PROJECT GOALS

Natural gas is a domestically available resource. The U.S. 

Department of Energy (DOE) supports natural gas vehicle 

and infrastructure R&D through its FreedomCAR and Vehicle 

Technologies Program to help the United States reduce its 

dependence on imported petroleum and to pave the way to a 

future transportation network based on hydrogen. Natural gas 

vehicles can also reduce emissions of regulated pollutants 

compared with vehicles powered by conventional fuels such 

as gasoline and diesel.

The goal of this project was to evaluate the safety implications 

of refueling natural gas vehicles at home with a home refueling 

appliance. Widespread use of natural gas vehicles has been 

hindered by limited availability of refueling stations. Home 

refueling could change this by making convenient natural gas 

refueling available to everyone who has residential natural gas 

service. The home refueling concept also may be applicable to 

the hydrogen-powered vehicles of the future. 

FUELMAKER’S HOME REFUELING APPLIANCE

Toronto, Canada-based FuelMaker Corporation has manufactured 

compressed natural gas (CNG) vehicle refueling appliances 

for more than 15 years. These appliances are designed to provide fast-fill or time-fill (slow-fill) CNG refueling for small 

fleets such as school buses and government vehicles. FuelMaker’s new home refueling appliance—known as Phill—brings 

CNG refueling capability to the homes of individual drivers 

(Figure 1). 

Connected to standard residential 

natural gas and electric service, 

Phill can be installed inside a garage 

(Figure 2) or outdoors. It provides 

automated time-fill CNG refueling 

at a rate of approximately 0.4 gasoline gallon equivalents per hour, 

appropriate for overnight refueling 

of commuter vehicles. 

In 2005, FuelMaker will begin selling Phill in California for approximately $3,400 per unit, and there are 

plans to market it in Canada and 

Europe as well. Phill is certified and 

listed by CSA International (a group 

that includes the Canadian Standards Association, CSA International, CSA America, and others). It is labeled with a safety 

listing similar to that used for other natural gas household 

appliances such as water heaters and clothes dryers. 

SAFETY EVALUATION 

A team was assembled by DOE’s National Renewable Energy 

Laboratory to conduct the safety evaluation of Phill. Various 

investigations and analyses were used to generate data for 

accurate safety incident probability calculations. References 

and databases pertaining to component failure and human error 

statistics were used. CNG vehicle refueling experience was 

surveyed to determine which kinds of accidents have occurred 

in the past. CNG vehicle fuel system designs were analyzed to 

support estimation of gas releases from vehicles. Experience 

with water heaters and other garage-installed natural gas 

Evaluating the Safety of a Natural Gas Home 

PROJECT IMPACT Refueling Appliance (HRA)

This project evaluated the probability of safety incidents 

related to use of FuelMaker Corporation’s natural gas home 

refueling appliance, Phill. The results indicate that an individual is 10 times more likely to be struck by lightning than 

to experience a safety incident resulting from normal use of 

Phill. The safety evaluation produced an added benefit—

FuelMaker incorporated suggestions from the study into its 

final design of Phill. Home refueling has the potential to 

increase the availability and convenience of natural gas 

vehicle refueling infrastructure. It is anticipated that this 

would lead to more extensive use of natural gas vehicles, 

resulting in reduced U.S. petroleum consumption.

Figure 2. Phill Provides CNG Refueling inside Residential Garages NREL/PIX 13714

Figure 1. FuelMaker’s Home 

Refueling Appliance, Phill 

 NREL/PIX 13800
appliances was considered. Thirty-three residential garages 

were evaluated to determine design, construction, and air 

infiltration characteristics. Computer modeling was used to 

calculate garage gas concentrations resulting from possible gas 

leak scenarios. In some cases, probability estimates were based 

on failure or error statistics for generically similar components 

and situations. Data from all these sources were used to generate the calculations and probability results discussed below.

The evaluation employed standard risk-assessment methods. 

First, scenarios that create potential safety risk were identified. 

These risk scenarios were grouped into the following categories:

• Equipment failure (e.g., gas leaks from Phill)

• Human error (e.g., driving out of the garage with 

the refueling nozzle connected to the vehicle)

• Misuse (e.g., attempting to use Phill to inflate a 

swimming pool toy)

• Maliciousness (e.g., a disgruntled neighbor shuts off 

Phill’s gas supply)

• External event (e.g., a vehicle hits Phill).

Next, the probability of each risk scenario occurring was calculated. Finally, the probability of each risk scenario resulting in a 

potential safety incident—a structure fire or deflagration—was 

calculated. The following is an example of these calculations. 

The annual probability of a vehicle hitting Phill and breaking 

the gas line was calculated to be approximately 1 in 1.4 million. 

The probability of a structure fire due to this scenario occurring 

was calculated to be approximately 1 in 17. Thus, the overall 

probability of a structure fire occurring because of a vehicle 

hitting Phill and breaking the gas line was calculated to be 

approximately 1 in 23 million 

(1 in 17 multiplied by 1 in 

1.4 million).

RESULTS AND CONCLUSIONS

Table 1 summarizes the calculated probabilities of safety 

incidents related to using Phill 

inside a residential garage. 

It was determined that most 

misuse incidents were more 

likely to occur in the first year 

after installation of Phill, 

so the misuse probabilities in 

Table 1 are probabilities that 

the incident will occur in the first year after installation. The 

non-misuse probabilities in Table 1 are probabilities that the 

incident will occur in any year after installation of the unit. As 

Table 1 shows, the annual probability of a deflagration due to all 

non-misuse failures is 1 in 7 million. An individual is 10 times 

more likely to be struck by lightning than to experience a nonmisuse deflagration. Even if intentional misuse (e.g., attempting 

to use Phill to inflate a pool toy) is considered, an individual is 

still more than twice as likely to be struck by lightning than for 

Phill to cause a deflagration. Structure fires resulting from use 

of Phill are even less likely. 

The safety evaluation produced an added benefit: FuelMaker 

incorporated suggestions from the study into its final design of 

Phill. For example, in an early design, the air flow that cools 

Phill’s compressor was fed from and discharged to the inside of 

the garage. The safety analysis revealed that this design limited 

beneficial ventilation if a gas leak occurred. Based in part on 

this analysis, FuelMaker refined the installation instructions to 

ensure that cooling air is discharged outside of the garage. 

RELATED PUBLICATIONS AND WEB SITES

The report, Safety Evaluation of the FuelMaker Home 

Refueling Concept, which describes the safety evaluation of 

Phill in detail, is available from the Alternative Fuels Data 

Center at www.eere.energy.gov/afdc. Hard copies are available 

from the National Alternative Fuels Hotline at 1-800-423-1363 or 

hotline@afdcweb.nrel.gov. The evaluation was part of DOE’s 

Natural Gas Vehicle Technology Forum. For more information, 

visit www.nrel.gov/vehiclesandfuels/ngvtf. For more information 

on FuelMaker and Phill, visit www.fuelmaker.com. 

Send Questions or Comments to

Richard Parish

National Renewable Energy Laboratory

1617 Cole Blvd., MS 1633

Golden, CO 80401

Phone: 303-275-4453

Fax: 303-275-4415

E-mail: richard_parish@nrel.gov

Neither the United States government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific 

commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States 

government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or any agency thereof.

Produced by the 

National Renewable Energy Laboratory (NREL)

NREL is a U.S. Department of Energy National Laboratory

Operated by Midwest Research Institute • Battelle

DOE/GO-102005-2084 ■ April 2005

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WHAT’S A DEFLAGRATION?

A deflagration is a rapidly moving flame 

front, which creates high pressure when 

it occurs in an enclosed space such as 

a garage. Deflagrations range from small 

flash fires, causing no property damage, 

to explosions, which may cause significant damage. The severity depends on 

how much of a flammable gas-and-air 

mixture has accumulated. This study 

considered the probability of scenarios 

such as gas leaking, accumulating in a 

garage, and being ignited (e.g., by an 

electric garage door opener) to cause a 

deflagration.