Typically a temperature and pressure relief valve as found on most water heaters for domestic water use. For boiler use I believe you only need the pressure relief type set to release pressure at 30psi - At least that's what I used on my systems.Excuse my ignorance...
what is a t&p valve
The 30 psi makes it a rare bird:I did see that on different sites , but I’m sure mine was temperature and pressure . Could be wrong though. I did find one online set at 30 psi but only rated for 105 k btu/ hr and my boiler is 300k.