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Question DetailsAsked on 12/4/2016

when water heater turns on furnace pilot goes low

getting low gas supply to furnace when water heater comes ;burners are low and won't light

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2 Answers


Unless you are quite capable in the gas piping end of DIY work, this would be something for a gasline certified plumber or HVAC tech to remedy, because unless there is something obviously wrong that a visual inspection catches, it is likely to require a water column gas pressure determination at the furnace, to see what pressure it is getting both with water heater firing and with it off. Causes for this in my experience that I can recall:

0) gas shutoff valve on appliance got bumped and is partly closed, or kid messed with the gas shutoff valve at the meter and closed it partway. Also make sure the burner control know is fully to ON - some can be partly opened between the "PILOT" and "ON" (or similar phrasing) settings.

1) low gas supply pressure to house fro the utility, though usually only in very high extreme winter cold conditions when the gas utility cannot keep up with demand so line pressures drop

2) very rare, but can be found in very old building especially due to the drip leg never having been drained out, water and debris filling the dripleg (piece of gas pipe hanging down and capped off near the appliance - usually at bottom of supply line just before the tee to the shutoff valve for the appliance. If that gets up to point where it is completely filling the drip leg (the 6-12 inches sticking down from the tee), it can inhibit gas flow into the gas regulatoror plug up the inlet screen on the regulator.

3) new gas demands in house so the supply system within the house, or rarely the outdoor meter/regulator, cannot keep up with demand. This can happen in houses built originally with gas water heater and furnace but electric dryer, range, etc that then have gas range and dryer and maybe fireplace, barbecue, hot tub, pool, etc put in later, overtaxing the capacity of the lines to carry enough gas when multiple demands are on. Can also happen - rarely when a much larger or high-recovery or commercial new water heater is installed, very common if you try to put in a whole-house gas-fired tankless water heater and do not increase the size of the gas supply line to it (commonly 1" but I have seen high-recovery units requiring 1-1/4 or 1-1/2" piping from the meter), or tap it off the line to the furnace rather than direct from the meter supply to the house.

4) as gas control valves get older, rarely the valves stop opening all the way so let through less gas - in which case replacement is in order

5) also rarely but happens, a weak thermocouple or thermopile can result in a weak signal po0wer to the valve, making the gas control valve (pilot and/or main burner) open less than fully, making for low gas flow - though I would not expect this to vary when the water heater turns on

6) also rare but has happened on my own houses three times over about 5 decades - gas meter pressure regulator, as it gets old (typically 20+ years), starts stopping the house supply pressure down. Design is such (at least supposed to be) such that if the regulator is failing to work right, it is supposed to stop down the gas flow (because it is spring-loaded against the incoming gas pressure), not let it run amok - so that can reduce your household pressure.

7) very rare but happens enough I have heard of it/seen it a couple of times - complete frosting up of the vent screen on the vent opening/pipe coming out the bottom of the outdoor regulator at the meter can cause a pressure imbalance in the regulator, making the supply pressure to the house drop. Usually happens only when there is a direct vent appliance exhaust or dryer exhaust exiting the building close to the meter - one reason (along with fire prevention in event the meter is venting or leaking gas) why exhausts are supposed to be well away from the gas meter - typically 6-10 feet in various code areas.

Answered 3 years ago by LCD



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