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Question DetailsAsked on 12/6/2017

Both my furnace and water heater will not fire up are these issues related, pilot lights are both lit...

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I recommend you read all the below before starting to try to track it down - especially since some of the thought may or may not be things you are personally up to checking out, depending on any physical mobility issues and your level of home repair savvy.

00) paid your gas bill recently ? Though usually, in most areas, before they can turn noff your gas they have to send several past-due notices AND post a notice on the door with warning before they can nturn it off.

0) considering time of year (start of cold weather in a lot of areas) be sure the thermostats were not turned way down - though if you have been living in the house certainly the water heater would not have been unless you (other other resident) were away for a trip and turned it down to VACATION or LOW and forgot to turn it back kup again.

1) Sure they are pretty certain to be related - could remotely be a blockage (debris, crimped pipe, even icing or bird nest) in a common exhaust flue (if you have one) though they would fire up OK, just might self-snuff as the carbon dioxide and carbon monoxide and notrogen compound levels in the flue gases built up - which would be dangerous as anything as the gases would then be backing up in the flue and coming out of the (assuming a gravity flue) draft hood (the open-footed cone at the top of the gas or oil-burning device on a gravity as opposed to fan-driven direct-vent flue), coming out into the house. Normally if this is happening you would smell natural gas odorant smell- both raw and burnt, and usually some at leat minoreye and lung-stinging sensation and chemically stink when you breathe the gases in too.

2) Water in the line, from water in the gas (pretty rare but happens at times, especially with some small gas providers and ones taking gas straight off local wells without going through a gas processing plant), can also block the gas flow. The in-house pressure is typically only a fraction of a psi, so typically about 5-7 inches of water height accumulated in a low spot in the line can almost totally block the gas flow, though in some conditions enough gas might diffuse through the water to keep the pilots burning - rare, but has been known to occur. Usually occurs in lines which are empty of gas and open to the air seasonally and accumulate water that way - like in sporadic use summer houses and cabins and such.

3) If you have tanked gas (not pipeline supplied) check you are not out of gas - as the gas supply gets real low there may be enough pressure to keep the pilots burning, but not enough to allow the gas regulator on the appliances to kick the burners on, as most gas regulators have a spring system to prevent it turning on if gas pressure is too low or high for safety. Also check outside shutoff valve to see that some vandal did not turn it off - either almost all the way so enough gas is getting through for the pilots to work, or maybe the valve is "off" but leaking a bit of gas through.

4) If flat out won't fire up at all, especially if pilots won't keep burning, they are not getting gas - no gas supply, someone turned off a gas valve, or remotely possibly blockage in the gas line - from rust (real unlikely to be that heavy) or they share a common dripleg/sediment trap which is full of water and rust, etc. [Dripleg is a piece of pipe sticking down about a foot below the gas line, normally right before the appliance shutoffs at gas taps for the appliances - looks like in following link and may be one per appliance, or one on the incoming gas line to the appliances at a tee which then serves both close-by adjacent appliances.


5) Since you say pilots are burning, could be an almost complete blockage - but more likely to be a gas regulator failure, where the gas supply line comes to the outside of the house.

6) If you have any other gas appliances in the house - range, clothes dryer, unit heater, etc - if any of them work normally, then the problem is in your interior piping and your problem to get a Plumber to fix. If no gas appliances work (or those are the only two you have) most gas supply companies (at least pipeline supplied gas utilities) do checks of things like this for free - call and find out, they will likely come out and check out the regulator and do minor diagnosis of the problem for you for free. (Note some, especially city-owned or skinflint ones like ConEd, which are looking for every buck they can get out of the customers, do charge for any service call unless there is actually a gas leak, so don't assume it will be free. If they are going to charge for a visit, I would instead call a HVAC tech to find the cause and can fix anything other than a gas supply problem. If it does turn nout to be a gas supply failure, then the supplier would be called to provide free repair of it - so you only pay one service call charge.

7) Couple of other possibilities - check that they were not turned down to pilot for some reason - perhaps there was a problem so some other adult or a maintenance person in the house turned them to PILOT for some reason and either intentionally left them that way, or forgot to turn them back to ON, and another adult in the house forgot to tell you about this action. Or if on tanked system, for some reason they were turned off during tank maintenance or refilling (or if in an apartment maybe manager let someone in to work on a gas issue), and the pilots were relit but the burners not turned back on - so the gas valves on the appliances would be set to PILOT in that case. I have heard of at least one gas company which, after shutting off the gas, will turn pilots back on but for liability reason will not turn it to ON, so if a manager or whatever let a gas utility guy in after a shutoff, that might be the case.

8) If you have small child in house, maybe they played with the gas control knobs and turned them to PILOT, which usually has a lock button or has to be firmly pushed down to turn back on so the control knob stayed that way regardless of whether the child tried to turn them again. I have heard once of a teenager with zero home maintenance or mechanical smarts doing this, thinking turning that knob would turn up the hot water temperature, not recognizing that the temperature control dial is much larger and is marked with heat graduations. Ditto to the thermostat dial maybe being turned all the way down, so it is not calling for heat - though real unlikely because furnaces never, that I have seen, have a temperature dial at the thermostat.

9) If you have direct-vent appliances (they vent horizontally through the wall through plastic flue pipe rather than metal pipe rising up through ceiling and roof), then if the power is turned off to the appliances (power out in house, tripped breaker, etc) they will not fire up, or will do so for a few seconds then the safety sensors will shut them down. Actually, since both are out BUT you have pilots burning, this is possibly the most likely cause.

10) ditto if you have direct vent appliances, some installers put the pipe in sloping slightly to the outside, then vertical upwards - without putting an open ended dripleg off the bottom outside, so condensate (and there is a lot) can build up in the line, especially if the appliances have not operated at heat for a long time so the condensate from the pilot is acculating for a long time, causing a water blockage at the flue outlet. For both to go out at the same time, unless house has been vacant and appliances only on pilot for a long time, wouldhave to be on one common flue pipe - or you had a lot of rain or a kid with a hose filling up the outside vertical riser.

If the above do not help you track it down, try your gas supplier to see if they will diagnose a possible regulator failure, otherwise Heating and A/C would be your normal Search the List category for a repair tech to do some diagnosis - typically $75-150 for a service call depending on local labor rates for a diagnostic visit, which if a simple fix might include solving the problem.

Answered 2 years ago by LCD

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