You have about 25 gallons in the radiation, also have water in the plumbing and in the boiler itself, so you're probably getting close to 30 gallons of total system water, which means the EX15 is marginal unless you keep the system temp under 150F. The EX 30 is definitely the better choice. If you fill and pressurize the system to 12psi with cold water you'll likely hitting 30 psi and releasing water at the PRV if the system ever reaches 180F.
To make any sense of comparative temperature readings of metals with an IR instrument you need to put a dab of highly emissive paint or wrap of tape of high emissivity (the black tape used for hockey sticks is good) on the surface, and measure the high-E material, not the bare metal.
It's possible that the leak point is pressure sensitive, and only occurs during the higher temp operation when the pressure is highest, but find it you must! A steady diet of fresh water in a heating system causes excess corrosion on the metals in the system (iron pump impellers can't really take it.)
If the direction of the pump flow is away from the system pressure meter ( and toward the higher impedance of the radiation or boiler), that would cause the measurement to drop when the system first fires up. If the pressure drops too low on a low mass boiler you will often hear the sizzle & bang of larger bubbles of water vapor collapsing at the heat exchanger (sometimes called "kettling"). This is really bad for efficiency, since the foamy water and water vapor in contact with the heat exchanger are less thermally conductive than micro-boil water, so less heat gets transferred, and more heat goes up the flue. On low mass boilers it's best practice to pump TOWARD the boiler, (and away from the expansion tank) so that the pump pressurizes it slightly above the average system pressure. If it's pumping away from the you may need to raise the pressure to 15-18psi to reliably avoid the sizzle, which would then give you even less margin on the EX15 expansion tank.
To make any sense of comparative temperature readings of metals with an IR instrument you need to put a dab of highly emissive paint or wrap of tape of high emissivity (the black tape used for hockey sticks is good) on the surface, and measure the high-E material, not the bare metal.
It's possible that the leak point is pressure sensitive, and only occurs during the higher temp operation when the pressure is highest, but find it you must! A steady diet of fresh water in a heating system causes excess corrosion on the metals in the system (iron pump impellers can't really take it.)
If the direction of the pump flow is away from the system pressure meter ( and toward the higher impedance of the radiation or boiler), that would cause the measurement to drop when the system first fires up. If the pressure drops too low on a low mass boiler you will often hear the sizzle & bang of larger bubbles of water vapor collapsing at the heat exchanger (sometimes called "kettling"). This is really bad for efficiency, since the foamy water and water vapor in contact with the heat exchanger are less thermally conductive than micro-boil water, so less heat gets transferred, and more heat goes up the flue. On low mass boilers it's best practice to pump TOWARD the boiler, (and away from the expansion tank) so that the pump pressurizes it slightly above the average system pressure. If it's pumping away from the you may need to raise the pressure to 15-18psi to reliably avoid the sizzle, which would then give you even less margin on the EX15 expansion tank.