Aquastat Fraction

This is the point in the entire primary storage volume where the temperature trigger (aquastat) for cycling on the heat pump water heaters is placed. The default is to place this aquastat so that 40% of the primary storage is used when the heat pump equipment is triggered to cycle on. This is controlled by where the aquastat port is on the storage equipment that is being purchased.

Decreasing the fraction will result in the HPWH running more frequently and a smaller primary storage volume.

Increasing the fraction means less storage is available to provide hot water while the HPWH equipment is recharging the primary storage, and a larger storage volume.

During load shift scenarios, you may decrease the fraction to decrease the total tank volume. The Ecosizer will notify you if the aquastat is too low to allow for the minimum cycling time of a HPWH.

Design Cold Water Temperature

This is the coldest water temperature supplied by incoming city water.

Increasing this value means the HPWH has a lower temperature lift to the hot water supply temperature, and the required heating capacity will decrease.

Hot Water Storage Temperature

This is the setpoint temperature of the primary heat pump water heaters, or the hottest temperature provided by the primary heat pump equipment. There is no default. If you are designing a swing tank system you should use at least 140F.

Input Method

These options allow you to toggle between two ways to provide number of people (building occupants) and number of apartments information.

Using Total People & Apartments requires just those two values.

Using Apartment Size & Occupancy Rates allows more control over apartment types, occupancy rates (people per unit size), and gallons of hot water used per day per person. Suggested values are also provided from California and ASHRAE occupancy data.

Load Shift Sizing

The load shift function allows you to block out part of the day when the heat pump water heaters will not run. The storage volume and heating capacity necessary to meet the load is sized similarly as in the primary system but accounting for period(s) where the heat pump water heaters will not run as requested by utility signals. This necessarily means that the storage volume must be higher so that the volume in the primary storage can provide the building occupants with hot water for the period when the heat pumps are prevented from running, and can meet any peaks after the end of the load shift period(s).

Load shift systems can grow quite large, in part due to the aquastat fraction being larger than needed. The aquastat fraction can be turned down in many load shift cases to account for a much larger than needed volume for the HPWH to cycle through.

Number of Apartments

Total number of apartment units in the building.

An increase in the number of apartments in a building will result in increased temperature maintenance losses for a given Recirculation Loop Heat Loss.

Number of People

Total number of people in the building served by the heat pump water heater.

One Cycle Off Time

This controls the temperature maintenance tank volume and is the expected time (in hours) that the temperature maintenance equipment will be off between heating cycles. This is related to the difference between the Temperature Maintenance Setpoint and Turn On temperatures, and the Recirculation Loop Heat losses. In other words, the time that will elapse as the loop losses reduce the tank temperature from the setpoint to the temperature that will trigger the heat pump water heater to cycle on.

Parallel Loop Tank

Single-pass heat pump water heaters are most efficient when heating cool city water to hot storage temperatures, whereas multi-pass equipment can still operate efficiently when incoming water temperatures are around 120F. A parallel loop configuration is one strategy used to isolate the temperature maintenance task from the task of heating the primary storage. A parallel loop tank is an electric resistance element or a multipass heat pump that is piped in parallel with the primary system, specifically to handle the temperature maintenance load.

Peak Gallons per Day per Person

Use this area to provide information on the maximum gallons of hot water used per day by each building occupant.

The User Defined option allows you to choose a value of your choice between 20 and 49 gallons per day per person. Using the California By Bedroom option is only possible when entering the number of units by size (input method), as each unit will have a different value aligned with the Title 24 Software CBECC-Res. Using this method means also using the CA occupancy ratios.

ASHRAE Low is described as an apartment building with a mix of demographics: all occupants working; one person working, while one stays at home; seniors; couples; middle income; higher population density.

Ecotope Market Rate with Low Flow Fixtures is based upon measured data from an Ecotope Inc. project, Stream Uptown, a building with 118 units. Data uses the 100% quantile for the peak draws during the given time periods.

ASHRAE Medium is associated with apartment buildings with a mix of occupant demographics: families; singles; on public assistance; single-parent households.

The number of people is needed to define how much hot water is used daily.


Percent of Load Shift Captured

This adjusts the percent of days to meet the hot water demands of load shifting. For example, turning this down to 70% implies that only 70% of days will meet the hot water demand from load shift. This is achieved by scaling down the daily gallons per person to the 70th percentile value following a distribution acquired from 4 years of data from a market rate building in Seattle, WA. The median value at the 50th percentile is about 70% of the 100th percentile load.

Note that your storage requirement increases exponentially with increased load shift, represented by the curve behind the slider. So the closer to 100% you get, the larger the impact will be on storage requirements.

Primary - No Recirculation

Primary - No Recirculation is used for sizing a primary plant only.

Recirculation Loop Flow Rate

The recirculation loop flow rate is the rate at which a recirculation pump moves hot water through the distribution loop in gallons per minute.

This is used along with the recirculation loop return temperature to calculate the recirculation loop heat loss rate.

Recirculation Loop Heat Loss

These are the losses associated with the recirculation loop expressed in Watts/apartment unit. Previously studied buildings show a median loss value close to 100 W/apt, with a 25th percentile of ~66W/apt, and a 75th percentile of ~175W/apt. Ideally, building and plumbing design will minimize this load. This is an area of building science that deserves additional research to better understand typical recirculation losses, and how to minimize them.

Increasing the recirculation loop heat loss rate will increase the temperature maintenance load expected and thus increase the temperature maintenance heating capacity and storage volume.

  • Kintner P., and B. Larson (2019). Literature Review of Multifamily Central Domestic Hot Water Distribution Losses. Prepared for NEAA.

Recirculation Loop Return Temperature

The recirculation loop return temperature is the temperature of hot water when it returns to the temperature maintenance plant.

This is used along with the recirculation loop flow rate to calculate the recirculation loop heat loss rate.

Storage Efficiency

This is the fraction of the primary storage volume that is filled with hot water at the storage temperature.

The storage efficiency is used to check that the primary storage volume between the aquastat and the bottom of the effective storage volume is large enough that the primary HPWHs can cycle for at least 10 minutes without any hot water draws. Having a cycling volume greater than this minimum helps build a robust CHPWH system and adds some safety in the storage volume.

Supply Water Temperature

This is the water temperature supplied to the building occupants from the hot water system/plant. The default is set to 120F, an industry standard. If you are aligning sizing efforts with CA Title 24 software this should be increased to 125F.

Increasing the supply water temperature raises the reference temperature for what the occupants use. This will increase the storage volume as the hot water stored has less potential energy over the supply water temperature.

Swing Tank

A Swing Tank design is a proven technique to use the primary heat pumps to support the temperature maintenance load (Banks et al., 2020), while keeping the heat pump equipment isolated from the warm water returning from the recirculation loop. This design strategy is best suited for buildings with low temperature maintenance loop losses (< 60W/apt) and relies on increased storage volume (with tanks piped in series) to ensure storage stratification. Swing tank systems have an electric resistance element in the temperature maintenance tank as a backup safety factor. Sizing a swing tank system also means increasing the heating capacity and storage volume of the primary system. The temperature maintenance storage volume for the swing tank can be small.

  • Banks, A, Grist, C., and J. Heller. 2020. CO2 Heat Pump Water Heater Multifamily Retrofit: Elizabeth James House, Seattle WA. Prepared for Washington State University Energy Program, under contract to Bonneville Power Administration

Temperature Maintenance Setpoint

This term, the Temperature Maintenance Turn On, and the Cycle Off Time are critical to sizing a multi-pass heat pump water heater system. The Temperature Maintenance Setpoint is the temperature of the water serving the recirculation loop supply.

Increasing the temperature maintenance setpoint while holding the temperature maintenance turn on temperature (at 125F) will decrease the temperature maintenance tank volume as the stored energy density of the tank is greater.

Temperature Maintenance System Safety Factor

The safety factor the temperature maintenance heater will have over the temperature maintenance load.

A higher safety factor will increase the output of the temperature maintenance heater. If the safety factor was set to 1, the temperature maintenance heater and the temperature maintenance load would be in perfect balance.

Temperature Maintenance Turn On

This term, the Temperature Maintenance Setpoint, and the Cycle Off Time are critical to sizing a multi-pass heat pump water heater system. The Temperature Maintenance Turn On is the temperature of the water that will trigger the temperature maintenance to begin a heating cycle. In other words, the lowest temperature to which the temperature maintenance tank should be allowed to drop. This cannot be less than the supply temperature.