Unitary HP/ER Heaters in Systems with Recirculation

Scope of Proposed Code Change

Justification for Proposed Change

In California approximately 22 trillion BTUs are used for water heating in commercial buildings (CBECS microdata 2018).  The majority of the SHW energy use (79%) is from natural gas with just 19% from electricity (CBECS microdata 2018). Utility incentive programs for HP/ER HPWHs have attempted to capture a portion of this opportunity, but configuration and operation issues are demonstrating lower than expected performance (in terms of coefficient of performance, COP) when HP/ER HPWH are installed as the heat source for central DHW systems. 

One key component contributing to reduced COP is the activation of the ER heating elements. ER heating can be triggered for several reasons including but not limited to: 

• HPWH supply air temperature is too low for compressor operation as a result of 
  • Insufficient ventilation in installation location 
  • Cool winter temperatures 
  • HPWH operating mode: some models have 10°F lower compressor cut out temperature in efficiency (HP only) mode versus Hybrid mode setting 
• HPWH control algorithm is attempting to avoid a perceived potential hot water runout event from: 
  • The use of master mixing valves or pump controls that improve tank stratification with more cold water in the lower part of the tank but trigger more heating activations 
  • Elevated HPWH temperature setpoints that may trigger more heating activations from the algorithm identifying a larger temperature difference from the top to bottom of the tank which can be related to draws or return temperatures from a recirculation loop 
• HPWH set point temperature is higher than the HPWH can achieve using HP only 

Ensuring that installed Hybrid HPWHs comply with the manufacturer’s installation guidelines will minimize the chance that HPWHs will overcool the space in which they are installed and reduce excessive ER heating. Enforcing a specific HPWH compressor cutoff temperature will improve efficiency and help designers understand the HPWH operation characteristics to ensure that it can deliver hot water in all conditions. 

By accomplishing the above goals, installations of central systems with unitary hybrid HPWHs will be more successful and demonstrate the capability for electrification without the high energy costs associated with ER heating. 

Relevant Documents

Round One Utility-Sponsored Stakeholder Meeting Materials

• Unitary HP/ER Heaters in Systems with Recirculation  – Proposal Summary