California Institute for Energy and Environment (CIEE)

Advanced, colorfully-graphic touch-screen thermostats entered the market over ten years ago, providing a much-needed improvement to the control of home heating and cooling systems. And in the last few years, climate change mitigation concerns and rebates from the Inflation Reduction Act (IRA) have accelerated heat pump installations in homes across the U.S. But the relationship of smart thermostats to typical air source heat pumps (ASHP) has been rocky at best: ASHPs with variable speed drives require a deeper integration with a thermostat, and most third-party smart thermostats have step-level staged control. Proprietary thermostats that come with the ASHP units are typically not as easy to understand and use as the popular smart thermostats on the market. Moreover, heat pumps with supplemental electric resistance heating require a different control approach that respects the lower temperature/long cycle heat transfer of heat pumps and understands the impact on the electrical grid, especially with high power peaks on cold winter mornings. This paper combines a literature review of the smart thermostat+heat pump relationship and interviews with stakeholders to outline trends, issues, barriers, and areas for further study in the relationship between variable capacity heat pumps and smart thermostats.