What ampacity circuit breaker would I need for an 18 amp electric heater?
Here's a link to the model (IEP-4024), it's North American, 240 volts, 4000 watts, installing it outdoor.
After reading the user manual for this heater, as per National Electrical Code.
It appears that it has subdivided heating elements, which requires 2 separate 240 volt 9 ampere branch circuits. It also states that "the electrical connection must be performed by a qualified tradesperson.".
Select the Conductors
Since you're dealing with space heating equipment, you'll want to check article 424 of the NEC. In article 424, you'll find that the conductors have to be sized to 125% of the load served.
So you'll have to do a bit of math to figure out the minimum ampacity of the conductors.
Now that you know the minimum ampacity required, you'll want to check article 310.15 to figure out what size wire you need. A quick look at table 310.15(B)(16), shows that 14 AWG wire is good for 15 amperes at 60°C.
Select the Overcurrent Protection Device
Now that you've selected the wire size, you can choose your overcurrent protection device. In this case, you'll want 2 15 ampere dual pole circuit breakers.
Wiring the Circuits
When it comes to wiring the circuit, you have a couple choices. You can pull two 14/2 with ground cables from the panel to the outlet, or you can pull a single 14/2/2 with ground cable.
Two 14/2 with Ground
Pull two 14/2 with ground cables from the panel to the outlet, and connect the heater as follows:
Don't forget to reidentify the white conductors, by marking them in an approved manner.
One 14/2/2 with Ground
A 14/2/2 with ground cable consists of a black, red, white, white with red stripe, and a bare conductor. The only drawback here, would be that since there are more than 3 current carrying conductors, the conductors have to be derated by 80%. Even with the derating, the conductors are still rated for 12 amperes at 60°C. Which is still above the 11.25 amperes required by the circuit, so there should be no problem using this cable (as long as the run is not overly long).
Again, don't forget to reidentify the white conductors.
You're asking the wrong question.
The circuit breaker size is based on the wiring size, not on the application size. The purpose of the circuit breaker is to protect the wiring from fire.
A better question would be "what size wiring do I need for an 18 amp load?" Then you can work out the circuit breaker, receptacle, etc. afterward, based on @HerrBag's comment.
If you just swap out an existing 15 amp breaker for a 20 amp one because your electric heater keeps tripping it, it's likely that your wire will be undersized for the application and you've created a very serious fire risk.
In America the NEC requires 125% protection of circuit so if you have an 18 amp heater then you would need to account for 23.4 amp. Meaning you would need a 30 amp circuit breaker and 10 gauge wiring.