Calcium build up in your water pipes can lead to slow drains, cause your hot water to run cold, and other plumbing issues. If you have especially hard water, you may need to clean your water pipes every 3 to 6 months to avoid heavy calcium build up.
Turn off your water while working on your pipes.
Purchase a chemical product such as Lime-A-Way or CLR. Mix in a 50/50 solution with water, and flush it through your pipes. You may also have success with a product such as Drano Foaming Pipe Snake.
Go green. If you are looking for a "green" solution to this problem, flush the water pipes with white vinegar. The vinegar will dissolve the calcium build up.
Use a filtration system. The most expensive option is to install a water filtration system that uses reverse osmosis or a similar method.
Clean your shower heads and faucets to remove calcium deposits. Detach the fixtures, and allow them to soak for several hours in Lime-A-Way or white vinegar.
A more expensive option is to install a water softener system in your home. This will be a much more effective solution to your problem.
Be careful when using any sort of harsh chemical to clean your water pipes. If the water flows to a reservoir for drinking water, you may contaminate the portable water supply.