Talking to an elderly neighbor this morning and he tells me they have had no heat for two weeks. I am NO HVAC person, but thought I would look into it and see if it could be something simple. Gas fired heat/AC central unit. When he turns the heat "ON" and raises the T-stat, the unit's heater forced air fan starts to run. About 30 seconds or so later there is a "click" at the gas valve as if it is opening. Then nothing happens other than a rich natural gas smell coming out of the gas vent on the side of the cabinet. The unit runs like this for about a minute then shuts down. There is a green LED light on one of the control boards which blinks in a "heartbeat" mode according to the info on the panel. This should mean "system working, no faults". I did confirm there is a gas supply to the unit also. I called my HVAC guy to come out tomorrow as we are expecting 22º tomorrow night. BUT- could it be the ignitor's? I see what appears to be two (2) ignitor's. One has an orange wire coming from what appears like an old auto coil, and the other is a white wire coming from a control board.
If you want to try an easy possible fix, check flame sensor. There's only one and it's near one of the combustion burners where flame would normally be.
Search this HVAC section for flame sensor because pros have included pics. If you mark where it connects to control board and maybe unscrew 1 screw to remove item, note angle and position first. If it's all black, it needs a cleaning.
Wear gloves and clean sensor with steel wool, not sandpaper.