As described the PRV is on a T piece off the cold feed, after a NRV from the pump. If the new calorifier has a built in NRV on the inlet as, some do, then there is no PRV available to release pressure in the calorifier, which may have stood up to an immersion heater at 60 degrees but the 80+ degrees it gets from the engine was just too much, hence the split. The PRV must be on the hot / outlet side of a calorifier, or an expansion tank must be fitted.