Reducing carbon emissions has become an operating model for homeowners, building owners, companies, organizations, governments, and moreover the past few decades. We hear about environmental strategies, such as net-zero energy and sustainability, daily as we go about our business.  

Net-zero energy strategies traditionally employ renewable energy products to reduce energy use and carbon load. Biodiesel, in particular, has become a heating industry norm. According to the National Biodiesel Board, biodiesel offers the following benefits when compared to petroleum-based fuel:

• Reduces life cycle greenhouse gases by an average of 74%;

• Lowers hydrocarbon emissions by 67%; and

• For every unit of fossil energy it takes to produce biodiesel, 4.56 units of renewable energy are returned — the highest return of any fuel in use in the U.S.1

Typically, biodiesel is blended into the traditional No. 2 heating oil for oil-burning appliances at low percentages to maintain the equipment’s reliable operational performance. Research has been conducted and progressed to determine an acceptable level of blending to mitigate concerns with the safe operation of the equipment.   

In 2007, UL conducted a fact-finding investigation for the National Biodiesel Board. The fact-finding investigation developed data on:

• Compatibility of B5 biodiesel2 with materials employed in residential oil-fired heating appliances3 (including metals, refractory/combustion chamber liners, thermosets, thermoplastics, and elastomers);

• B5 biodiesel combustion characteristics; and

• General performance comparisons between B5 biodiesel and No. 2 fuel oil.4

The investigation focused only on examining relevant safety and performance criteria for B5 biodiesel. It did not address biodiesel blends over 5% or blends with other ASTM D396 fuel grades (Nos. 1, 4, 5, or 6), nor did it investigate blending with other similar Class II combustible liquids, such as ASTM D975 diesel fuel oils or ASTM D3699 kerosene.

ASTM D6751 specification biodiesel fuel blend stock (B100) was used to create the B5 biodiesel blend used for most tests. An exception to this included a synthetic aggressive fuel denoted as “UL B5 biodiesel.” This fuel blend was used for gasket and seal testing and endurance testing and was chosen for its more aggressive properties (increased acidity and moisture content). The No. 2 fuel oil used to create the biodiesel test fuels was in accordance with ASTM D396. The investigation did not consider any base fuels or biodiesel blends outside of the aforementioned specifications.

In general, test samples were considered and selected to represent fuel oil heating systems available in the field as of 2007. The test program was developed based on the safety requirements contained in UL 296, the Standard for Oil Burners; and UL 157, the Standard for Gaskets and Seals, for No. 2 fuel oil-burning components and appliances. Using B5 biodiesel fuels, components, and/or heating appliances intended for use with No. 2 fuel oil yielded acceptable results when tested as required in the scope of the UL standards. The results observed also typically showed lower NOx levels when firing B5 biodiesel compared to firing No. 2 fuel oil. Notably, the duration of the testing contained in the aforementioned UL standards is limited and does not reflect the equipment’s useful life span.

Utilizing the data from the UL fact-finding investigation along with other testing, data, and experience from the fuel oil heating industry: the ASTM Committee D02 on Petroleum Products, Liquid Fuels, and Lubricants, announced in Oct. 20085 that ASTM D396-08b (specification for fuel oils used for home heating and boiler applications) was revised to include an allowance for up to 5% biodiesel.   

ASTM D396 was updated in 2016 to include an additional biodiesel-related fuel grade for blends consisting of up to 20% biodiesel. At the request of burner manufacturers, UL developed a preliminary draft of biodiesel blend requirements to be added to UL 296. To ensure maximum consistency with traditional fuel oil requirements, the new requirements did not go into a separate standard as other biodiesel or ethanol standard requirements have. Instead, the base standard persists for all oil burners. The B20 specific requirements were placed in a new normative supplement to UL 296. By using this method, requirements were only added or modified when specific requirements for B20 blends were required. While this approach differs from traditional UL standards, international standards have successfully applied this format for many years. Structuring the requirements in this way does have drawbacks: The requirements are located in a separate part of the standard, and it requires a reader to reference two different sections (the body of the document as well as what modifications those requirements need). However, this ensures consistency for the construction and test requirements across all fuel types. In general, the construction and test requirements at the burner level are the same across the fuel used. Additional requirements included component references, corrosion resistance, cold oil, and markings.

When using B20 blends with traditional on-road products (fuel pumps, metering devices, etc.), certain material-level potential hazards were identified. As a result, a test fuel was developed to simulate accelerated aging for gasket materials. This test fuel has a slightly higher biodiesel content of 25% with additional decanoic acid for a final 1% acid number. After immersion in this fluid for 1,000 hours, the gasket’s physical properties are checked for volume change and any effects due to compression. This ensures gasket materials will not deteriorate and leak when exposed to a nominal B20 blend. Due to the potential for galvanic corrosion, which could result in fuel leakage, the maximum amount of lead and zinc in alloys is identified.

FIGURE 1 - Cloud point properties

The general properties of the biodiesel blends depend on the feedstocks used to make the biodiesel. These biodiesel feedstocks generally have higher cloud points than traditional fuel oils. Palm oil and tallow feedstocks result in biodiesel with the highest cloud points6,7. The National Renewable Energy Laboratory (NREL) has an ongoing program that tracks the properties of B100 sold in the U.S. One of the properties it identifies is the cloud point of the biodiesel. In 2017-2020, the average cloud point has been near the freezing point of water (µ=0.75-1.0°C, σ=3.7-3.8)8, see Figure 1 for details.

“Cloud point” refers to the point at which hydrocarbons (HCs) in the fluid start to crystalize. This may lead to reduced flow through filters and can also affect oil atomization and ignition. Tests have existed in UL 296 since the 1960s to ensure burners would consistently ignite when fed cold fuel oil and ignition components at reduced voltage. The fuel oil fed to the burner should have a temperature of 35 ±5°F (1.7 ±3°C). The current requirements in the standard identify that the cold oil tests should be conducted with a 20% biodiesel blend where the biodiesel component had a cloud point of not less than 3°C. This specification was confirmed to be commercially available. It would represent a condition that could exist with an oil heating system where the tank is not located in a conditioned space or with integral preheaters. Note that commercial products can be added to the fuel oil to improve the cloud point and other cold properties of the fuel oil.

The fuel oil specifications: ASTM D396 (fuel oil) and ASTM 6751 (biodiesel fuel blend stock) do not currently specify a maximum cloud point for the fuels. ASTM D396 provides guidance with a 10th percentile minimum temperatures table for several geographical areas in the U.S. The determination of the fuel oil properties are left to the ultimate users and the fuel oil distribution/delivery companies.

UL 296 was also updated to clarify the burner should be marked with each type of intended fuel and the volumetric flow rate, with a resolution of 0.1 gallons per hour. The flow rate is marked on the ultimate end appliance (furnace, boiler), and the marking should follow with the rate indicated on the burner. In some standards, the end product appliance markings may indicate the rate be identified in Btu/hr. In addition to clarifying that rates should follow the burner and be marked in volume, there is an opportunity to update these standards further to address modern applications. Biodiesel has a lower energy content than No. 2 fuel oil, so a blend of the two fuels will have a slightly lower heating value than traditional fuel oil. Because of this property, if the same burner is used and coverage expanded, there should be no need for retesting to add a B20 fuel. However, that assumption will need to be confirmed by the certification agency once the specifics of the product are detailed.

UL 296 was updated, knowing that some components standards used in those systems did not contain B20 requirements. The applicable standards were referenced where it was possible to reference B20 component requirements. Where no specific component requirements existed, UL 296 identifies the minimum requirements for metallic and nonmetallic materials, as noted previously. These requirements are included in the standard for two reasons:

1) Provide flexibility to the end product manufacturer on the usable components; and

2) Indicate the specific requirements to the component manufacturer.

This provides the most flexibility for the supply chain because it ensures, regardless of who is conducting the evaluation, the same requirements are applied. This parallel approach also allows burner manufacturers to leverage their existing supply chain, potentially without changing the parts or component manufacturers. Let’s look at an example of this with regards to an integral oil pump. Consider the situation where a manufacturer is producing a burner that is rated for No. 2 fuel oil and wanting to expand the rating for that product to include B20. They may have a stock of existing pumps or, due to supply chain concerns, cannot source an oil pump that is rated to B20. The requirements in the end product standard allow that manufacturer to evaluate the materials used in the current pump and show they are suitable for the B20 blend.

Component standards often have limitations on what can be tested. For example, temperature rise on specific electrical components cannot be identified until the orientation and ultimate electrical enclosure are identified in the end product. However, there are ways to establish some conditions with component level testing. Going back to our electrical component example, a minimum enclosure size, ambient temperature, or maximum component temperature not to be exceeded can be communicated to the end product engineering staff through what UL terms the conditions of acceptability (CofAs). Use conditions outside the component-level standard can also be communicated using a CofA. Suppose a component manufacturer knew it wanted its product to be used in a B20 system. In that case, the company could conduct and document the material testing specified in UL 296, and traceability would exist that the construction complied with the end product requirements. This would permit the component manufacturer to have access to a greater market share.

UL 296 will allow for the use implementation of fuel oil heating using biodiesel blends up to 20% (i.e. ≤ B20) in the U.S. Currently, there are limited requirements for products in Canada. The National Resources Canada (NRCan) website9 recommends that if an oil heater is to be used with a blend greater than 10%, the appliance’s manufacturer is to be consulted on the suitability. There are ongoing discussions to incorporate biodiesel requirements into the CSA B139 code for Installation Code for Oil Burning Equipment. At this time, the Canadian codes and appliance standards do not contain specific requirements for biodiesel blends.

At the last standard technical panel meeting for UL 296, discussions began to identify requirements for burners to be tested with fuels up to B100. The requirements are following the approach as used for the B20 as explained before: Gasket and seal requirements will need evaluation during an accelerated aging program with a representative test fuel. Cold oil tests will also need a revised test fluid that represents expected operating conditions. B100 only has 85% of the heating value of traditional fuel oil, which can affect the overall performance of the burner over a range of blends. There has been discussions on the inclusion of a test for the burner to ensure continued operation over an intended range of blends. Additional end product appliance tests may be required to ensure a burner properly fires in a combustion chamber over the range of fuels. Biodiesel does not burn with the same luminosity as traditional fuel oil, which can limit the technology used for flame detection and/or proving. Ignition and combustion tests will confirm if the components of the burner’s management system operate as required. The requirements have not yet been proposed to the standard, partly due to the fuel oil specifications not currently covering blends greater than B20.

All UL standards for the U.S. follow the ANSI standards development process. As part of this consensus process, there is an open public comment period when revisions are proposed to UL standards. During the comment period, anyone can provide input on the proposals via UL’s Collaborative Standards Development System.10 If the proposed revisions do not sufficiently address the safety hazards or language in it needs clarification, UL would welcome feedback from ABMA members.

There continues to be innovation in the industry for a variety of fuels. Hydrogen, both as fuel or blended with natural gas, is currently being implemented. Installations in both Europe and North America are exploring the suitability of this fuel when introduced to a traditional gaseous supply system. There continues to be a need to understand the impacts fuel changes could have on the performance of currently installed appliances. In the past, flammable gases generated from a biological decomposition process were seen as a waste product and burned via an atmospheric flame discharge. There are challenges to using this gas as a fuel source in a heating appliance: water content, impurities, and lower Btu content. Technology exists to help address some of these issues. Still, updates to standards are needed to ensure the compatibility with materials and that system safety and controls function as intended.

Updates to UL standards will continue as new technologies are developed and implemented. As the combustion industry continues moving forward with alternative fuels, UL will continue to ensure the requirements in the standards address the evolving safety hazards and help to make the world a safer place to live.


“B5 biodiesel” is comprised of 5% by volume ASTM D6751 (Standard Specification for Biodiesel Fuel Blend Stock (B100) for Middle Distillate Fuels) and 95% ASTM D396 (Standard Specification for Fuel Oils) No. 2 fuel oil.  

UL defines residential oil-fired appliance as an appliance with volumetric fuel flow burn rate of less than 3 gallons per hour (gph).

”No. 2 fuel oil” is as defined by the Standard Specification for Fuel Oils, ASTM D396.,

6 Alleman, T. L., McCormick, R. L., Christensen, E. D., Fioroni, G., Moriarty, K., & Yanowitz, J. (2016). Biodiesel handling and use guide (No. NREL/BK-5400-66521; DOE/GO-102016-4875). National Renewable Energy Lab.(NREL), Golden, CO (United States).

7 Knothe, G., Krahl, J., & Van Gerpen, J. (Eds.). (2015). The biodiesel handbook. Elsevier.,,,