This step may actually be your "Part 1" if you're not planning on installing a Hydrotherapy tub with jets and added features requiring electrical connections. If you're simply installing a soaker tub with no electrical connections, start with this video.
Walk-in Tub Plumbing
IMPORTANT: Before you remove your old tub, be sure that you've turned off the water either at the source or at a valve that controls the room in which you'll be working.
When installing a new walk-in tub, it's important to make sure you have all of your tools and hardware with you before you begin. This will not only save you time from running back and forth to the hardware store, but will aid in the ease of installation.
Figure out what type of plumbing you currently have...
Decide whether you're home is currently using Copper, Cast Iron, or Pex (plastic) plumbing. This will be the deciding factor in purchasing the hardware that you'll be using to attach your new walk in bathtub.
Thermostatic Mixing Valve
These valves can sometimes be problematic and depending on the situation, may sometimes be left off of the installation. Thermostatic mixing valves can cause problems with water not getting hot enough, or sometimes may prevent hot water from coming out at all. If you have any problems like this after installation, be sure to check your mixing valve. (These valves are particularly useful for users with diabetes or lack of nerve sensation in their feet)
Before Applying Glue
Remember that you should always do a test fit on the plumbing before you actually glue it all together. This is a mistake that newbies and even experienced plumbers will make when they're overly confident with their work. However, a mistake made in the measuring can add hours of unnecessary work to your project. It's better to spend a few minutes of time and find it that you were right the first time, than to spend a few hours trying to sort out the problem you've created for yourself.
Fill and Drain Times
Many people complain about the time in which it takes to fill or drain their walk in tub. This is directly related to the size of the plumbing going into and out of the tub, but can also be relative to the amount of water pressure your home receives from the main water line coming into the house.
Problems in Older Homes
Many times, older homes may have problems supporting a walk-in tub and take a lot longer to drain or fill than a newly built home. The reason for this is typically caused by corrosion in the lines that prevents the full flow of water from coming into or draining out of the tub. A simple solution is to have a plumber clean your drain lines prior to installation to insure the fastest possible drain time.
Filling it quickly
The larger your incoming lines are, the faster it should fill. Check your lines for any corrosion on the inside and do a bucket test to figure out how many gallons per minute your existing lines can produce. To do a "Bucket Test", simply grab an empty gallon jug and place it under the faucet. Start a stop watch and see how long it takes to completely fill the jug with water. Now, multiply the amount of time that it took to fill the jug by how many gallons your new tub will take to fill. This will give you a pretty good idea of whether or not your existing plumbing is up to the task.