Today I had a new washer installed and now the hot water pressure through out the whole house is very low and no hot water at all in the showers

toyandre, did the installer shut any shut-off valves? Check them, if there good I would flush out the water heater sounds like might be debris in the lines.
Speedball gives good directions on how to flush it proper here, good luck

Did you leave the pressure on when you opened the boiler drain?
For long life and fewer troubles you should keep your heater clear of mineral build-up by flushing on a regular schedule. Attach a hose to the boiler drain at the bottom of the tank. With the pressure on, open the boiler drain and let it run until the water runs clear. You will see a spurt of red,(rust) followed by white or yellow grains,(lime or calcium carbonate). This shouldn't take more then a few minutes. Do this monthly to keep it clear. Now flush out your hot water lines on ALL fixtures that are affected . Now pull each aerator and clean the screens. Be sure you put them back togather the same way you took them out. Don't forget to flush it out every month. Your heater will thank you for it. Hope this helps, Tom