Draft Report: 

November 6, 2011

Input Rate of Pilot Burners on Gas Fireplaces 

Prepared for: 

Alex Lekov 

Lawrence Berkeley National Laboratories 

By 

Jim Mullen 

EER Consulting, LLC 

 
General

The following ignition system discussion is relevant to Hearth Products that are usually certified to ANSI 

Z21.50 Vented Gas Fireplaces (Decorative) and Z21.88 Vented Gas Fireplace Heaters. While it is not 

mandatory that this equipment be certified to these standards nearly all units are. 

In comments filed with the Department of Energy (DOE) on March 21, 2011, the HPBA (Hearth, Patio 

and Barbecue) Trade association states that Decorative vented gas fireplaces are 70% of all vented gas

fireplace sales. 

Requirements

The respective certification standards for these appliances require a gas Ignition system which is 

designed to ignite and reignite the main burner. Based on ANSI Standard Z21.20, Automatic Electrical 

Controls for Household and Similar Use, such systems are required to: 

a. Ignite gas at the main burner or gas at the pilot burner so the pilot can ignite the main 

burner 

b. Prove the presence of either the ignition source, the main burner flame, or both 

c. Automatically act to shut off the gas supply to the pilot and main burner, when the 

supervised flame or ignition source is not proved. 

The ANSI Z21.50 and Z21.88 standards include both requirements for construction of the ignition system 

and performance tests based on Standard Z21.20. The ignition system and its’ components generally 

must conform to the requirements of, or be certified to, the requirements of Z21.20 in order to obtain

certification of the appliance to the Z21.50 or Z21.88 Standards. 

Units Not Requiring a 120 Volt Power Source

The basic ignition/flame safety system is generally referred to as a Millivolt, Thermopile or standing pilot 

control. Under Z21.20, it meets the definition of a Continuous Pilot with a pilot. When placed in 

operation, it is intended to remain ignited continuously until it is manually interrupted. The system

generally consists of: 

a. A suitable automatic gas valve 

b. a pilot burner assembly with pilot burner, spark electrode, thermocouple and 

thermopile components 

c. a Piezo-electric electric generator 

d. a high temperature limit 

e. an electrical control switch to turn the main burner gas on or off. 

 The automatic gas valve performs several functions: It starts and stops the gas flow. It regulates the 

outlet pressure of the main burner gas. And it contains a manual knob with three selectable positionsOff, On and Pilot. 
The pilot assembly generally includes a spark rod, pilot burner, thermocouple and thermopile. The 

spark electrode is used to light the pilot flame. The thermocouple is used to monitor the presences of 

adequate pilot flame. The Thermopile is a number of electrically connected thermocouples in a 

common housing in the pilot flame. The heat on the thermopile generates a small amount of electricity, 

sufficient to power the operator in the gas valve. The voltage generated is small, around 750 MilliVolts. 

Pilot Input Rates

The gas Input to the pilot is primarily a function of the pilot orifice, gas valve pressure drop and gas line 

pressure. With the move to conserve energy over the past 10-20 years, pilot input has generally been 

reduced. The pilot input on fireplace units, particularly the decorative units, may be considerably higher 

in order to provide proper operation of the thermocouple and thermopile under varying field 

conditions. 

While no published literature documenting a large number of input rate measurements has been 

located, one field measurement indicates a nominal rate of 1500 Btuh. A range of inputs can be 

estimated from several sources. 

 Literature on the NRCan website1 reports the following: “In many gas fireplaces, a pilot light ignites the 

main burner as the unit is turned on. A pilot light is a small gas flame that, on its own, can consume from 

600 to 1500 Btu of gas per hour . . . “. The Duke Energy website2 gives an estimate of approximately 

900 Btuh. 

One manufacturer’s literature for the pilot assemblies recommended for use with thermopile gas valves 

indicates “nominal” rates of 500, 1000, and 1500 Btuh as standard offer product. A second 

manufacturer indicates 716 Btuh is the “nominal” input rate. However, the manufacturers do not define

several variables that were used to determine “nominal” flow. 

Examining the Flow equations for an orifice, the flow of gas in cubic feet per hour is proportional to the 

discharge coefficient of the orifice(Cd), the diameter of the orifice (D), the specific volume of the 

gas(V’n) and the pressure drop(dp) across the orifice. Specifically, the flow is calculable as 

CFH=358.66*Cd*D^2*(V’n*dp)^.5. 

Using nominal values of .7 for Cd, and V’n for natural gas at 7.5 inches water pressure drop, the flow

equation becomes CFH=3236*D^2. Using this as the basis for a standardized “normal” flow, the Two 

manufacturers supply an orifice of D=.020 inch for natural gas and the third manufacturer uses D=.018 

 

1

http://oee.nrcan.gc.ca/publications/infosource/pub/home/all_about_gas_fireplaces_chapter3.cfm?attr

=4

2

http://www.duke-energy.com/kentucky/savings/gas-usage-faq.asp
inch, resulting in inputs of 1449 and 1174 Btuh respectively. The .018 diameter is an alternate for the 

two manufacturer’s with the .02 inch as standard. The smallest natural gas orifice offered is .012 in

which would produce a nominal flow of 522 Btuh. While not exact proportional, the orifices offered for 

propane provide similar input rates to those for natural gas. 

Based on the above, it can be anticipated that the majority of units will have pilot inputs in the 1000 to 

1500 Btuh range when the pilot is burning. The main factor affecting annual energy consumption of 

these pilots is whether or not they are turned off between fireplace usages. Most installations will fall 

into one of three patterns: On continuously, on when the fireplace is in use, or on during the heating 

season, and off the remainder of the year.