question: A left his water running [or flushed his toilet and the water overflowed],it seeped through and ruined B's apartment, is A liable?

A is a Mazik ,כחו כגופו .it is כח ראשון .Now, if A is a אונס ,say, that he NEVER or extremely rare had his toilet overflowing,he is exempt,otherwise we follow the rule אדם מועד לעולם.. בין אונס וכו .

if his child or someone else(unknown) did it,we may consider the water אש since בהדי דקאזלי מזקי & it is not תחילת עשייתן לנזק .Here a שמירה פחותה is sufficient ,so if his child does not do this on a regular basis, we can not consider it a רוח מצוייה and A is פטור .this שאלה was asked by יעקב מארקאוויטש בשם יעקב גליק יתברכו ממעון הברכות