A Survey of the Cleanest Home Heating Options

Please click on the image to enlarge it.

Jeff Rubin

The problem of climate change is so large and complex that it’s easy to become overwhelmed. The past decade has seen great strides in reducing greenhouse gas emissions from electric generation and transportation, but heating doesn’t get much attention, even though buildings emit nearly a third of U.S. carbon dioxide.

In many parts of America, heating is our biggest opportunity to take control of our carbon footprint. The opportunity for carbon savings is greater in colder climates, but even as far south as Virginia, heat and hot water account for more home energy use than all household lighting, electronics, refrigerator, laundry, and air conditioning combined.

Let’s look at an example of a typical 2,000-2,500 square foot home, built between 1900 and today. It has a central heating oil or gas boiler (which distributes hot water) or furnace (which distributes hot air.) Our example starts out on the dirtiest side of the clean heating continuum.

Our cleanest and most affordable heating retrofit option will depend on:

• The comparison of our limited heating fuel options in any particular region;

• The practicality and affordability of cleaner heating appliances; and

• The heat-loss profile of the building we’re heating.

Let’s tackle heat loss first so that we size our heating appliances to be as small as possible in order to lower our heating appliance installation costs and minimize fuel costs over the 20- or 30-year life of the system.

Heat Loss Explained

The two variables of heat loss are geographic location and building performance. Your geographic location determines your design day, the theoretical coldest day of the year. Your design day is fixed, but your building’s performance is variable. Building performance is a measure of the rate of air leakage through the building’s exterior walls, roof, doors, windows, and other openings. Most of America’s twentieth century housing stock was designed to require pumping a lot of heat into leaky buildings, because fuel was cheap and pollution wasn’t something people were worried about.

Weatherize First

We can save money and reduce our carbon footprint by creating a continuous thermal boundary around the building with air sealing and insulation. Whether we’re weatherizing an existing structure or a new building, we seek to establish a high-performance thermal envelope before we consider adding heat with a heating appliance. The EPA provides resources to connect with state efficiency utilities and other energy efficiency program sponsors.

Now that we’ve taken the first step by reducing heat loss with weatherization, let’s explore our climate-smart options for adding heat.

Clean Heat Option 1. Heat Pumps

Heat pumps are electric appliances that achieve their efficiency by leveraging a thermodynamic cycle that works on one of three technologies: air-to-air (air source), geothermal (ground source), or new air-to-water. Each can be paired with either a ducted whole house delivery system or ductless mini-split room heaters.

Can a heat pump completely replace my oil or gas central heating system?

Generally speaking, heat pumps provide energy-efficient, affordable heating in the winter down to temperatures around 10-30°F. The performance of your particular heat pump will depend on the heating capacity of the system and your building’s heat loss profile.

The point at which a heat pump can no longer keep up with the building’s heat loss will vary by location. Our newly weatherized high-performance thermal envelope should allow us to completely replace our oil or gas heating with a heat pump in Energy Star zones 2, 3, and 4. In zones 5, 6, and 7, we will likely need some auxiliary heat for the winter’s coldest days.

Even if you can’t completely replace oil or gas heating, a heat pump can still save money and lower greenhouse gas emissions.

Heat pumps are a good choice for reducing reliance on the home’s oil or gas central heating system during the shoulder heating seasons (the fall and spring days before and after the really cold winter months). When the temperatures drop below the point where a heat pump can keep up with the heat loss, the home’s central heating system can take over. How and when we switch from the heat pump back to the oil or gas system should take place before temperatures drop to a level where the energy and cost savings performance of the heat pump is defeated.

Is a heat pump cleaner than an oil or gas central heating system?

Maybe you’ve heard that natural gas is a clean bridge fuel. In reality, natural gas is as bad as coal when it comes to total greenhouse gas emissions. By switching to a heat pump, the system runs on electricity so your exact answer will depend on the fuel sources of the electricity powering the heat pump. If your local utility generates electricity mostly from coal, your heat pump (and your Tesla) is essentially running on coal. Electric utilities are required to publish an environmental disclosure statement detailing the fuel sources they use to generate electricity.

How about heat pumps paired with photovoltaic solar panels?

Grid-connected PV solar arrays are designed to push more electricity out to the grid during the summer than the home consumes. During the winter in the northern hemisphere, the sun is too low to keep up with the household electric consumption. This means that electric consumption is cost neutral for the homeowner on an annual basis. We can also say that at worst the home’s electricity use is carbon neutral. The fuel mix composition of the electricity delivered to the home in winter will determine how close we can come to our goal of getting to totally carbon-free heating.

Clean Heat Option 2. Partially reduce fossil fuel heating by adding a pellet or wood stove

The newest EPA-approved wood stoves and wood pellet stoves are a great way to offset CO2 from the home’s central heating system, when they are installed in the room where the appliance is located. This type of retrofit is particularly well-suited if our example home is one with an open floor plan or great room.

Retrofitting a wood stove requires a hearth and clearances to all combustible surfaces as required by the manufacturer. It also requires venting into a masonry or Metalbestos chimney and adequate, convenient firewood storage.

Pellet stoves also require a hearth and adequate clearances, but a pellet stove’s exhaust can be direct-vented vertically or horizontally and has the advantage of longer run times without refueling. Pellet stoves utilize augers and other moving parts that can create noise that is bothersome to some people. Many retailers such as hearth and farm stores offer delivery of bagged pellets. When planning a pellet stove retrofit, consider pellet storage. Bulk wood pellet storage and delivery may also be an option, depending on your location and nature of your home site. Look for bulk storage solutions that allow you to fill a five-gallon bucket for ease of carrying the fuel.

Both wood and pellet stoves require ash clean out and regular chimney cleaning, and a pellet stove should have annual maintenance service.

Clean Heat Option 3. Completely eliminate fossil fuel heating with a pellet boiler or furnace 

In zones 5, 6, or 7 the only renewable fuel that can achieve the high temperatures needed to keep up with the heat loss from our example home is wood, but until recently, wood couldn’t compete with the automated convenience of oil and gas. Today, renewable wood pellet heating fuel has matured from stoves and the forty-pound bags usually used, to boilers, furnaces, and hands-free automated delivery with bulk trucks that handle the fuel pneumatically.

Upgrading from an oil or gas boiler or furnace to one that runs on clean, renewable wood pellets avoids the cost and disruption of changing the heat distribution system around the house. All the radiant floors, baseboard heaters, or ductwork can remain exactly the same.

It’s more difficult to combust a solid fuel than a liquid or gas fuel. That makes pellet boilers and furnaces more expensive than their liquid- and gas-fossil fuel counterparts. Fortunately, many states have incentives to help make pellet boilers and furnaces more affordable. Low-interest green energy loans are also widely available.

Wood pellet delivery. ©2020 Sustainable Heating Outreach & Education, Inc.

What about the climate, forest, and health effects of wood heat?

This issue is too important to settle for by following easy assumptions. One answer can be found in the article “Why Wood Heat is Essential to Carbon Sequestration” (https://www.sustainableheating.org/carbon-sequestration/):

• Heat from renewable wood pellets reduces greenhouse gas emissions by 54% compared to oil and 59% to natural gas.

• The EPA requires a wood pellet boiler to meet a higher standard for particulate emissions than oil or gas heating appliances.

• Preservation alone doesn’t work. We are currently losing over one hundred acres of forest a day to development; that’s the real threat.

• We need a mix of preservation and conservation of working forests. More than 60% of U.S. forest are privately held.

• 20% of forest harvest is sequestered for generations in our buildings, furniture, and other wood products.

• That leaves tons and tons of waste wood. It’s those markets for waste wood that drive sustainable forest operations.

Since a dead tree is going to give up its carbon anyway, what’s the best use for all that waste wood? If our example home has a heat loss profile that demands a combustion fuel other than electricity, our only options are oil, gas, or wood.

The bottom line for our example home is that the cleanest heating option will depend on the geographic location, the building’s existing heating system, available resources that might be invested in energy improvements, and a survey of which fuels are locally available, including an understanding of the fuel mix of the electric utility.

Jeff Rubin is Executive Director at Sustainable Heating Outreach & Education, a nonprofit 501(c)(3) sustainableheating.org.