Rust-Oleum CombiColor Original is heat resistant up to 90°C (dry heat), so as long as the surfaces being painted didn't exceed this temperature it should be OK.

Rust-Oleum CombiColor is however an industrial product designed for exterior use or interior use in industrial environments so it is not specifically intended for internal home use and will smell during curing.

As we don’t know what actual surface temperature the storage heaters will get up to we can’t say for sure if the paint will be OK on any particular storage heater, it could depend on the model, type etc.

You will need to check with the manufacturer to find out what the surface temperature of their particular heater will get to during use.