how do i know what the boiler relief valve should be?
The 'relief' valve is a temperature/pressure safety device, that opens when a given temp or pressure is attained...keeps the system from exploding. These are brass valves with a spring lever on the end; hose bib for outlet. Although you can use it to 'bleed' off air from the boiler using that, some systems will have air gap for the production of steam: if that is your system type. Some systems are not steam, but hot water only. One device that can bleed air automatically is a Taco (brand) air scoop placed in the lines. Floor radiators can also have small bleeder vents; automatic or manual. If you have steam, you should get a qualified boiler man to look at it...very dangerous to have a steam system un-inspected and you not being familiar with it. Pay the bucks to get inspection ask questions as he walks through it so you know what to do and look for, plus what the daily operational procedures are. Write them down, paying special attention to heating season 'fire-up' and verifying all components are safe and operational. When it's shut down time, there likely are steps to take that insure the next fire-up goes smoothly. Refer to your notes to assure you're doing it right each season.
The pressure of the relief valve should be stamped on the valve itself. if it is not there, Then give the details of your boiler to a Plumbers Merchant, and they can supply the correct valve.