If I’m on an emergency service call for no heat after hours, I often find old hydronic boilers that haven’t been serviced in years, and I have trouble purging the zone(s) or draining a zone due to non-functioning isolation valves/drains. I keep a self-piercing copper saddle valve handy to serve as a quick temporary bleeder valve or drain. It gets the heat back on, then I can return for another appointment to correct the issues during regular service hours.