree days Established practice Capacity (tonnage) of residential air conditioning often based on contractor experience, availability and of course cost No economizer cycles south of Other construction practice: basements, envelopes, 3

4 Economizer cycle, Free cooling Commercial building ventilation strategy to reduce energy use Use cool/dry outdoor air to cool building instead of chiller Increases first cost Don t always work as intended What if climate changes and other thoughts Ventilation requirements do not depend on weather Lower outdoor air ventilation rates More hours close to design cooling Less economizer design and operation Unofficial pressure on operators to reduce costs, reduce outdoor air, run system less System selection and design Trend to right-size instead of oversize for energy savings More efficient cooling equipment often has higher SHR More efficient buildings operate at higher SHR 4

5 More other thoughts Mechanical cooling in many areas of world with no outdoor air, adding outdoor air will increase humidity load Current trends towards more natural ventilation; hotter and wetter conditions more challenging for natural Are current filtration approaches up to challenge of more and different outdoor species? Much of design is qualitative, experienced-based; unlikely to respond quickly to significant changes in climate We currently don t design buildings well for moisture control; more challenging climates likely to increase moisture problems 5