I had a boiler installed almost a year ago. The house is nearly 80 years old and this is the first boiler replacement...cast iron in the old one rusted through and cracked.
The new one was installed with 1 1/4" black metal pipe. A lot of off the shelf nipples were used and a bunch of pipes were cut and threaded on site. They used a blue pipe dope, not sure which one, but I've had many leaks since the installation. The company who installed it has been back 4 or 5 times, several times taking half the new pipes apart and putting them back together with bigger wrenches. The last couple times they've used teflon tape and pipe dope but still some are leaking.
The water pressure is about 18 pounds.