Air-to-water heat pumps (AWHPs) are a vapor compression HVAC technology that provides space conditioning and, in some cases, domestic hot water (DHW) heating. AWHPs feature electrically powered compressors that utilize outdoor air as the heat source (or sink) and deliver heated or cooling water via a refrigerant-to-water heat exchanger to meet the space conditioning (and in some cases DHW) load. As a hydronic-based system, space conditioning delivery options include standard or low-profile fan coil units, ceiling or wall mounted fan coils, radiant floor systems, and radiant ceiling panels (Chiltrix 2019) (Frontier Energy 2019). In addition to enhancing the current CBECC-Res modeling of AWHPs, this measure proposes to recognize radiant ceiling panels as a viable hydronic system delivery option and adds necessary HERS verifications.
As an all-electric HVAC equipment type, AWHPs can play an increasing role in supporting the state of California’s movement to a low-carbon future. Since all refrigeration components are contained in the outdoor unit, AWHPs have significant advantages in terms of precise factory refrigerant charging and avoidance of any field refrigerant connections which are more prone to refrigerant leakage issues and potential contamination or mischarge when future refrigerant servicing or recharging is needed. The factory charged and sealed refrigeration system has long term performance advantages over the lifetime of the equipment, with these benefits ideally recognized in future updates to the Title 24, Part 6 Standards relative to field charged systems. Since the refrigerant is fully contained in the outdoor unit, this product type is also more amenable to converting to low global warming potential (GWP) refrigerants which may not be appropriate for indoor use (such as flammable refrigerants). Additionally, hydronic AWHP systems coupled with a supplemental space conditioning storage tank provides for the opportunity to integrate thermal storage for off-peak charging during times of excess photovoltaic or wind energy which thereby can mitigate evening peak electrical demands. All of these beneficial attributes contribute to interest in this technology as an increasing viable HVAC solution in the years ahead.
The proposed change for the 2022 code cycle is to improve the existing AWHP space conditioning modeling algorithm currently used in the CBECC-Res compliance software. The existing algorithm was added in 2016 as an interim method to allow the technology to be explicitly modeled within Title 24, Part 6. The current software implementation is overly conservative and represents a significant barrier to the technology’s advancement in the marketplace.
Extended field monitoring has been undertaken over the last five years at a research lab house located in Stockton, California. Data has been collected for multiple AWHPs coupled with radiant ceiling panels for thermal delivery to the house. The collected data from the site represents the most detailed California specific residential AWHP dataset and forms the basis of the proposed enhanced AWHP modeling algorithm. The findings from the first few years of monitoring can be found in two technical reports hosted at the Emerging Technology Coordinating Council (Frontier Energy 2018) (Frontier Energy 2019). These two reports provide significant detail on the basis of developing this code change proposal.
The measure is limited to single phase residential AWHP equipment used in single family and low-rise multifamily building types, as well as additions and alterations in all 16 California climate zones.
New verifications are necessary to verify AWHP and radiant ceiling panel installation. For the AWHP, the verifications are simple with the HERS Rater (or possibly building official) required to verify make and model number and confirm whether the installed system has a fixed or variable speed compressor(s) and whether the information is consistent with the CF2R compliance documentation. For the radiant ceiling panels, the HERS Rater shall verify that installed panel area is consistent with the compliance documents. If the CF2R specifies a “hydronic delivery in conditioned space” radiant ceiling panel installation, then the following must be HERS verified:
- Prescriptive ceiling insulation levels are installed;
- Ceiling quality insulation installation (QII) requirements are met; and
- All hydronic piping shall be properly insulated and all piping supplying the radiant panels be in conditioned space (excluding up to ten feet of piping from the outdoor unit to conditioned space).
Radiant ceiling panels are a delivery option that is not common in this country, especially in residential applications. However, these systems have been popular in Europe for decades. The popularity of radiant floor heating in the United States has increased since the 1980s, particularly in heating-only climates and high-end custom homes. Radiant ceiling panels offer advantages over floor delivery in that floor surface coverings do not impact heat transfer and ceiling panels can also be retrofitted to existing homes. Cool ceilings also improve summer comfort by transforming the normally warm ceiling surface (from mid-day attic roof heating) into a cooled radiant surface. Despite these benefits, radiant ceiling systems continue to face initial market entry barriers in the United States. The greatest of these barriers has been the risk adverse nature of the construction industry and homebuyer market. Although concerns about panel leaks may be an immediate reaction, sprinkler systems and plastic potable attic water piping systems are no different in terms of risk. Panels offer the potential for condensation from the panel surface if the surface temperature falls below the indoor dew point temperature for an extended period. In California’s dry climate, this is a rare condition. Nevertheless, production builders would likely be initially cautious in this regard and may consider the need for supplemental dehumidification. Appendix G provides some additional background information on the condensation issue and advocates for not requiring any additional radiant panel supplemental dehumidification energy given the dry California climate and rare need for dehumidification.
Software enhancements would be required to modify the existing AWHP modeling algorithm. Given that the AWHP technology currently has very limited market share in the California residential sector, the Statewide CASE Team proposes an easy to implement modeling approach within the CBECC-Res compliance software at this time. The simplified approach proposes applying fixed heating and cooling energy savings factors to the hourly CBECC-Res calculated energy use of a prescriptive standard air source heat pump (minimum efficiency heat pump meeting the Federal Standards).
The proposed software approach is recommended for several reasons. First, the Energy Commission’s software team’s resources are limited and a measure such as AWHP (with limited market penetration) is lower on the priority list as a software enhancement need. Second, the Energy Commission’s 2019 approval of variable capacity heat pump compliance credits through the performance path utilized a similar approach given the observed variability in field performance due to a variety of factors (California Energy Commission 2019). Third, the currently limited number of residential AWHPs listed in the MAEDbS suggests that if the technology gains traction in the years ahead, product offerings and participating manufacturers may change significantly. If that is the case, it would be prudent at that time to revisit AWHP modeling and better reflect the available products and observed installation configurations.
 The DHW performance of AWHPs is currently recognized in CBECC-Res. The Statewide CASE Team is not proposing any changes to the existing DHW methodology in the compliance software at this time.
 QII requirements ensure that the insulation has been properly installed, with minimal installation defects.
Measure proposals, supporting documents, and other outside references will be made public as they become available.
- Final CASE Report – Single Family Enhanced Air-to-Water Heat Pump Compliance Options
- Draft CASE Report – Single Family Enhanced Air-to-Water Heat Pump Compliance Options
- Notes – Oct. 10 – Single Family HVAC Utility-Sponsored Stakeholder Meeting
- Presentation – Oct. 10 – Single Family HVAC Utility-Sponsored Stakeholder Meeting
- Single Family Enhanced Air-to-Water Heat Pump Compliance Options Submeasure Summary
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The Statewide CASE Team values input from all stakeholders engaged in the Title 24, Part 6 code change process. We encourage the open exchange of code change comments and concerns.
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