The first thing to do is to make sure your existing central gas heating system is as efficient as possible.
- The efficiency of the furnace, which is rated in AFUE. 90% or higher is considered efficient, with multi-stage furnaces over 95% AFUE being best practice for gas heating.
- The condition of the existing ductwork. It is critical to make sure you properly size, install, air seal, and insulate ductwork.
- Opportunities for air sealing and insulating your attic, walls, and crawl space (as relevant and accessible) since it will be a much better investment to create a good "shell" first before exploring supplemental space conditioning.
Once you have ensured these items, then supplemental heating options can be explored.
Given the climate zone of Walnut Creek, I'd say that a ductless heat pump (DHP) system would be the best practice option.
- DHPs are the most efficient form of heating, and also can run backwards to provide air conditioning.
- Many people enjoy having the supplemental heating in their Master during Winter and really enjoy the nice side-effect of air conditioning during Summer.
There can often be utility and government rebates/financing to help decrease the upfront cost of projects like this that save energy while increasing comfort and health.
For more information:
Read "I can't afford geothermal. Is an air-to-air heat pump my best option for heating?" a Q&A answered by Randy Potter.