I have a Hayward h100id pool heater. It is 5 years old. Recently it heats poorly. The burner comes on, the fan comes on, but the water seems to heat about 40%. The burner and fan continue to run, but the water barely heats when I feel it entering the pool. It also smells like the natural gas is not fully burning? Where do I start to troubleshoot?

Pool & Spa - Hayward