To avoid confusion over debits and credits, avoid thinking of them in the way that they are used in everyday language, which often refers to a credit as increasing an account and a debit as decreasing an account. For example, if our bank credits our checking account, money is added to it and the balance increases.
One way to reduce the frequency of this occurrence is to maintain an air pocket at the top of the water heater tank. This air pocket will form in the tank by design - however, it will be reduced over time by the everyday use of your water heater. To replenish this air pocket: Turn off the water heater.
pump (circulator) on a hot water heating system has the same function as the fan on a forced air heating system. We are pushing the heat transfer medium (water) from the boiler out to the rooms we want to heat. Just like the fan on a forced air system, the pump pushes the heated water out to the system and pulls the cool water back.