What large bomb? Steam boilers are the large bomb waiting to happen.Closed system boilers have a 30# relief valvePressurized?? How much water does it hold? The only OWB pressurized system I have ever seen way the old clean fire from Woodmaster and it was a disaster.
You will require oxygen barrier pipe and there is more involved in pressurized systems. Not to mention on I don’t like the thought of a large bomb.
There is actually less involved with a closed system,mine is 10 years in use,zero water testing required,zero water leavle to worry about,unlike melting your open system down if you happen to steam your water away.No chemicals to treat your water to keep it from rusting your boiler away.