You get your first Heal over Time spell at 19. They have changed the mana cost to make it worthwhile now. This is an important thing to be aware of.
Always know the relative efficiency of the spells you have available. Some are good, some are awful.
As a general rule HoT spells are useful for two things.
Low aggro healingHealing before contact HoT spells do not generate anything like the same aggro as a direct heal. If you have several mobs in a pull and do not want them all over you then HoT can stop that.
However:- There are problems with this
1. They may not be enough healing to keep the tank standing under multiple attack
2. You may actually want to pull mobs off him so you can root them for later.
The other use is to cast one on the puller before they go out to pull so they can recover from a certain amount of damage while they are pulling, or on the tank before the mob starts hitting him so you have a buffer on when you first need to heal directly.
Complete Heal is the most powerful tool you have. Learn how to use it and don't be tempted when you get much faster heals later on. They have their place but if I have one more group where the cleric uses fast heals on a 6k+ tank and then makes us wait while he meds I think I may say something I'll regret (not like me as I am usually so tactful
)
The math is fairly simple. Superior healing costs 250 mana and does 610 or so. Focus effects might improve this (Mana pres and Improved Healing for example). Complete Healing costs 400 and does whatever is needed. Once you start getting to the stage where tanks have over about 1500hp then it is cheaper to CHeal than to do two Superiors.
It depends on the type of fight and some hard-hitting mobs will get you into panic heals but as soon as you have the buffer of time fall back on CHeal.
One nightmare heal is Ethereal Light. 1600hp healed for 525 mana. We failed an 50's LDoN the other day because the cleric insisted on healing a well-equipped (c. 5.5k) tank with this. Instead of using CHeal when he was around 50% to land when he was maybe 20-30 and heal 3-4k he spammed with Ethereal Light and we kept having med breaks and getting further and further behind.
By the way do not confuse CHealing by which I mean Complete healing with Celestial Healing. Celestial is a nice heal over Time but HoT spells DO NOT STACK. Here is a
useful training video for clerics which subtely emphasises that point.