4″ waste line can support 90 DFU's, Toilets can be 3–4 DFU's depending on how many gallons per flush. So, 22 toilets if they use a lot of water. 30 if they're the low flow type. There can be no other fixtures attached to this pipe.
Here are some standard toilet drainpipe sizes and where you'll find them: 3-inch drainpipes: The standard drainpipe used in single-family homes. 4-inch drainpipes: These are used in larger residential buildings (multi-family units with up to 500 fixtures) and high-occupancy spaces (like commercial bathrooms).
A large diameter is useful for a drainage pipe because it can carry extra water. A 4-inch pipe can accommodate considerably more flow than a 3-inch pipe. Four- and 6-inch pipes are commonly used for French drains. Regarding speed, the 4-inch pipe is ideal for French drains.
The relationship between pressure, flow and pipe diameter:
Pipe diameter and flow rate: Flow rate is proportional to the fourth power of pipe diameter, which means that small changes in pipe diameter can cause significant changes in flow rate. As the pipe diameter increases, the flow rate increases significantly.
Common Toilet Drain Pipe Sizes
The standard drain line size for a toilet is typically 3 inches in diameter. This size is recommended to ensure proper drainage and to help prevent the inconvenience of clogging. For households with multiple toilets, it is possible to have up to three toilets on a 3-inch common drain.
If you install the toilet flange flush with the finished floor, or even below the finished floor, leak paths will form, because the flange won't be at the correct height to accept the horn at the bottom of the toilet.
The trench should be about 18 inches deep and 9 to 12 inches wide.
1 1/4 inch – This is the standard size pipe for most bathroom and kitchen sinks that are used an average amount of time throughout the day. 1 1/2 inch – This size pipe will usually be used for a large kitchen sink or any sink that is used more heavily than average.
The horizontal float is typically installed at a 1º gradient, 17mm/m (1:60). It is possible to connect up to 7 WC pans either side of the soil stack before secondary ventilation is required (fig. 2), this is based on BS 5503 WC pans with a spigot outlet height of 190mm (centre) from floor slab.
Under the Uniform Plumbing Code, a 2" vent can handle 24 fixture units(F.U). In a residence, a lavatory sink= 1.0 F.U., bathtub/shower= 2.0 F.U. and a toilet 1.6 gallon or less= 3.0 F.U, greater than 1.6 gallon= 4.0 F.U. So, 3-lav sinks, 3-1.6 toilets and 3-tubs/showers could be on one 2" vent.
Most houses have more than one toilet, and if they are on the same side of the house, their waste lines can usually tie into the same stack. If they are on opposite sides, however, each may need its own stack. Since the stacks must be vented, this would mean two vent openings on the roof.
Regardless of your plumbing code, the minimum drain size for a toilet is 3 inches. Water closet's flushing 1.6 gallons (or less) are rated at 3 DFUs (drainage fixture units). Some older model toilets, flushing over 1.6 gallons per flush, are rated at 4 DFUs.
A toilet trap sounds like something you'd want to steer clear of, but it's actually a key component of a toilet drain that helps keep sewer gases out of your home. There are two types of toilet traps—P-traps and S-traps—that accomplish the same goal with different designs.
It's important to note that the connection between the bath and toilet can't exceed a six-foot radius. Failing to do so prevents the valve from transporting water into the drainpipe. If you can't maintain a six-foot radius, your bath and toilet can't share the same drain.
A drain with 4-inch piping, a 1% slope, and a 1' x 1' gravel trench can transport about 5 GPM.
While you can indeed run a horizontal vent pipe, you can't do it indefinitely; this is designed to provide your pipes with the ability to drain into the sewer system. The typical rule of thumb according to code requires you to run a horizontal pipe for no more than one-third of the total length.
Does Increasing Pipe Size Increase Water Pressure? Generally, a larger pipe can increase water pressure, but there are limits. Larger pipe diameters move more water, and this can create stronger pressure, especially if you have multiple faucets and appliances competing for water.
There is a common concern about the restriction caused by the inserts (stiffeners) needed at joints. These restrict the bore, but they are so short that the the affect on flow is much less than it may seem. The above graphs show relative changes in flow rate that would result from a fixed pressure.