During freezing weather, pipes can freeze over, which can happen to the condensate pipe of a central heating boiler. This can lead to a very cold house, as the boiler wont function like this. The simple fix is to pour boiling water over the pipe until it defrosts.
What options are there to deal with this if it happens on a Shabbat morning, for example?
I am interesting in this halachic situation. Using hot urn water to defrost it, even if that is itself allowed, would lead to the boiler computer activating and the system heating up (if it is currently "on" by timer). Is the only way to deal with this by asking a non-Jew? What situation would warrant this? Severe cold? If someone in the house is sick?