I'm curious, do you have low pressure (ounces) or high pressure (pounds) in your 1 1/2" gas pipe. Look to see if there is a regulator on the pipe right before it enters the heater.
I've found all Mfg's LoNox heaters really like the dual pressure gas systems where you bring 2psi right to the appliance and then regulate it back to ounce pressure.
My guess is that you should ask the utility company to convert you to this set up.

See ya,
Kelly