Part 1 - The Basics
Custom showers are multiple-head showers. They consist of valves, water outlets, volume controls, shower heads, hand showers, or body sprays.
Custom showers require plumbing changes and/or major considerations before installing. You cannot just simply add more shower heads or sprays to an existing shower stall or tub enclosure. Custom showers should be installed by a licensed plumbing contractor.
Water Pressure
Water pressure is a critical factor in a custom shower. GROHE recommends at least 50 psi for a system with 3 or more outlets. If water pressure is below 45psi a water booster pump should be installed.
Hot Water
The bigger the custom shower, the more hot water will be required. A water heater of sufficient size, dedicated to the custom shower is recommended.
A dedicated 50 gallon hot water heater will supply a four-outlet custom shower for approximately 8 minutes (assuming shower heads are restricted and all are turned on for the entire duration). For best performance, GROHE recommends a 100 gallon water heater as a minimum.
Water Softeners
Water softeners can restrict the flow of hot water needed for maximum shower performance. If you must use a water softener to control hard water conditions, choose a model designed for high water flow installation.
The Drain
Drain capacity is critical. For example, two Grohtemp 34.902 valves can flow 32 gallons per minute ... enough to fill a typical 5' bathtub to the very top.
Kohler's Full Body recirculating shower systems (carwash showers) offer a water-saving alternative.
In the shower, two 2" floor drains or one 3" drain is recommended, especially when an accessible shower pan is installed.
Note: A water well or cistern might not be able to service these showers, and your current septic tank might not be able to handle the additional demands. Make sure to discuss this with your plumber before deciding on a custom shower.
Ventilation System
Custom showers produce high levels of humidity. An adequate ventilation system must be provided in order to maintain proper household humidity.
The Valves
The valve can be a single handle, which controls both water temperature and water flow, or a dual handle valve - with one control for water temperature, and one for water flow.
Thermostatic Control Valve
If a single valve is used, it should be a single handle thermostatic temperature control valve, and preferably a 3/4" thermostat valve.
Temperature control in a custom shower is much more than a performance consideration, it is a safety consideration.
A pressure balance valve is not a high flow valve, and cannot be expected to replace a thermostatic temperature control valve, especially in a multiple shower outlet installation.
It is not necessary to have a 3/4" supply line to use a 3/4" valve.
A 1/2" supply line plumbed into a 3/4" valve will still effectively increase water flow, but a 3/4" supply line is preferred.
Given adequate water pressure, one 3/4" thermostat valve can supply enough water to simultaneously power 6 outlets ... a shower head, a personal hand shower and up to four body sprays... while maintaining a specific, pre-selected water temperature.
If the number of shower outlets desired is more than one valve can accommodate, it will be necessary to use two (or more) thermostat valves.
When more than one valve is necessary
GROHE recommends the water supply line be 1" or larger.
Each valve should be plumbed directly from the main water supply. Using a "tee" can cause inconsistency in both water flow and temperature.
Volume Controls (water flow controls)
A volume control is a valve that turns the water on and off. Starting at the "off" position, you can steadily increase water flow through to the "full on" position.
Custom showers are generally designed with separate on/off (volume) controls for each outlet . Usually. one volume control for the shower head, one for the hand shower, and one for three body sprays.
Volume controls, of course, have their own flow rates. Manufacturers recommend using 3/4" volume controls throughout the system. If cost is a concern 1/2" volume controls can be used, however, the flow rate is reduced to 8 gpm at 45 psi.
If the combined flow rates of the outlets exceed the volume control flow rate, there will be a reduction in performance.
Diverter Valves
Because diverter valves can only change the direction of water, and not turn water flow on or off, they are rarely used in custom shower systems.
A diverter valve can channel water to the showerhead OR the handshower OR the body sprays...but not all at the same time. If a hot supply and a cold supply were plumbed into a diverter valve, the water would run constantly with no way to stop it.
How Many Shower Outlets?
The sum total flow rate of all shower outlets (shower heads, hand showers, or body sprays) combined cannot exceed the maximum flow rate of the valve at the given water pressure
Exceeding the capacity of the valve will only lower the overall performance of the shower system.