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Question DetailsAsked on 6/19/2017

Water heater pilot stays on but goes out when burner goes out. ThermoC assy & gas valve have been changed.

Pilot-thermocouple assembly and gas valve have been changed out with new factory parts. The pilot lights fine and will stay lit indefinitely in pilot light mode with the knob released. Once the knob is turned and the burner lights, it also stays on fine and heats the water. Once water reaches temperature, the burner and pilot go out. I believe that sometimes they may go out before the water reaches temp. I have two water heaters plumbed in series with the exact same symptoms. I believe it's the gas line input but the gas company came out and said that pressure is fine.

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Assuming it is not factory-defective gas control valves, my guess would be the gas supply to the heaters is not large enough for two to fire at the same time - so when one is firing and the other kicks on, the gas pressure drops enough that the safety in the gas control valve drops out and shuts the gas off.


Knowing if the heaters are reaching full temp each time (by measuring water temp a nearby draw point several times) might also help - if always reaching full temp, that makes it odder - if shutting off early, or only when both are firing, then simpler to guess the cause.


Try setting one of them to pilot only and the other to normal operation and see if the other one works OK that way alone for a half day or day or so, then maybe vice-versa. If both work OK alone, then it pretty much has to be a pressure drop in the line at the heaters because the gas supply line is too small for the demand on it.


When the gas company came, did they measure the static pressure or the line pressure at the heaters with both running - I am guessing the first. It may well be the pressure at the heaters is dropping way down when the second one fires up because the line is too small to conduct enough gas to two of them (comes into building at only 1/4 to 1/3 psi normally).


It is also possible the problem is household capacity (plumber or gas company should be able to tell you line size needed for total simultaneous gas BTU/hr demand in the building) - maybe when range or clothes dryer or furnace/boiler or such is kicking on that is drawing the pressure down enough that it is causing the gas control valves to kick out. Gas pressure should be checked with both water heaters on, and if that is OK then maybe try using other gas appliances too at same time to see what pressure drops to.


If this is for an apartment building or condo or such with dubious pipe sizing, this is not uncommon on workdays - residents waking up or coming home at about the same time and using a lot of hot water for showers, gas range for cooking, gas dryer for laundry - all at about same time. Most common at AM in cold weather (people getting up and using gas PLUS cranking thermostat up (manually or auto setback) and increasing heating load with doors opening as people go out, more commonly in evening/night around dinner time in normal or hot weather because of showers/cooking/laundry. Some areas also have peaks on a certain day of the week, commonly in neighborhoods with a concentration on one ethnicity or religion which leads a lot of people to use a lot of gas at one time - the proverbial Saturday night bath for instance, but also evenings around dinner time due to laundry (especially infant/baby lanudry) and child baths all around same time of evening. Obviously, if this is the case, your shutdowns would be tied to a certain time of day. For instance, if you relight at night and they do not shut off overnight but do in the morning about 7AM in in the evening about 6-8PM, that is a likely cause - not large enough pipes, or an undersized gas meter or building feedline size maybe.


On the gas line feed thing - do you have any other standing pilots in appliances, furnace, boiler, etc - if they are going out too then a gas meter problem is probably more likely, because the water heaters shutting down due to too low an input pressure would not be expected to drain the gas pressure in the rest of the line enough to shut down other appliance pilots.


Of course, if this has been the setup for some time and you have not added new gas appliances in the building (or they are not being used when it occurs) then a meter problem becomes a whole lot more likely.


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One totally different possibility - firebox drafting - either blowing out the pilot (so the gas burner shuts down too because thermocouple/thermopile goes cold)) due to excessive drafting of the flue due to wind, or a flue failure due to partial flue blockage backing the gas into the firebox and tripping out a safety sensor (firebox overheat or flame rollout for instance) - or if high efficiency water heaters with direct venting, perhaps the venting is wrong and the flue eduction fan or its sensors are not working right, the two are run into one flue so the eduction fans are backdrafting or excessively drafting the other one, etc.


Or if other appliances are on a gravity flue, maybe you have too many appliances connected (or connected wrong way) for the flue size so when the furnace/boiler for space heating kicks on, the flue drafrting is too strong so it is blowing out the pilot or overcooling the thermocouple, causing the gas to shut off at the water heaters.


Sounds to me like you need a diagnostic-inclined technician from a Heating and A/C contractor, rather than a plumber, to look at this - gas supply and flue issues are more in their line than a plumber's.

Answered 1 year ago by LCD




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