excel spreadsheet for diminishing temperature in a buffer tank

Description

I need to calculate the diminishing temperature of a buffer tank that is fed with hot water from a boiler and itself feeds radiators or underfloor heating. when the heat requirement is greater than what the boiler can provide, the temperature of the buffer tank will start to decrease and the flow to the radiators will need to increase to compensate...inputs will include flow and temperature from the boiler, delta T from return to boiler and supply from boiler, flow/temperature for the radiators/underfloor heating, Kw in and out, how long the tank will take to cool to a set temperature, what the tank temperature needs to be to last for a set time, and any other things that might be useful...:) I think it's similar to the built-in mortgage calculation in excel?