Bob
Propane : Natural Gas
2550 Btu : 1000 Btu per cubic foot
That is the ratio.
You will have to add a 3.5″ w.c. pressure regulator to convert the existing gas valve or replace the valve totally.
The pilot and burner orifice(s) will have to be drilled out or replaced with Nat.Gas orifice(s).
A label affixed to the appliance denoting the conversion to Nat. Gas.
Conversion kit may be available from the appliance manufacture. A gasfitter would look at a orifice tables chart and take into consideration the number of burners and input of the appliance.
In all honesty this is a job that you should hire a tradesperson. He (or she) could check the appliance out, perhaps the heat exchanger is cracked, safety pilot is not working properly, the application of the job, etc.
Maybe it is not worthwhile to convert.
John Boy
J