Understanding Your Home's Plumbing System
Your home's plumbing consists of two completely separate subsystems: one brings pressurized freshwater in (the supply system), and the other relies on gravity to take wastewater out (the drainage system). Vent pipes allow air into the system so water flows smoothly.
The Water Supply System
Fresh water enters your home under significant pressure from the municipal main or a well. This pressure allows water to travel upstairs and around corners.
- Water passes through a meter that records usage.
- The main shutoff valve controls all water entering the house.
- The supply line splits, sending cold water directly to fixtures and to the water heater for hot water needs.
The Drainage System (DWV)
Unlike supply lines, drainage pipes do not rely on pressure. Instead, they pitch downward, relying entirely on gravity to pull wastewater to the sewer.
- Drain pipes are larger in diameter than supply lines to handle waste.
- Wastewater flows into the main soil stack, which carries it to the main sewer line.
- Because they lack pressure, drainage pipes must be kept clear of obstructions to function.
The Importance of P-Traps
Look under any sink, and you will see a curved, U-shaped pipe. This is a P-trap, and it is a critical safety feature.
- The trap holds a small amount of water at all times.
- This water barrier prevents noxious and explosive sewer gases from backing up into your home.
- Every fixture connected to the drainage system must have a trap.
The Venting System
Drainage systems require air to flow smoothly, much like you need a second hole in a juice can to pour it quickly. Vent pipes stick out of your roof to provide this air.
- Vents prevent the vacuum effect from slowing down drainage.
- They stop rushing water from siphoning the water out of your P-traps.
- Blocked roof vents (often by leaves or bird nests) cause slow drains and gurgling toilets.
When to Call a Professional
Call a professional if you experience widespread low water pressure across the house, or persistent slow drainage in all lower-level fixtures.
What a Plumber Will Do
Plumbers can diagnose systemic issues, trace complex venting problems causing bad odors, and identify whether a problem lies in the supply or drainage system.
Frequently Asked Questions
Why do my pipes bang when I turn off the water?
This is called "water hammer," caused by fast-moving water crashing into a closed valve. It can be fixed by installing water hammer arrestors.
What is a PRV?
A Pressure Reducing Valve (PRV) steps down the high pressure from the city water main to a safe level (usually 50-70 psi) for your home's pipes and appliances.
Are supply lines and drain lines made of the same material?
Typically no. Supply lines are usually copper, PEX, or CPVC to handle high pressure, while modern drain lines are almost always PVC or ABS plastic.
What is a cleanout?
A cleanout is a capped pipe located on your main drain line that provides plumbers easy access to clear severe blockages with an auger.