Empty a bladder type expansion tank.

i assume a bad one? drill a hole in it.

When the bladder has failed the easiest thing to do is use a Schrader valve core removal tool to unscrew the valve core and drain the tank from the Schrader valve. These tools are commonly used in refrigeration and automotive repair, it's the same tool used to remove the valve core on a tire.

Excellent idea. If you are pressed for time. Add a regular valve and go from there. What @pecmsg suggests above is really the way to go.

First, if you know the tank is bad, replace it. If you're not sure, what is the date code on the label, serial #? Maybe the air charge is diminished and all the tank needs is a proper air charge. I have had these tanks last 20 years and some have failed in less than 5 years.

If you have no way of isolating the tank, you have to drain down the sys. Do you have anti freeze in the sys? If you are lucky then the installers put isolation valve on each heating loop (probably not). As my Dad use to say,"If I didn't bad luck, I wouldn't have any luck at all". Isolate the zones. If you replace the tank, put a Webstone valve on the tank so it will be much easier to service next time. Use this valve. It's directional so install the right way,

I bought for a modest sum at Home Depot a small 120 Vac tire compressor that I use to put air in these tanks and of course my car tires.

https://www.supplyhouse.com/Webstone-40612-1-2-Threaded-Pro-Pal-Full-Port-Ball-Valve-w-Hose-Drain

https://www.homedepot.com/p/Husky-120-Volt-Corded-Electric-Inflator-HY120/301753174

@mattmia2 , I don't want to work that hard. Dual use.

Good idea maybe you should suggest that to them??

But maybe they won't. Why sell one tool when you can sell two. (their perspective)

When there's no isolation/drain valve assembly on x-tanks, it can be a huge pain. Removing the schrader valve helps, but with a good bladder, there can be an unexpected weight when released with a potential for lots of water. Maybe someone will come out with one that has a small release valve tapped into the tank itself.

I've got both the cordless Makita and Milwaukee compressors and like the Makita trigger action vs. the push button Milwaukee. Easier to let go of the trigger to stop air flow than pushing a button. Also got the dual presta/schrader fitting for bike tires/x-tanks.