Hybrid heat pumps can be a stopgap to an electric future

Electrifying home heating is critical to a net zero future, and we’re seeing a growing trend of homeowners swapping fossil-fuel powered systems for electric heat pumps. In fact, with current and proposed government programs and proper accounting of all sources of savings, the net cost of fully electrified heating in many homes can be as cheap as your Netflix subscription (case studies from my team coming soon).   

However, what if you have a newer gas furnace and an air-conditioner that needs replacement? We’re seeing some promising results from homeowner case studies showing that hybrid heating systems can fill the gap. Hybrid heating systems look the same as conventional furnace-A/C systems, only there is an air-source heat pump (ASHP) instead of A/C. The ASHP is used for heating in milder (or moderately cold) conditions and provides all of the cooling. This approach can have a much lower upfront cost compared to many fully-electrified options. A new hybrid system may only cost $1,000 to $3,000 more than conventional systems.  

The downside to hybrid heating is that it risks locking in natural gas infrastructure. For that reason, installing a new gas furnace within a hybrid system should never be the first option considered. Homeowners should always first look at fully-electrified heating or, even better, deep building envelope upgrades that nearly eliminate the need for heating and cooling entirely – as in the EnerPHit standard.  

Hybrid heating systems should only be used when these other lower-carbon options are not feasible from the homeowner’s perspective due to upfront cost or other reasons. Homeowners should also always consider lower-cost building envelope upgrades, like air-sealing or adequate levels of attic insulation.   

The conventional approach for controlling hybrid systems is to use the ASHP exclusively when the outdoor temperature is above a set-point value, and the furnace exclusively below it. At a set-point value of -6^oC, theoretical calculations estimate a reduction near 75% of the natural gas consumed for space heating (assuming recent climate data). Note that various factors constrain the set-point and this should be discussed with a qualified installer. 

Let’s look at the data from three hybrid systems in the GTHA.  They all incorporated relatively small (2.0-2.5 ton) single- or two-stage ASHPs alongside a new high-efficiency furnace. The homes ranged in age from relatively recent construction to century homes, both detached- and semi-detached. The homes each used different set-points for switching. 

We evaluated the utility bills, gas savings, and carbon reductions. We also asked about the homeowner’s experience. This is what we found: 

• The systems reduced the natural gas for space heating by 51-74%. 
• Slight utility savings may be possible today, but net utility savings (due to carbon pricing) is generally expected starting 2024/2025. 
• There were no issues with the comfort of occupants and, in some cases, improvements in comfort from the previous system.  
• Homeowners were satisfied. 

The case studies are available on STEP’s website here.   

To be clear, fully-electrified heating and deep building envelope upgrades are the key approaches for net zero. However, hybrid systems are proving an important option for some homeowners today, especially when furnaces are new and when the upfront cost of other options is a major barrier. These homeowners are reducing carbon but also “hedging their bets” as gas rates are quickly increasing. 

For policymakers, we recommend providing incentives for heat pumps that satisfy part of a home’s heating needs, in addition to incentives for whole-home heat pump systems. These examples show that smaller lower-efficiency ASHPs can create significant natural gas reductions, and a small rebate can make these systems cost competitive.  

It is still early days in the transition to zero carbon. The overwhelming majority of HVAC installations in the GTHA are like-for-like replacements of conventional systems. This commits homeowners to high-carbon heating for the lifetime of the equipment. Low-cost hybrid systems are an important tool to help prevent this in the near term.  

Lastly, when one of the homeowners in the pilot was asked why they chose a hybrid system, they said:

“I wanted to significantly reduce our household fossil fuel consumption. My goal was to do this at little to no extra cost in the long term, and demonstrate to others how feasible this is to do today without sacrificing comfort or budget.” Their retrofit, and the others like theirs, have done just that. 

Find out the full results from our case studies.  

This work is led by the Sustainable Technologies Evaluation Program (STEP) of the Toronto and Region Conservation Authority (TRCA), with funding support from TAF. Base funding for STEP projects is provided by the City of Toronto, The Region of Peel, and York Region.  Find STEP on Twitter and LinkedIn, or connect with Erik directly via LinkedIn or email.  

STEP gratefully acknowledges the insights shared by homeowners Liisa Repo-Martell and Eve Wyatt, installing contractor Imperial Energy, and the Sustainable Neighbourhoods Action Program (SNAP). The contents of this post and associated case study documents does not necessarily represent the policies of supporting agencies. Mention of vendors, trade names, and commercial products does not constitute an endorsement of products or services.