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Question DetailsAsked on 2/29/2016

I have a 40 yo forced air gas heater in an 1800 sf, 4 br house. What is the cost- in therms, say- to run the pilot

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General numbers - a pilot light uses about 1-5% of your gas consumption on an appliance when they are used 365 days a year - of course more if you have "dead" seasons like when furnace is not kicking on at all. In a hot water heater as much as 80-90% of that might be beneficially utilized in keeping the water hot. In a furnace, a smaller portion contributes to heating the house unless the fan runs continuously, so more goes up the flue unrecovered - and of course if your area gets above desired room temp in the summer, then the portion burned during those hours is undesired heat and might increase air conditioning demand.

A normal standing pilot uses about 1/2 to 1 cf (cubic foot) of gas/hr. Natural gas has about 1028 Btu/cubic foot. A Therm is 100,000 Btu by definition. So, running the math, if you say (on the conservative side) 1 cf/hr gas consumption then 1 cf/hr x 1028 Btu/cf = 1028 Btu/hr, x 24 hr/day x 365 day/yr = 9 million Btu/yr, /100,000 Btu/Therm = 90 Therms/yr. Or about 87 hundred cubic feet of gas (ccf) if your utilkity bills that way. Depending on your local gas cost, that could run from about $20-150/yr on a larger size pilot like a furnace. Hot water heaters and ranges and dryers with standing pilots commonly about half that demand or less.

That is on the higher side, by my experience and by my utility's website (with gas at $0.90/therm) - they say about $20-35/yr for gas water heater and furnace/boiler pilot lights combined, which matches my computation when I checked over a 30 day vacation with water heater and boiler both on pilot only, when it worked out to about $2 worth of gas consumed over a month, or about $25/year, or almost exactly 2% of annual gas cost with gas water heater and boiler, no other gas appliances.

If you look on the web you will see blogs and articles claiming 40-80% of gas use is due to pilots - but if you run through the math you invariably find they blew the math at some point along the line, or very commonly mixed up ccf pricing with cf, or figured gas energy at a therm/cf instead of 1/100 of a a therm per cf, etc. Some of the propane calcs get even worse with conversion error in figuring how many gallons of propane are used - some even figuring 1 gal propane = 1 cf gas, so they erroneously get numbers into the many $100's /yr,

Certainly, the $20-150 number is not nothing, but not a major factor in considering whether to get a new furnace or not - which is presumably your reason for the question. Also, remember probably half that number (ballpark) is productively used to heat hot water or becomes contributory heat to the house heating (more like 80-90% in my area). Plus, your older furnace is simpler and does not have all the electronics to fail so it's reliability is likely much higher than a new furnace - although it is likely only about 60-70 efficient versus 80%+ with newer models.

If looking at efficiency/gas use as a consideration in getting a new furnace, two things to note:

1) it is very difficult to get a standing pilot furnace these days, so pretty much anything you buy will have an on-demand pilot ignitor, so all the gas used is effectviely part of your combustion gas

2) generally speaking, unless you are pretty certain you will be staying in the house for over 20 years or you are in a very cold, long heating season area, the energy savings of getting a new furnace will rarely pay off for you. Granted, with a 40 year old furnace if you go to sell the buyers are likely to demand a new furnace so getting one now so you get some of the benefit from it might make sense, but my philosophy is unless this is your forever home, it rarely pays off to get a new furnace as long as the old one is still functional or can be kept that way with available parts. Typically, on an older furnace or boiler, as long as the heat exchanger/boiler is intact, you can replace ALL the other components (motor/fan or pump, controller, gas control valve, pilot, burners) for $1-2,000 range over time - about half or less what a new one costs installed. Plus avoid the chronic electronics and sensor controls and heat exchanger failures which seem to plague newer units.

You can find more discussions and some ballpark cost and payback period info in prior questions in the Heating>HVAC link in Browse Projects, at lower left.

Answered 4 years ago by LCD


Great Answer! Thanks so much.

Answered 4 years ago by casalingo

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