Expert Advice on Green Buildings

Electric vs Gas Heat: What Is the Best Choice?

 
Question:

Diane asks: We are remodeling and want to improve energy efficiency. We are improving insulation and also replacing heater. We are debating merits of Mitsubishi Comfort (electric) http://www.mitsubishicomfort.com/ versus gas furnace. How do we go about determining which is most efficient?

Answer:

Hi Diane, thanks for the question. It sounds like you’re in need of a home HVAC system in an area that still has a cold climate (based on the mention of the gas furnace). So what I’ll be comparing and contrasting here are two systems: Central AC with a gas furnace (probably the most common home HVAC system) versus a ductless AC multi-zone system with electric heat.

Central AC with Gas Furnace:

The first option is relatively simple and familiar. A furnace located in the basement circulates warm or cold air throughout the house via a series of ducts and registers. This air is then returned to the furnace via more ducts and grilles. A gas-fired heat exchanger in the furnace provides heat, and a DX cooling coil (coupled to an exterior condensing unit) provides cooling. A single thermostat controls the furnace, so all rooms in the house are heated or cooled similarly.

Ductless AC Multi-Zone System with Electric Heat

These systems aren’t as widespread in the US as they are in other parts of the world. Indoor heat pump units are located in several rooms throughout the house. These units include a fan and cooling/heating coil and can be supplied with electric back-up heat in some instances. Typically for every three indoor units, you have an associated outdoor unit. This condensing unit is smaller than the one coupled to the more traditional system option (the first option described above) and typical dimensions of 30”x35”x12” wide coupled with a rectangular shape.

Each indoor unit is controlled by its own thermostat, so different spaces could have different set-point temperatures. Since ductless mini-splits have no ductwork, they don’t experience the energy losses associated with air transmission in central forced-air systems. However, during exceptionally cold winter months, the heat pump will likely not be able to keep up with heating demand, and would need to be supplemented with electric heat. And while electric heating converts electricity directly to heat, it is often more expensive than heat produced by combustion appliances like natural gas.

Seasonal Energy Efficiency Ratio (SEER), is most commonly used to measure the efficiency of a central air conditioner over an entire season. Energy Efficiency Ratio (EER) is a measure of how efficiently a cooling system will operate when the outdoor temperature is at a specific level (95°F). Both SEER and EER are simply a ratio of energy in to cooling capacity. Therefore, the higher the SEER or EER, the more efficient the system. On the heating side, annual fuel utilization efficiency (AFUE) is a measure of a furnace’s heating efficiency, in amount of fuel supplied to amount of heat delivered to the space, so higher AFUEs are also preferable.

More Research and Suggestions:

In terms of overall efficiency, both system options could be quite economical and environmentally friendly if properly designed. To further complicate your options, instead of just a split system for cooling, a traditional gas furnace could be supplied with a heat pump for added energy benefits. If you’re looking to make easy, energy-efficient choices for your home, the Energy Star website would be a great place to start. You can search for Energy Star products for the home, including appliances, building products, electronics as well as HVAC equipment, lighting and plumbing components. All of this equipment has earned the Energy Star label by demonstrating that it helps reduce greenhouse gas emissions and other pollutants through energy-efficiency without sacrificing performance, features, or comfort

Also on the Energy Star website, you can find information about federal tax credits for energy efficiency which you can use to help set targets for your new equipment’s efficiency. For central AC split systems (which both of your options could quality for) must be a minimum of SEER 16 & EER 13. Gas-fired furnaces must have an AFUE of 95% or greater.

The best way to find out what tax credits are available and compare to system costs and energy savings is to get several quotes from a local HVAC Contractor. To verify tax credit eligibility, make sure the contractor provides the Manufacturer's Certification Statement for the equipment you plan to purchase.

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Expert Advice and Comments
bkurczak's picture

remodeling and want to improve energy efficiency

Dianne,
If your goal is energy efficiency, start from the outside in, before you get to the mechanical system. Check out: http://www.buildingscience.com/doctypes/designs-that-work

These guys are smart...really smart. Lots of good info on that site.

Now, with respect to the furnace vs. mini-split system, Sarah did a great job giving you the basics. Provided you get a mini-split with a low-ambient kit (assuming your are in a cold climate), multiple zones and the control capacity to move heat energy around your house, your energy consumption will likely be less (because you can balance the energy used in the house, before having to rely on the exterior heat pump.

However, the amount of money you pay for utilities will be skewed by the (always) fluctuating cost of gas vs. electricity. There is also the question of the impact of the electricity you consume (i.e., GHG emissions). If you go electric, you are set up to fuel switch to a greener source of energy (on site PV, green power); with gas, you're pretty much stuck with gas.

For the mini-split system, you will also have to do something for ventilation in the house - consider an ERV.

But remember - it all starts with the insulation (and the windows!), so don't skimp on those items!

my 2 cents

Braden Kurczak
LEED AP BD+C
Enermodal Engineering Limited
http://www.enermodal.com