* Blue flame: Indicates complete combustion. This means the gas is burning efficiently and producing the most heat with the least amount of waste products.

* Yellow flame: Indicates incomplete combustion. This means the gas isn't burning completely, leading to:

* Less efficient heat production.

* Production of carbon monoxide, a dangerous gas.

* Soot buildup in the boiler, which can cause damage.

* Orange or red flame: Suggests serious problems with the boiler, such as a lack of oxygen or a malfunctioning burner.

If your gas boiler's flame is not a steady blue, it's important to contact a qualified technician to diagnose and address the issue.