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Can anyone provide a method to estimate the gas flow rate to sustain a pilot light, using just the apparent size (flame width and length) and reasonable assumptions for things like gas line pressure?

Note that we have our gas (propane) delivered, not continuously metered, so no response that amounts to "don't use any other gas, and see how much is used" is going to work. I can't go days or weeks not using gas :).

Once I have a flow rate for the pilot light, in addition to determining the actual gas use per time period, I will convert it to equivalent energy units with electricity and compare it to other "vampire" or continuous baseline loads.

Edit to answer comments:

Some good ideas there, but unfortunately not applicable. HighlyIrregular's suggestion is not feasible because there is always a safety to shut off gas flow if the flame is out, and it is not accessible enough otherwise. As to weighing, I have a 500 gal underground tank! And for EnergyNumbers, unfortunately it is not a water boiler but just heating air.

The best solution may be inspired by Flyto, along the lines of the "I'll trade the barometer to the building superintendent in exchange for the building plans" solution to "how high is the building" - i.e., try to look up some specs for the device and see if there is burn rate info.

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  • I've never seen a pilot light; how accessible is it? Could you blow out the flame and stick a balloon over it perhaps? Could you heat a small container of water over it?
    – Highly Irregular
    Nov 25, 2013 at 2:19
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    One workaround would be to assume that pilot lights are similar, and have somebody with a metered supply do the measurement. I don't know how valid that assumption is, though.
    – Flyto
    Nov 25, 2013 at 8:18
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    Can you weigh the gas container?
    – sharptooth
    Nov 25, 2013 at 9:39
  • was thinking about wieghing the container, too - difficulty could be that the gas use of the pilot is low compared to the total mass, and thus is hard to measure. Maybe try it when the container is almost empty.
    – mart
    Nov 25, 2013 at 10:02
  • I did locate the installer's manual; it has no info on flow rate but does specify 11-14 water column inches of pressure, which converts to .4-.5 psi.
    – half-integer fan
    Nov 28, 2013 at 1:32

3 Answers 3

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In general: "Various sources report that a pilot light burns from 5 to 12 therms of a gas per month depending on the particular appliance and how the pilot is adjusted."

For your purposes: Flame size does indicate fuel use but it isn't a simple or 100% accurate measurement. If you know the pressure of your propane in the line and can measure the width of your pilot orifice.

Check out some flow rate calculators here:

http://www.pipeflowcalculations.com/

Also if you are worried about wasted energy keep in mind: in the winter most of the waste is dumped into your house as heat for many appliances, such as the stove and water heater.

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  • 5-12 therms per month converts to 5.5-13 gal of propane per month, or 145-350 kWh/mo. That is a lot! It is the equivalent of 200-480W continuous usage. Assuming my device is in this range, I will start shutting off the pilot whenever it will not need to be used. It is true that the waste heat goes into the house, but it is still going right up the wall to the ceiling so it may not be a very useful heat.
    – half-integer fan
    Nov 28, 2013 at 1:30
  • Indeed that may be best! Although for hot water heaters (assuming they are designed correctly) the pilot can serve to keep the water at temperature, so it isn't strictly wasted. For the stove you could consider replacing it with an ignition system.
    – Meep
    Dec 2, 2013 at 16:51
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Some options for you:

  1. Flyto's suggestion in the comments of assuming that pilot lights are similar, and have somebody with a metered supply do the measurement, is very good.

  2. Install a gas meter. Not only will it allow you to answer your question about pilot lights, but will give you data that you can use to assess the efficiency of each of the uses of your propane.

  3. Measure the heat flux out of the boiler, after it's been on only the pilot light for many hours. If you know the heat flux through the boiler case per unit area, then you can multiply that by the surface area of the boiler, to calculate the heat lost by the boiler per unit time. Assuming the boiler is in thermal equilibrium, the rate of heat loss will equal the rate of heat input from the pilot light.

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I'm a natural gas technician, and I have the answer. All you need is the orifice size and pressure. Do you know the orifice size for the pilot? You can look in your paperwork/on a plate on the unit, or you can size it by hand by sticking drill bits in the orifice to measure it.

11" wc is usually what propane is set to, but your appliance might cut it down even further, check again for a info plate. You can be sure by hooking a manometer to the pilot, opening the pilot valve, and reading what it reads.

There's a book, NFPA 54, and I believe annex f-something has a chart. You just line up oriface size, and delivery pressure, and it tells you how much gas it will pass... simple math.

Sorry it's copyrighted and I wouldn't know how to go about putting a chart on here anyway, but if anyone's interested, check it out!

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  • Welcome to Sustainable Living! Thanks for your answer. I guess you mean this book?
    – THelper
    Aug 3, 2016 at 13:51

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