Central Valley Research Homes: Phase 2 Assessment of Residential Radiant Ceiling Panel Space Conditioning Systems

This project builds on the 2016 Emerging Technologies report (https://www.etcc-ca.com/reports/field-assessment-residential-radiant-ceiling-panel-space-conditioning-systems) based on testing two types of radiant ceiling panels coupled with air-to-water heat pumps (AWHPs). Performance in the Stockton, California residential test site was compared to two separate high-quality reference forced air HVAC systems with ducts in conditioned space. Phase 1 of the project covered summer 2015 and winter 2015/2016 with the report available on the Emerging Technology Coordinating Council (ETCC) website.  Key objectives were to evaluate comfort performance, energy use, panel thermal performance, and develop an industry best practices guide. Phase 2 of the project detailed in this report spans the period from summer 2016 through summer 2017 and evaluates the comfort performance and energy usage of additional AWHPs and delivery configurations. Additionally, Phase 2 evaluated dehumidification using an integrated hydronic fan coil, impacts of that fan coil on setback recovery, how radiant zoning impacts comfort, and how ceiling fan operation affects panel cooling performance.

Phase 2 Key Findings

1. Panel condensation risk:

Monitored panel surface temperatures and indoor dewpoint temperatures were compared to evaluate the risk of panel condensation. No condensation was observed, and no panel surface reached or went below the dewpoint temperature during the testing. Two supplemental dehumidification methods were evaluated to explore the performance impacts: In Phase 1, a free-standing dehumidifier and in Phase 2 a hydronic fan coil coupled with the AWHP. On average, the dehumidifier drew 470 W, using 11.2 kWh/day to remove 2.6 gallons of moisture per day. The fan coil drew 10 W, using 0.01 kWh/day to remove 0.1 gallons/day[1].

The dehumidifier overdried the indoor air while fan coil operation resulted in slightly higher humidity than the reference systems, although still within acceptable ranges. This demonstrated that for those cases where dehumidification may be prudent, sufficient dehumidification can be achieved without the significant energy use associated with the free-standing dehumidifier.

2. Thermostat setback recovery (cooling mode only):

The ability to recover indoor temperature from setback was evaluated by not operating each system from midnight until 5 PM and then comparing how quickly indoor temperatures returned to setpoint. The Phase 1 report finding of slow radiant recovery rates (51 minutes per °F drop in indoor temperature) could be improved by use of the dehumidification fan coil operation in Phase 2 (recovery rate was three times faster).

3. Compare radiant to conventional system comfort:

Air Conditioning Contractors of America (ACCA) Manual RS comfort standards were used to quantify and compare comfort across system types. For Phase 2, zoning was added to the radiant system. From the perspective of labor and equipment, adding zoning is a relatively minor cost. With zoning, the Radiant system’s comfort performance under the Manual RS criteria became equivalent to the finely balanced Reference air conditioner system and superior to the zoned Reference heat pump system.

4. Energy use comparison between Radiant and Reference system configurations:

Phase 2 testing revealed much about the energy use impacts of various elements and components of the Radiant systems. The standby energy use of the single speed Aermec AWHP was unexpectedly high, while the standby energy use of the variable capacity PHNIX AWHP was the lowest of any system tested to date. Buffer tank sizing had a large impact on system energy use, as the undersized buffer tank used in Phase 2 caused excessive short cycling of the single speed AWHP.

The radiant system using the variable capacity AWHP without a buffer tank performed best for energy use, achieving levels comparable to the Reference heat pump.

5. Ceiling fan impact on panel cooling performance:

The impact of ceiling fan operation on panel performance by comparing two summer days with similar weather conditions: one day with and one day without ceiling fan operation. Panel cooling delivery rate from these two days was improved by 7% for the xLath panels and 24% for Ray Magic panels, with negligible resulting impact on overall radiant system EER. The Ray Magic panels exhibit superior heat transfer characteristics to the xLath panels and operate at lower surface temperatures for the same inlet water conditions, and therefore derive greater benefit from the addition of forced convection via ceiling fans.

Key Project Takeaways

• Continue Stockton laboratory house testing of additional AWHPs to identify and reduce existing technical market barriers. There are opportunities to test additional equipment, panels, and control strategies at Fidelia, which are proposed in detail in the Recommendations section. These tests will be conducted using the CEC and AHRI-certified Chiltrix CX34 AWHP.
• While California has taken a major step in requiring AWHPs to be tested and certified under Title 20, there continue to be no minimum performance requirements. Additionally, there are no standards or testing and certification requirements in the United States for radiant panels. It is difficult to locate publicly available, accurate data and compare performance of different brands.
• The radiant systems tested in this project can be successfully installed in occupied homes. A field study comparing the energy costs for a house with ducts in attic to be retrofitted with a radiant delivery system will be undertaken in 2019 in Sonoma County under an Energy Commission funded project. This project will document system costs, occupied home performance, and customer satisfaction at multiple sites.
• It is recommended that the best practices guide from Phase 1 be revised to reflect the results of Phase 2.