As a Disc/Holy spec, I heal alot in BC.  I found that healing in Hellfire Citadel Blood Furnace is MUCH easier than healing Ramparts.  I think I'm decently geared (+580 healing, 6k mana) here is my experience thus far:

Ramparts has alot of burst damage, and suboptimally equipped tanks take too much damage for me to effectively heal.  The dragon boss is the worst, he always goes for me immediately upon landing, and tanks cant seem to get a clue until we wipe 2-3 times.  Then they stupidly stand in the fire and /cry when I cant keep them up with renew and spamming flash heal. (renew ticks for 350+ and flash heal hits for 1200+ thats 900+HPS and still not enough  :( )

Ran Blood Furnace twice with similarly equipped tanks and renwe healed most of the instance.  The loot and EXP here are better that ramparts as well.