What is causing the pressure relief valve on our gas-fired hot water baseboard heating unit to leak?
The pressure relief valve should not be leaking any water. If it is leaking water, there is a problem with the water heater. The most likely cause is a faulty thermostat that is heating the water past the recommended temperature and not shutting off the water heater. This causes pressure to build in the water heater, and the relief valve will open to relieve that pressure. You can install an expansion tank that goes on the cold water line that helps with pressure changes in your water heater. Honestly, if your water heater is 20 years old, you should think about replacing the whole thing.