Gas smell when heater runs

[Ctzhenderson]

I have a 2001 ford explorer sport trac with 118k on it. The trouble that I am experencing, is that when the heater is on, a gas smell comes into the cabin.This does not happen when the air is on. The heater wasn't used much in the last 10 years. Any thoughts on this trouble would be appreciated.
Thanks

 

[RNGRDVE]

When running heat you are probably set in fresh air mode and when running a/c you are probably in recirc mode. Either way it sounds like you have a fuel leak and need to get that checked asap.