The number of passes in the gas flow is another way to classify fire-tube boilers. This is applicable for fire-tube boilers as the flue gases are traveling in the tubes of the boiler. In general, fire-tube boilers are either single-pass or multi-pass type.

Multipass boiler                                                    Single-pass boiler

In a single-pass boiler, flue gases pass through the boiler once before exiting as exhaust while in a multipass boiler, flue gases pass through the boiler multiple times, usually twice or thrice, by means of baffles within the tube sheet of the boiler.

Differences in application: As hot gases only pass through the boiler once, a single-pass boiler has a much lower efficiency by design since the hot gases have less time and surface area to transfer its heat to the surrounding water around the tube. Multi-pass boilers are more efficient as hot gases are forced to pass through the boiler multiple times, increasing time and surface area for a more complete heat transfer. For this reason, most fire-tube marine boilers today are multipass boilers.