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Rural USA. We just had a gas furnace installed. The new furnace has a 1-gang junction box with a switch and a fuse.
It's the 21st century, here, I swear. And not England.
The furnace circuit is tip-top. It's punched down into a modern service panel with perfectly typical 15A breaker, and if it needed something special, that really wouldn't be a problem.
So why does the furnace also need a fuse?
You have an electrical connection to the Furnace - even if it is gas. Blower motors and control circuits require electricity; should you have a short - you will need the overload protection to prevent a fire that only a pyromaniac could enjoy.
The fuse is most likely of a lower amperage than your Circuit Breaker - it guarantees the safety of the furnace - regardless of your Circuit Breaker.
Think on this Why do Microwave Ovens have Fuses in them or other appliances such as TV's have fuses in them when we have these wonderful circuit breakers installed?
In Summary: The consuming unit may not trip the breaker, and if not a fire could easily erupt first before that breaker ever went into overload.
Newer furnaces might not need an SSU if they have internal motor overload protection.
The fuse is sized to protect the blower motor from overload. It should be about a 3 amp fuse.
The 20 amp breaker is there to protect the conductors from ground fault and short circuits. The breaker alone would allow the motor to overheat and burn up if a bearing jammed.
You also need a fuse to protect the hot surface ignitor in case it attempts to ignite a non-existent gas flow. I've blown mine twice in the fourteen years we've owned this house, and in both cases it was when the furnace was turned on while the gas was turned off.
Not entirely sure where the fuse is you are talking about. Most new furnaces have a 3-5 amp fuse on the low voltage side to protect the circuit board and transformer from a possible short or overload. If someone were to put too many accessories on the existing transformer the fuse should protect it. Also if a thermostat wire were to ground on the cabinet it would save the electronics.
I'm not American (nor British), anyway I still like to have fuses on expensive appliances 'just in case main breaker fails'. Or, if on a bigger circuit, to limit the ampacity available to the specific appliance (eg: all my thermostats valves have a 1A fuse, and are connected to 10A light circuit). So in case of failure (that once occurred me) I have just to replace a few cent fuse and not a thousand Euro thermostat with its thermo-electric actuator,
Also fuses usually can break bugger currents than 'standard' switch-like breakers. So if a 10kA failure occurs (AKA short circuit) it's more probable a fuse will fuse than a standard breaker will trip because standard breaker cannot extinguish the arc well as a old-style fuse.
