Last week my boiler was running fine, but you could hear water sloshing along with banging noises. This is a manual system and does not auto fill the boiler. I checked and found that the lever allowing the water into the boiler was left partially open, by my granddaughter. I used a hose to drain all the water from the boiler and then refilled it until it appeared at the right level in the sight glass. Now the boiler is running out of water much too fast. I have no idea why. By the way I am a great-grandmother, so please don't make the answer too technical.

You have a leak, an open drain or open bleed valve. System should use minimal water. Check to see if your safety release is leaking. What pressure is on the boiler?