Quick Answer:
If two fixtures are flowing into each other when only one should, you have a crossover or backfeeding issue. Start by isolating supplies: turn off each fixture stop sequentially until backfeeding stops to find which valve or line is allowing flow the wrong way. That will tell you whether it’s a simple valve problem you can fix or a plumbing crossover that needs a pro.
Why This Happens
Backfeeding happens when water finds a path from one fixture to another because of a cross-connection, failed valve, or incorrect piping. Pressure differences make water move from the higher-pressure side into the lower-pressure side, so a sink filling while a nearby toilet is used can push water the wrong way. Sometimes shared supply lines or a faulty mixing valve allow this. Problems like a stuck shutoff, worn washer, or an unintended connection inside a wall can all cause a Cross-connection causing pressure drops.
Step-by-Step What to Do
Step 1: Stop any active flows
Turn off the fixtures you’re using (faucet handles, shower valves, etc.). Stop any appliances that use water, like washing machines or dishwashers, so you can test calmly.
Step 2: Turn off fixture stops, one at a time
At each fixture, there is usually a shutoff valve under the sink or behind the toilet. With everything else off, close one fixture stop and watch the other fixture.
- If the backfeeding stops when you close a particular stop, that fixture or its supply valve is the source.
- If the backfeeding continues, reopen that stop and close the next one.
Step 3: Narrow down the problem
Once you find which shutoff stops the backflow, test the suspect fixture’s valve by opening and closing it a few times. A valve that leaks internally or a damaged mixing valve can allow flow between hot and cold or between fixtures.
Step 4: Check visible connections and appliances
Look for flexible hoses, appliance connections, and shutoffs. If any hose is loose or a valve handle turns freely without stopping flow, note it. If you’re able and comfortable, tighten a loose hose connection or replace a leaking flexible line with a new one rated for potable water.
Step 5: Test again and document what you found
With suspected fixes in place, repeat the sequential shutoff test to confirm the backfeeding is gone. If it stopped only when a certain supply was shut off and your repair didn’t fix it, you’ve likely got a hidden crossover or a failed internal valve.
Step 6: If detection is needed, follow a careful process
If you can’t find the source by visual checks and shutoff testing, consider tracing the lines and using simple tests like turning fixtures on one at a time while watching pressure and flow elsewhere. For more detailed troubleshooting, see guidance on How to detect cross-connection.
What Not to Do
- Don’t leave crossover unresolved — it can damage fixtures, waste water, or contaminate supplies.
- Don’t remove pipes or make permanent changes unless you know how to properly shut off and drain the system.
- Don’t assume the main shutoff is the only solution; isolating individual fixtures is safer for diagnosis.
When to Call a Professional
- If you cannot stop the backfeeding with sequential shutoffs or you can’t identify the source.
- If you suspect contamination (odd tastes, smells, or visible discoloration) or a cross-connection in concealed piping.
- If repair requires cutting pipes, soldering, or modifying water appliances — these are tasks for a licensed plumber.
Safety Notes
- Shut off water to the affected area before working on fittings. Use the individual fixture stop valves first.
- Avoid electrical contact when water is present. Turn off nearby electrical devices if there is any risk of splash into outlets.
- Wear gloves and protect surfaces. If you suspect contamination, avoid using the water for drinking or cooking until the issue is resolved and the supply is flushed or tested.
Common Homeowner Questions
Q: How quickly should I act if fixtures are backfeeding?
A: Act immediately to isolate supplies with fixture stops and avoid using the water until the cause is known.
Q: Can I replace a faulty shutoff valve myself?
A: If you have basic plumbing skills and the correct parts, you can replace a visible valve; shut off the main and drain the line first. If you’re unsure, call a plumber.
Q: Could backfeeding cause contamination of my water supply?
A: Yes. Cross-connections can allow contaminants into your potable water. Stop using affected water for drinking and call a professional if contamination is possible.