1.1.3 Baseline systems 2 and 4

Default autosizing for pre-defined Heat Pumps in ApacheHVAC sets the heating capacity at ARI rating condition of 47 °F  equal to the full heating load assigned to the HP in the Heating Design Sizing run. Using the default performance curves, the capacity actually available from the HP alone at the design heating outdoor condition will be significantly less (on the order of 30 to 60% of the rated capacity, depending on climate, etc.). The remaining load will be met by the backup heat source, which, by default, is electric resistance heating with infinite capacity.

For ASHRAE 90.1 PRM Baseline Systems, check that the default autosized capacity does not significantly conflict with the following considerations:

Generally, the ASHP heating capacity should be within about +/- 10 to 20% of the cooling capacity to mimic the behavior of actual equipment. Heating capacity for actual equipment is often, but not always, on the order of 15% less than cooling capacity. Using this as a guideline, rather than exactly matching the sizes or sizing the heating capacity larger to meet more of the winter heating load without backup (as such equipment can be selected), will cause the electric resistance heat to begin sharing the load at a temperature closer to the 40°F outdoor maximum permitted for electric backup operation.

For ASHRAE 90.1 PRM Baseline Systems, the ASHP heating capacity should must be sufficient to maintain the space heating setpoint without solar or internal gains when the outdoor temperature is 40 ° F, such that the backup electric heat never supplements the heat pump when the outdoor temperature is 40 ° F or higher.

If the required heating capacity to maintain the 40°F upper limit for backup heat source operation is significantly greater than the cooling capacity (e.g., ), the cooling capacity should be increased so that the heating and cooling capacities are more fairly representative of actual equipment. This will cause the simulated DX cooling mode to operate at a lower part-load fraction and may require revision of the cooling mode COP if the cooling capacity is shifted to a higher range with respect to ASHRAE 90.1-2007 tables 6.8.1B and 6.8.1D.

For actual applications, this will typically be a function of the equipment selection and sizing—often, but not always, prioritized for Design Cooling Loads—with the heating and cooling capacity both determined by sizing at 100 to 125% of the cooling load. However, for heating, the NRCAN recommended outdoor temperature balance point (OA temp at which capacity equals space-conditioning load) should be in the range of 23 to 32°F (-5 to 0 ° C). 

To maintain dehumidification capability in humid climates, and thus consistency with industry best practice, heat pump cooling capacity should be oversized no more than 125%.

As noted in the more general section describing these pre-defined ApacheHVAC systems, the 90.1 PRM requirements for sizing and operation of air-source heat pumps (ASHPs) are based in part upon DOE-2 parameters and default inputs to eQuest rather than standard industry practice for sizing and operation of ASHPs. These defaults and the sizing approach that they imply can lead to unrealistic simulation results, regardless of what simulation tool is being used. The following considerations are therefore essential to using these pre-defined systems in the context of the PRM baseline requirements.

This discussion addresses the following section G3.1.3.1 text from ASHRAE 90.1: 

Heat Pumps (Systems 2 and 4). Electric air-source heat pumps shall be modeled with electric auxiliary heat. The systems shall be controlled with multistage space thermostats and an outdoor air thermostat wired to energize auxiliary heat only on the last thermostat stage and when outdoor air temperature is less than 40°F.

IES has provided a new set of default values for the Air-Source Heat Pump (ASHP) component as of VE 6.3 (default values are shown in the figure below); however, in light of the complications with ASHRAE requirements vs. both real-world and simulation-based sizing considerations detailed below, fully automating the sizing and performance curve inputs to meet the ASHRAE-90.1 PRM requirements for ASHPs in PRM baseline systems 2 and 4 will come in future versions. The current method therefore combines autosizing with manual inputs to size baseline ASHPs within the current version of the VE.

 

 The sizing process for the ASHP should be as follows (if needed, see further explanation below):

·       For these systems, the ASHP component and DX cooling provide the heating and cooling modes of the reversible heat pump. The electric resistance backup heat source has unlimited capacity.

·       Complete the standard design sizing runs for spaces and then for the system(s) with the oversizing factors set to their default values of 1.15 for cooling equipment and 1.25 for heating equipment, as required by the 90.1 PRM. This will autosize the capacity ASHP and DX cooling to meet the full extent of the oversized loads at the respective design conditions.

·       For each zone with an ASHP, compare the autosized capacity for the ASHP component in the range of typical balance point temperatures (e.g., 25 to 40 °F, depending on the climate) with the sized capacity of the corresponding DX cooling coil. These capacities are shown in the Heat Pump component dialog or, for many zones at once, in tabular edit view thereof, and in the Cooling Coil dialog or tabular edit view thereof. Note that the connected DX Cooling type covers a range of possible baseline sizes having a common COP, and therefore the capacity indicated in the type dialog may be overridden by autosizing for each instance of that type (if permitted).

·       If the ASHP capacity within the range of typical balance points does not and DX cooling coil capacities do not differ significantly* (e.g., by 20% or more), adjust as indicated below.

·       If the ASHP component and DX cooling coil capacities differ significantly* (e.g., by 20% or more), adjust as indicated below.

o   If the autosized cooling coil capacity is significantly greater than the ASHP capacity, adjust the ASHP capacity upward to match the cooling capacity. If the ASHP is within 20% of the DX cooling, it is probably not worth making an adjustment, as this serves only to scale the ASHP performance curve with respect to efficiency.

·       If the heating design day outdoor temperature is 40°F or higher, check the simulation results to ensure that the backup electric

System heating Capacity from sizing runs, modified as required to accommodate the following considerations:

For actual applications, this will typically be a function of the equipment sizing as prioritized for Design Cooling Loads, with the heating and cooling capacity both determined  by sizing at 100 to 125% of the cooling load. However, for heating, the NRCAN recommended outdoor temperature balance point (OA temp at which capacity equals space-conditioning load) should be in the range of 23 to 32°F (-5 to 0 C). 

For ASHRAE 90.1 PRM Baseline Systems, in order to have the electric resistance heat share the load at a temperature closer to the 40°F outdoor maximum for electric backup operation, the ASHP heating capacity should be thr great of:

A) the sized cooling capacity

B) sufficient to maintain the space heating setpoint without solar or internal gains when the outdoor temperature is 40°F.

If the heating design day OA temp is significantly below 30°F, and additional design sizing run with OA temperature between 30 and 40°F may be justified to determine an ASHP capacity and balance point that will engage the electric backup below the design temperature when internal and solar gains are not present.

If the required heating capacity to satisfy B is greater than the cooling capacity, A, the cooling capacity should be increased up

To maintain consistency with industry best practice, the cooling capacity should be oversized no more than 125%, unless this is required to match sizing of heating capacity in a PRM baseline system to avoid operation of the backup electric heat when the OA temperature is above 40°F. 

The sizing process for ASHPs tends to differ significantly from that of other HVAC equipment. The ASHRAE PRM specification is unfortunately unclear with respect to the sizing this type of equipment for baseline systems (more on this below). Thus there is a degree of interpretation required here, and you are free to re-interpret this as you see fit. The numbers you are seeing in the ASHP component dialog are simply a starting point, and will require modification.

If the ASHP were sized for typical design heating indoor conditions (zero solar and internal gains) and an outdoor temperature between 23 and 40°F, this would be consistent with both.   However, if the outdoor condition for the heating design day is significantly below 32°F, heating this is inconsistent with the way in which equipment is normally sized in a simulation environment—i.e., it’s a departure from the standard procedure of sizing of heating equipment to meet heating loads at the standard design heating conditions.

The 90.1 PRM requires adequately sizing the ASHP plus backup heat source to avoid excessive unmet load hours, and does not permit any electric backup heat when the outdoor temperature is 40°F or higher. Thus the “balance point” (the outdoor temperature at which the full output of the ASHP just meets the space heating load) needs to be below 40°F under all circumstances of varying internal gains, solar gains, etc. The PRM goes on to specify that the electric backup should be the last resort with respect to maintaining the desired room temperature when the air-source heat pump can’t fully meet the load. The specified incrementally lower thermostat setpoint for engaging the backup heat is but one means of accomplishing this. Addressing these together requires a bit of a sizing balancing act. And, the 40°F ASHRAE is referring to is very different than the Minimum source temperature in the ASHP dialog. Both of these are explained below.

It is important not to be misled by the PRM reference to 40°F as the maximum outdoor temperature for operation of the backup electric resistance heating. The PRM is not saying that the ASHP should be off below this temperature, only that the electric resistance heat should be off at above it. Assuming the former, which may be implied by the combination of the PRM language regarding the and the eQuest default for “Minimum HP Heat Temp” in the eQuest Supplemental Heat dialog (see below), can significantly skew results for the model.

The following bar graph shows annual energy consumption by end use for autosized heating and cooling as predicted by eQuest version 3.63 with location set to Minneapolis, MN and HVAC system type set to PTAC and heat source set to heat pump (i.e., system type is PTHP). All other inputs, including the building geometry, remained at pre-set version 3.63 default values.

 

In this particular illustration of how the default eQuest version 3.63 ASHP input values can lead to unrealistic results for a baseline system, the default building in Minneapolis, MN used 75 times as much energy (390 MBTU vs. 5.2 MBTU) for “supplemental” electric resistance space heating as it did for heating via the specified PTHP systems. 

The screen captures above show how the default Minimum HP Heat Temp has been reduced to 10 °F to avoid this particular problem. Because we have found no justification for cutting the heat pump off at even 10 °F when the backup heat source is electric resistance, the default value in ApacheHVAC as of VE 6.3 is 0 °F.

For one particular zone (the top floor core zone of the default 2-story office building), the default ASHP inputs and sizing beget particularly skewed results, as indicated by the annual load served by the Supplemental heat source vs. that served by the total load on the Heat pump unit in the SS-Q report above.