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Question DetailsAsked on 2/19/2018

After hours of just having the pilot light on in my gas fireplace I can smell gas. Smell it at no other time.

I read there can be too much odorant for the pilot light to burn off. Is that the problem?

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1 Answer


Not "too much odorant", which is only present as a very small fraction of a percent by volume in the gas, but because the pilot light is very small and does not create a hot firebox, it is possible for a pilot light to have unburned gas / odorant escape good enough burning to eliminate the odor - either by the fringes of the flame not totally burning or by flow from the pilot orifice which does not pass directly to the pilot flame. Also, again because the firebox is not real hot from the pilot light, some fireplaces will not draft properly under pilot-only conditions.

Therefore there may not be enough heat from the pilot to cause a distinct gravity-induced draft up the flue (hot gases are lighter than cold, so should rise) to exhaust the odor of the combustion gases, so instead of the fireplace being in a "negative pressure" situation (slight vacuum relative to the surroundings) you can have a cold air draft down the flue (especially in cold weather when the outside air is denser) which can put a very slight positive pressure on the firebox, letting some of the combustion gases leak into the room.

This is the same sort of thing which happens, again especially in very cold weather or sometimes in windy weather, with gravity flue furnaces and water heaters with draft hoods right above them - when they initially fire up you can get a few second burst of not totally burned gas coming out of the draft hood, causing a gas odorant smell near the appliance for just a few seconds until the unit puts enough hot air into the flue to cause it to create a draft which makes it rise through the flue, and starts pulling enough air from the surrounding room to consume that small volume of partly burned gas products.

The problem is, you do not know for certain WHERE the gas smell is coming from - from unburned gas from the pilot flame, from unburned gas from the pilot assembly which is not getting to the flame to burn, from a leaking gas pipe leading to the pilot, a slight bypass leak in the valve to the fireplace main burner which might not be igniting because there is not enough gas concentration there, or from a leaking gas control valve (the latter is pretty common after 15-20 years or so as seals age).

It may also be that you have a significant leak in the firebox which is letting gas out into the living space.

Certainly, you should not be able to smell it some distance away - maybe right up close to the fireplace, but if smelling it across the room or such I would be concerned.

First place I would check, if accessible to you, is sniff right at the gas control valve while the pilot is buring - seal leaks around the valve stem is probably the most common form of failure of gas control valves, usually age related. Cannot legally be repaired - valve (not a real pricey item - almost always under $100 and commonly about half that for manual ones, commonly around $100-125 give or take $25 or so for electric thermostat controlled ones, plus service call labor charge) has to be replaced.

My recommendation - get it checked out by a gas fireplace specialist (preferred because he should be better trained in gas fireplace leak detection) or Heating and A/C contractor for probably $100-200. Better safe than sorry, plus there is some evidence that unburned or partly burned natural gas or propane is a health hazard both to elderly and very young persons, and to the population in general with prolonged exposure. Make sure he has a gas sniffer to trace where the gas is coming from - amazingly, way too many techs working gas appliances and fixtures trust just their nose to trace a leak so cannot accurately locate your type of potential leak source, even though the sniffers are only about $100-170 range for that sort of relatively low accuracy device.

Better safe than sorry - plus if this is a gas leak in the gas control valve area, there is a possibility that an explosive concentration of gas could build up in the walls or floor and eventually ignite with dramatic consequences.

Answered 2 years ago by LCD

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