I'm a new home owner. My house is heated with steam radiators attached to an oil fed boiler. The boiler is getting close to retirement. One concern I have is that I need to drain water from the reservoir every couple days. There's a glass tube on the front of the boiler with a sharpie mark made to indicate the level it should be at. It fills up slowly and I need to place a bucket under the drain and lift the lever to allow excess water out.
What happens if the reservoir fills up completely?