All E1 rxns are not E1cb. all E1cb are E1.

The conditions for an E1 reaction are a polar solvent, good leavin grp i.e weak nucleophile(e.g F^-, Cl^-) .

It usually takes place when a stable carbocation is formed ( benzyl, 3 degree etc.)

E1cb has all these conditions and also apart from these it needs to have a very strong withdrawing group (e.g F^-) .