Every time my radiator heat turns on, the pressure gauge on the boiler spikes into the red zone. What's causing this?

The most likely culprit is a waterlogged expansion tank, which can no longer absorb the expanding water as your system heats up. Depress the tank's Schrader valve briefly—if water squirts out instead of air, the internal rubber bladder has failed and your pressure relief valve will eventually spill onto the floor. Call a licensed hydronic heating contractor to replace it, as servicing pressure vessels requires proper system isolation and draining; total repair costs generally range from $200 to $600 depending on local labor rates and tank size.