If you don’t know the size of the heater (measured in BTU per hour), then you can estimate it by watching your gas meter when it runs. Simply run your heater for an hour and look at the volume dial before and after. Every cubic foot of natural gas is roughly equal to 1000 BTU’s. For example, a 400,000 BTU/hr pool heater will use a little more than 400 cu ft of natural gas (gas heaters are typically 80-85% efficient).
You can then update your signature with your heater’s model and size.