Why does my hot water heater have a blower on top of it?

High-efficiency units need fan-forced exhaust because the exhaust temperature is too low for exhaust gases to be adequately removed by convection alone.

I presume it's a gas fired high efficiency water heater.

As you pointed out, hot air rises, but since the exhaust "pipe" can run sideways for quite a distance in some installations a fan is needed to push the combustion products outside.

Capice?

Jeff

Not needing a chimney, they can be located more conveniently.

The power vent also prevents back draft which is common in some of the newer tightly constructed houses. the old style type furnaces are a safety hazard in new construction so count yourself lucky.

What will happen if my blower is not working. Should I unplug it until I order new one

You're at tremendous risk of carbon monoxide poisoning. I would turn off the gas supply to the water heater immediately and get the unit serviced (if feasible) or replaced today. Meanwhile, you can go a day or two without washing clothes or taking a shower. Dishes can be washed in the sink with cold water unless someone is very ill with a highly communicable disease, in which case, buy a small supply of disposable paper plates, cups, and plastic utensils

I agree, it's the exhaust blower and if it's not working, I would turn off the water heater until it can be replaced. I would think they must have an interlock of some kind there to prevent it from firing if the blower fails. If it doesn't have proper airflow it will generate CO.

Yes. the blower purges the exhaust - without it you risk carbon monoxife poisoning.