Natural gas smell when cutting on heater for first time

Is it heating and actually blowing hot air from the vents? Or is it blowing ambient temperature air?

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what did they do when recently fixed the heater

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Had to troubleshoot a lot and then found the issue. Had to replace gas valve

Could be the ignitor being a little slow. Are you sure you smelled gas and not burning dust odor since it's the first use of the season?

How old is it?
Sometimes the tiny orafice's (that light up one of the panels) get junked up and gas will ever so slightly seep through the hole

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Update:

A/c guy came back out and put his sniffer all over the new valve and parts by it. Said whatever we smelled was normal and it from it just starting up.

A poster above said something about a delay, maybe that was it.

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Just keep an eye on it. The air handler has a inducer blower that is vented outside - usually through the roof - that runs for maybe 60 secs to 90 sec or longer when the thermostat calls for heat to purge the furnace’s heat exchanger of gas and contaminants prior to the igniter turning on and gas being released from the electronic gas valve and ignited. The induced blower continues to run through the entire heating cycle, while the furnace is burning gas and generating heat, to remove combustion products outside. That’s a separate blower than the larger blower that distributes the air through your duct work.

If this only occurred the one time you the ran the furnace for the first time it’s likely ok, but if you still smell it every time the furnace runs, assuming it is indeed natural gas you are smelling, that would not be normal.

So any potential gas leak downstream of the gas valve should be vented outside the house because the electronic gas valve will not open unless the inducer blower is running and venting outside, but any gas leak upstream of the furnace gas valve would not be vented outside and you’d likely smell that all the time. Since the tech used a gas sniffer and didn’t find anything sounds like all is OK. There are multiple safety features in a gas furnace to prevent issues of gas leaks. .

I had a similar issue last year. Kept smelling gas bit fairly weak smell. Got the A/C guys out and I had a big wasp nest in the exhaust stack. They cleaned that out and no problem since.