Photograph of a gas meter cubic feet readout LP or Natural Gas Pressures & BTUH per Cubic Foot
     

  • GAS BTUH, CUBIC FEET & ENERGY - CONTENTS: How to Calculate, Measure, & Set LP "Bottled" Gas or Natural Gas Pressures & BTUH per Cubic Foot. What are the typical pressures in an LP or natural gas fuel system & how do they differ. What's the difference between butane & propane or natural gas? Can we just substitute butane for propane or LPG? [No.]
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Propane & natural gas BTUs and gas pressures:

Here we provide descriptions and photographs of unsafe gas piping, indications of unsafe or improperly operating gas appliances, gas meters, and other gas installation defects are provided. This document provides free sample draft home inspection report language for reporting defects in oil and gas piping at residential properties.

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LP "Bottled" Gas or Natural Gas Pressures & BTUH per Cubic Foot

General safety warning: improper installation and even improper inspection and testing methods involving natural or "LP" gas can involve dangerous conditions and risk fire or explosion. If you smell gas you should leave the building immediately and should do so without doing anything that could create a spark such as operating a light switch or telephone. From a safe location, call your gas company's emergency line and/or your fire department. The text provided here is a working draft and may be incomplete or inaccurate.

How to calculate the BTU capacity of LP or natural gas fired equipment, heaters, or appliances

Computing BTUH: Technical Note: you can compute the BTU's per hour of gas consumption of your gas-fired equipment. Making sure than only a single gas appliance is running, watch the gas meter and measure the time required to use one cubic foot of gas. The formula: (3600 x 1000)/seconds = BTUH.

The number you compute for BTU capacity for an appliance should approximately equal the appliance's nameplate "input" BTUH on the appliance.

How many BTU's are in a cubic foot of natural gas? How many BTUs are in a gallon of LP gas or propane?

  • One cubic foot (0.028 cubic meters) of natural gas contains about 1,050 BTUs
  • One cubic meter (35.3 cubic feet) of natural gas contains about 36,303 BTUs

How many cubic feet of natural gas or propane will a heating furnace or boiler consume per hour?

  • A 100,000 BTUh furnace will use about 95 cubic feet of natural gas (100,000 ÷ 1,050 = 95.21) in an hour of "burner on" time
  • A 100,000 BTUh furnace will use about 40 cubic feet of propane (100,000 ÷ 2500 = 40) in an hour of "burner on" time

How to calculate the conversion of gallons of propane or natural gas to pounds or BTUs

One gallon of propane contains about 91,500 BTUs and weighs about 4.20 pounds. One pound of propane contains about 21,500 BTUs. Calculation details are below.

One gallon of LP-gas (propane or C3H8) weighs about 4.20 lbs (at 60 degF), contains about 8.66 cubic feet of gas vapor per pound (at 60 deg. F), burns at 3,595 degF in air, and requires 23.86 cubic feet of combustion air to burn properly. The numbers for butane gas (C4H10) are different.

One gallon of LP-gas composed of butane weighs about 4.81 lbs (at 60 degF), contains about 6.51 cubic feet of gas vapor per pound (at 60 deg. F), burns at 3,615 degF in air, and requires 31.02 cubic feet of combustion air to burn properly. [Metric equivalents of these amounts are available from the website author or from the LP Gas Service Handbook cited below.]

Thanks to reader Fred G. Van Orsdol for correcting our weights and measures for LP gas.
Thanks to reader Bay Ground Control for additional technical editing.

How to Convert cubic feet of natural gas to BTUs/hour: multiply cubic feet per hour by 1,020/cubic foot of natural gas to obtain BTUH.

What are the common operating pressures of natural gas and LP or "liquid petroleum" gas in the building gas piping and at the appliance?

The Common operating pressure for natural gas is 3.5" of water.

Common operating pressure for liquid petroleum or LP gas is 10" of water.

Details about LP gas pressures and natural gas pressures as they occur at buildings and in gas fired appliancs are found
at GAS PRESSURES LP vs NATURAL GAS.

How much gas do various household appliances and systems use?

The following are approximate since there is a wide range in fuel usage rate among appliances and between conventional and "high efficiency" heating systems. But according to NFPA Pamphlet 54 and the LP Serviceman's handbook (cited below), common household gas appliances consume LP gas roughly at these rates:

  • Gas-powered clothes dryers, residential types, use about 35,000 BTUs per hour (BTUH)
  • Gas Ovens and Stove gas consumption rates: A residential gas range can burn about 65,000 BTUs per hour (BTUH), while the oven may consume about 25,000 BTUH.
  • Gas -fired residential water heater gas consumption rate: A typical residential water heater used to produce domestic hot water consumes about 45,000 BTUH for a 30 to 40 gallon water heater.
  • Gas-fired "on demand" tankless water heaters burn gas at a much faster rate, ranging from about 140,000 BTUH (2 gpm water flow and assuming incoming water at about 40 degF and outgoing at about 120 degF or less) and can burn up to about 430,000 BTUH at 6 gpm water flow.
  • Gas-powered refrigerators are much smaller consumers, using about 3,000 BTUH.

Gas service people and installers, in deciding on the total LP gas load at a property, may use "standard cubic feet per hour" or SCFH which is calculated by adding up the total anticipated gas appliance load (in BTUs per hour) and dividing by 2488.

The total gas requirements at a building are used to determine the necessary distribution piping sizes as well as the gas tank size most appropriate.

Other LP Gas Characteristics

  • Chemical Formula for LP Gas: C3H8
  • Ignition Temperature for LP Gas: 920 - 1,120 degrees Fahrenheit
  • Maximum flame temperature for LP Gas: 3,595 degF
  • Heat value per cubic foot of vapor of LP gas (atmospheric pressure): 2,516 BTUs
  • Heat value per pound of liquid LP gas: 21,591 BTUs
  • Heat value per gallon of liquid LP gas: 91,547 BTUs
  • Specific gravity of liquid LP gas: 0.509
  • Specific gravity of vapor-form LP gas (at atmospheric pressure, sea level): 1.52
  • Weight per gallon of liquid LP fuel: 4.24 U.S. pounds

-- National Propane Gas Association

Fuel Alternatives for Heating Appliances: Butane, Natural Gas, Propane: proper set-up required

At TANKLESS WATER HEATER INSTALLATION we noted that several readers had asked about using butane in tankless water heaters and we commented that several manufaturers, at least including Bosch, make conversion kits for their water heaters that include butane as well as LP gas and Natural Gas. The proper match of the appliance set-up to the fuel is important.

Watch out: do not simply hook up a water heater or any other gas fired device without checking the intended fuel against the heater's current set-up. While most heaters can be converted among fuels, using a fuel different than the current jets and air and fuel metering controls on the appliance would be dangerous, not to mention that the device won't work properly.

Butane is used as a fuel gas alone or in some areas may be mixed with propane. Adapting text from Wikipedia & Princeton University:

Butane gas is sold bottled as a fuel for cooking and camping. When blended with propane and other hydrocarbons, it is referred to commercially as LPG ["liquid petroleum gas" or LP gas]. Butane is also used as a petrol component, as a feedstock for the production of base petrochemicals in steam cracking, as fuel for cigarette lighters and as a propellant in aerosol sprays such as deodorants.

Very pure forms of butane, especially isobutane, can be used as refrigerants and have largely replaced the ozone layer-depleting halomethanes, for instance in household refrigerators and freezers. The system operating pressure for butane is lower than for the halomethanes, such as R-12, so R-12 systems such as in automotive air conditioning systems, when converted to butane will not function optimally. - Wikipedia (2104) & Princeton (2014) 

For example, even though many sources refer to both Butane and Propane as LPG or liquified petroleum gas, Butane is not an exact substitution for Propane in all applications as the latter form of propane is commonly sold for use in heating appliances. Quoting from Avantigas in the UK

There are two types of LPG - Propane and Butane. They have similar properties but different applications. They are not interchangeable due to the different operating pressures and burner settings required. Valves and fittings are also different to avoid confusion or accidental use of the wrong type of LPG. Propane has a lower boiling point than butane so it will continue to convert from a liquid to a gas even in very cold conditions, down to -45ºC. When stored as a liquid in a tank, it exerts a greater pressure than Butane at the same temperature.

... Propane, as an LPG, is most suitable for exterior storage and use. Its ability to operate in low temperatures makes it the most suitable LPG for many applications.

Propane is widely used as a fuel source for domestic and commercial heating, hot water and cooking....

Butane has the lower vapour pressure at equivalent temperatures and is suitable for interior use or outside during the summer. Butane is a very common fuel amongst leisure users and owners of portable heaters. .... - Avantigas (2014) www.avantigas.com U.K.

 

 

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