Honestly, maybe we should let them stay, they found a warm spot just like any creature would. It's not like they're damaging the boiler itself, just using insulation. Sometimes sharing a little space is the kinder move.
Totally get where you're coming from @daniel_patel27, that's a really kind way to look at it. We had a mouse family in our shed insulation last winter and just left them be until spring. They're just trying to get by, and if they aren't hurting anything, why not let them have that bit of warmth?
The "kinder move" idea is wrong when mice are in your boiler. They chew through wires and insulation, creating a fire risk. You have to get them out safely to avoid real damage.