ASHRAE Journal - October 2011 - 31

and schools do tend to have higher daytime internal gains.) Envelope-dominated buildings are subject to intermittent solar gain, by time of day and building exposure, and so have a need for distributed temperature control to deal with the vagaries of the sun coming and going. With lower core gains and less heat available from the core, and often lower internal gains, envelope-dominated buildings have higher no-load temperatures, the outdoor temperature below which heating is required and cooling is not required. This reduces the need and energy-savings potential for economizers within the HVAC system. Another characteristic of many envelope-dominated buildings is repeating room types, where the same type and size of room is repeated throughout the building. This is certainly true of hotels and apartment buildings, and to a lesser extent applies to homes, small ofﬁce buildings, and retail storefronts such as strip malls. And if room types do not repeat, then room types are often fairly simple, as is the case with small bank branches, single-story department stores and supermarkets, and warehouses. For either case (repeating room types or simple rooms types), envelope-dominated buildings are more typical for the application of fan coils or other simple single-zone terminal units, rather than central-station air-handling units. As with all buildings, ventilation presents challenges for envelope-dominated buildings. There is frequent reliance on natural ventilation (typically, windows), with associated control problems. Alternatively, constant-volume ventilation is not infrequent (exhaust fans and/or constant-volume makeup air). Modulating ventilation, timer-controlled ventilation, or demand control ventilation is sometimes provided, but these are exceptions rather than the rule. Ventilation options are many, including no ventilation, intermittent ventilation, continuouslow-level exhaust, continuous-low-level-balanced ventilation, and energy recovery ventilation. Envelope-dominated buildings tend to be smaller than buildings with a core, with some exceptions (for example, large apartment buildings, or large hotels). By extension, envelope-dominated buildings tend to not have full-time maintenance staff, and so can be more appropriately served with simpler HVAC systems.

for a speciﬁc building with a speciﬁc annual heat loss Q, the required energy is related to the heat loss by the system efﬁciency (Eg for geothermal heat pumps, Ea for air source heat pumps, Ef for fossil heating systems): Geothermal heat pumps: Q = Wg × Eg Air source heat pumps: Q = Wa × Ea Fossil heating systems: Q = Qa × Ef Geothermal heat pump efﬁciency is given in COP and is typically approximately 3.1 (ASHRAE/IESNA Standard 90.12007, ASHRAE/USGBC/IESNA Standard 189.1-2009).1,2 Air source heat pump efﬁciency ratings are given in heating seasonal performance factor (HSPF) and are typically in the range of 7.7 (Standard 90.1) to 8.5 (Standard 189.1). HSPF converts to COP by dividing by 3.412, so the equivalent or year-round COPs for air source heat pumps are 2.3 (Standard 90.1) to 2.5 (Standard 189.1). In this simple comparison, out of the starting gate, geothermal heat pumps are already ahead of air source heat pumps. Air source heat pumps traditionally have suffered because their capacity drops with reduced outdoor air temperature, just as the load increases. However, the advent of variable speed systems is allowing improved performance at lower outdoor temperatures. Now, how do fossil fuel heating systems stack up against these two heat pump systems? The comparison is a little harder, because we are comparing different fuels. Let us focus on natural gas, the most widely used fossil heating fuel. One basis for comparison is carbon emissions: a kWh of electricity is considered, on average, to generate 1.3 lbs (0.6 kg) CO2,3 while a therm of gas is considered to generate 11.5 lbs (5.2 kg) CO2. On this basis, per MMBtu of building load (Q), a 3.1 COP geothermal heat pump generates 123 lbs (55.8 kg) of CO2, a 7.7 HSPF air source heat pump generates 168 lbs (76.2 kg) of CO2, and an 80% efﬁciency fossil fuel heating system generates 144 lbs (65.3 kg) of CO2. In our horse race, geothermal is out front, fossil heating is in second, and air source heat pumps are in third place, using this simpliﬁed heating analysis. Now let us examine a fourth class of popular heating systems, frequently referred to as water loop heat pumps, or “boiler/tower” systems. Here the relationship is slightly more complex. The water loop heat pump delivers heat to the space to replace the same lost heat (Q) as in the examples above, using electrical power Wl, at efﬁciency El: Q = Wl × El But energy is also used by the boiler to supplement the electrical energy supplied by the heat pump. The supplemental input energy delivered to the heat pump is Qi, and an energy balance on a heat pump says that the delivered heat is equal to the sum of the heat from the loop Qi and the electrical energy used by the heat pump:
ASHRAE Journal 31

First Principles
How can we translate these general characteristics of envelope-dominated buildings into concrete guidance for HVAC system selection? Simpliﬁed analysis using the ﬁrst law of thermodynamics (conservation of energy) can help. We focus on four general types of HVAC systems: geothermal heat pumps, air source heat pumps, fossil heating and chilled water cooling systems, and ﬁnally water loop heat pumps (“boiler/ tower” systems). We initially focus on heating, because envelope-dominated buildings typically need more heating. We also ignore internal gains, because these are low. We also ignore solar gains and distribution system energy, for simplicity. For most systems, the relationships then become simple,
October 2011

ASHRAE Journal - October 2011

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