Remove big nut? I'm trying to remove the nut off a boiler to be precise...

I'm trying to remove the nut off a boiler to be precise kinda BIG so there be no sockets big enough for it. I tried using a vice grip but it wouldn't hold because the nut be in gaping and I could only hold it using the tip of the vice grip. I manage to find a wrench that fit but just similar to the vice grip, i couldn't get a firm grip of the nut. Any guidance?
How do I bring back my heat system...
Answers:    Use the proper tools!
Measure the nut across the flats.
You should either buy a socket or run it to someone that has one.
Some rental companies own large sockets for rent.
(There are sockets made for adjectives nuts.)
How do i remove this pvc top?
Ginanic channel locks they are resembling pliers and have the gift to open completely general to fit and then you flip down a minor handle that locks it onto the nut or anything, you should give the nut a blast of WD40 or some de rust type lubricant. Ok ---torque away dude!