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These doors are grey and usually (always?) open into a "weapon" room, i.e. there is a weapon on the minimap for them. I've gotten in two, but it was very non-obvious what I did to open the doors. For other doors, I've killed every mob on the floor, put out every fire, and dropped a bomb on the door and it still didn't open. Does anybody know what the rule is for these things?

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I've gotten in more doors since posting this question. Some seem to trigger when I kill all the mobs in the room while others when I pickup all the items, however, there are some that just don't ever open. I'm still at a loss as to why some open and some don't... – Morinar Sep 30 '11 at 17:24
up vote 19 down vote accepted

The doors are tied to your health. The door will only be open if you have full health. However, the temporary HP boosts (blue hearts) can serve as a equivalent of the missing red hearts.

For example: If you're missing one red heart but have one blue heart the door will open. But if you're missing two red hearts and only have one blue heart the door will still be locked.

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Does it require "full" health, or just 3 hearts of health? – Chris Rasys Oct 4 '11 at 17:47
@ChrisRasys Full health - You need to have all the heart containers filled up, or the equivalent of the missing heart containers in blue hearts. – mordi2k Oct 4 '11 at 17:56
Awesome! I had no idea. Thanks for the answer. – Morinar Oct 5 '11 at 14:27

If you get items such as Transcendence or Lord of the Pit, or the Hanged Man tarot card, you can actually fly through/over the door, regardless of your health level.

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Maybe the doors didn't open because they were secret rooms. Some rooms can only be seen with x-ray glasses. That could be it.

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That doesn't make any sense. As the accepted answer says, the door letting you in depends only on your health. – murgatroid99 Jan 5 '13 at 18:46

protected by fredley Jul 4 '13 at 14:44

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