I have two furnaces, one for upstairs and one for downstairs. They both turn on, but one doesn't catch fire..

How can I fix the one that doesn't light? I can hear it kick on and I smell gas, but no spark.

Answers:
hard to answer this is it electric? forced hot air? gas?
In any case, check the transformer. It is located inside the flap door of the furnace. There should be a red restart switch on top. once your furnace kicks in, hit the switch. If it fires up its probably the transformer.

Other answers:
do not use both of them at the same time.
do not use both of them at the same time.
it needs a new starter
Are they both connected to the same gas line?Well then your answer is simple..One has not got a control to start iotv a bit later and the other is closer to the gas line..When the one closer starts assuming they both suppost to start together has got the gas first which is a surge..and then theres electric also involved..Which all can diminish for the second furnace rying to start at the same time.I think maybe you have the gas control valve higher on one then the
other..(yOU know the part that feeds the pilot light with gas)Possibly..You may want to try and turn the formost
gas down on the one that starts (just a notch)and turn the gas feed up on the second furnace,that is further away from the incoming gas line feed.This is very dangerous "all"to mess
with )most furnaces come with instructions that re4quire you to turn off all electric conduits to it for five minutes all gas to it for or (both)and relight both pilot lights..There is a gas feeding valve (handle on all)...The gas line although on the forbearing furnace may still supercede the one further away..from the gases entrance to the house...You may not be able to remedy this.The one upstairs may be clogged or have reception of gas problems.
Your down stairs gas line valve may be on too high and is hoarding the equallness of the upstairs gas to ignite properly.It means downstairs is closer to the input gas line..it could make parts of down stairs too warm..
The gas has to bypass the first outlet which is downstairs andhave enough to light upstairs..but no..You may need also a better thermostats.(And check the gas feed)the one down stairs is probably turned up higher which does not need to be.Because all the gas (incoming is bypassing and exhausting on the first floor (naturally because its closer)then going up the pipes to the second floor..Are you a landlord ?Do people on the second floor seem to freeze..Well then you actaully have it backward..You are sheltered by the second floor..visa versa your first floor will remain warmer or as warm simply by being sheltered by the seconf=d floor..If the second floor is cold and you do have the gas valve higher on yours you should reverse this...it takes time to figure out but it's true.first floors are naturally warmer then second floors.
The pilot light is probably out.Have someone light the pilot light.
Call your gas supplier to verify that the gas lines are okay and verify that your spark igniter is okay. If it needs repair, then call and HVAC serviceman.
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