How to clear Major plumbing clogs
I am assuming that you are referring to a calcium or rust build up in the water heater.
If you are having this issue you will need to turn off the water to the heater. You will need a garden hose, access to a outside water soucre, cold water is fine.
Shut off the gas or power to the hot water heater before starting.
Now you will need to place the garden house on the drain fitting on the water heater, placing the end of the hose outside and lower than the drain valve. Open the valve and drain the heater, you will need to open a hot water faucet to help drain the heater. I don't suggest opening the TPR ( Temp Pressures relief )valve due to the fact that they sometimes leak or don't reseal after opening them.
Hopefully the drain valve is not clogged, you should hear and see water exiting the garden hose.
If you are not getting any water from the drain valve you are going to have a challenging task.
If no water is coming out, my recommendation is to get the required tools to remove the hot water line from the top or side or the heater, also a few towels to soak up water A bucket is helpfull also.
remove the hot water line, then you will have to use a small hose to siphon the water from the tank. Once the tank is empty you can then remove the drain valve and replace it.
once you have replaced the drain valve you should ensure that it's closed and use a Gallon of CLR. Poor this into the water heater through the hot water outlet where you removed the pipe from. Now make sure the faucets are closed and replace the hot water line. You can now turn the water back on to the water heater.
After a few minutes open a hot water faucet to let out any air until water flows out, than close the faucet.
Let the water heater set for a few hours, then drain the tank!
when the tank drains, close the drain valve again and refill the water heater. Once again open a a hot water faucet and let the air out till the water heater fills. Again let sit for a hour.
At this point open a hot water faucet at the farthest point from the heater and let the water run for 5 min. , than close it and move to the next closest faucet and repeat until all faucets have been opened and run for 5 minutes.
If you still have a slow flow in a faucet, you may need to repeat the CLR step.
I also suggest that you remove all faucet screens and clean them.
If you still have a clog than you are going to have to remove or replace a piece of water line in the house. The blockage may be hard to find but if you have access to most of the pipe or it's visable you may be able to hear the blockage with the water on. Otherwise this will result in using some water line taps and a pressure guage to find the blockage.