Are you oversizing the expansion tanks in your radiant heating systems?
All closed-loop hydronic radiant heating systems require an expansion tank to accommodate the increases volume of the heated fluid.
The classic method of sizing an expansion tank looks at the volume of the water when the system is at it highest temperature, and compares it to the volume of the water when the system is filled. A tank volume is then calculated that will accept the increased volume without allowing the system pressure to climb high enough to open the relief valve.