NRG Awareness - How to seamlessly blend a hybrid 4-pipe gas boiler and a Heat Pump This system shows one example of a hybrid interlink between a four-pipe gas boiler with Priority
Save on Monthly Utility Costs with a Hybrid HVAC System - Bob Vila
Hybrid Air Source Heat Pumps
REHVA Journal Hybrid heat pumps - saving energy and reducing carbon emissions
Decarbonising the Grid: Why Hybrid Heat Pumps Aren't The Answer - KHP