Negligence causes pipes to freeze. Who is at fault?

[Dan Crenshaw]

Every time my tenant would leave, they would turn their heat off (hwbb) to save money. I told them not to do that because you’re not really saving that much AND the cost of a frozen or ruptured pipe is WAY more that what they would ever save on a gas bill. SO......about a month later, they say they have no heat after being away for a few days. Come to find out, one of the sections along an exterior wall is frozen. It costs $400 to unfreeze and get the water moving again. There is no way that pipe freezes with 170° water moving through it, so the heat had to be turned off (yet again) for a period of time. Who is responsible for the cost of the service call?