Dual Fuel or All Electric?

When upgrading our home to a more efficient heating and cooling system, one that incorporates a heat pump, the issue of whether to use electricity or natural gas for back-up (alternate heat) during colder winter days, became the big question.

If we lived further south in a more temperate zone, a heat pump would be the only thing needed for both heating and cooling. However, our Pennsylvania winters require homeowners to have some sort of backup heat for days and nights below freezing, that threshold being 30-degrees-F with the typical heat pump. Sure, there are heat pumps that work well in more frigid temperatures, but their cost can become prohibitive for many homeowners.

Therefore, the big question was whether to upgrade from an 80-percent to a 96-percent efficient forced air natural gas furnace, or go ‘all electric’ and switch to an electric furnace that uses resistance heat. That form of forced air heat is similar to a handheld hair dryer and isn’t known for its energy efficiency.

Our specific situation

Even though we can provide a portion of our own winter electricity with rooftop solar panels (8.97 kW system) and a Tesla Powerwall2 backup battery, even during a grid outage, the feasibility of doing so becomes more challenging during mid-winter months, when solar energy production may be blocked by snow, or Powerwall energy storage (up to 13.5 kWh is usable) may be drained.

In a worst case scenario, the grid would be down, solar production would be zero due to rooftop snow cover, and the battery would be drained. In a slightly better scenario, the back roof solar panels could be cleared of snow cover, some energy is stored on the battery, and much less electricity is needed for home heating, used specifically to operate the fan on a high-efficiency natural gas furnace, as opposed to much more for a heat pump.

Operating with electric/gas dual fuel

Over the first four months of owning a SEER 15 heat pump and 96-percent high efficiency furnace, I’ve made the following observations:

  • During summer, the heat pump is far superior to our 15 year old central AC unit in both energy efficiency and humidity control.
  • The new gas furnace uses about 15-percent as much electricity as the SEER 15 heat pump for heating, and operates for shorter periods of time, making it a much more viable option in a grid outage situation when solar production and battery storage is minimized.
  • Early results on Fall energy use (home and EV charging, mid-Oct to mid-Nov) show our electricity usage dropped 10-percent. For that same period, our natural gas usage dropped 57-percent.

What remains to be seen

  • What effect this new dual fuel mix will have on our already low electric bill and monthly natural gas budget figure over a 12-month period.
  • Where the ‘sweet spot’ is when balancing electricity and natural gas usage for the lowest heating costs, since rising natural gas prices also have an adverse effect on electricity costs from the grid.
  • An accurate R.O.I. (Return on Investment) is complicated by ever increasing energy costs and changing weather patterns.

What we do know

  • As with our 2018 and 2020 solar energy installations, it feels great to become more energy efficient and energy independent, regardless of R.O.I. considerations. Besides, our grandkids’ future is at stake, and that’s what is most important to us.
  • Going to an “all electric” heating option would still use a large amount of natural gas, since our regional grid uses over 40-percent natural gas for electricity generation, and that percentage is steadily increasing, as coal fired plants are phased out and nuclear energy wains.
  • For those accustomed to forced air natural gas heating, a heat pump provides a more moderate type of heat. While we used to drop our nighttime temperature setting to 64-degrees, our HVAC installer recommended keeping the night and day heat setting within 2 to 3 degrees of each other, so the ‘make-up difference’ isn’t so great.
  • Energy Star rated HVAC systems qualify for various tax credits, so check with a tax professional for specifics.

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