Overflow after pressure change

Floor drain overflowing after sudden pressure change

Quick Answer:

If a floor drain starts overflowing after a sudden change in water pressure, check the static pressure at an outdoor spigot with a pressure gauge, inspect the pressure-reducing valve (PRV) and expansion tank for signs of failure, and watch for new leaks downstream. Slowly cycle indoor valves to relieve trapped pressure and see if the overflow stops. If pressure spikes continue or you find damaged parts, call a pro.

Why This Happens

Sudden pressure changes can send a short spike or surge through the house plumbing. That spike can open relief devices, push water into traps and floor drains, or reveal weak fittings and appliance connections. A failed PRV, a waterlogged expansion tank, or a stuck relief valve can make pressure swings worse or let high pressure reach parts of the system that aren’t designed for it.

If the situation followed an automated shutoff or turning water back on, similar events can create rapid pressure changes; see Overflow after smart shutoff trip and Overflow after turning water back on for related scenarios.

Step-by-Step What to Do

1. Stop the immediate overflow

  • Shut off the main water valve if the overflow is active and you can do so safely. This prevents more water from entering the drain and gives you time to inspect.
  • Contain and mop up as needed to reduce water damage.

2. Test static water pressure at an outdoor spigot with a gauge

  • Turn off all water-using fixtures and appliances inside the house so no water is flowing.
  • Attach a pressure gauge made for garden hose spigots to an outdoor hose bib. Open the spigot fully and read the gauge. Residential pressure normally sits around 40–60 psi; sustained readings above 70–80 psi indicate a problem.
  • Record the number and note whether pressure jumps when you open or close indoor fixtures.

3. Inspect the PRV and expansion tank for signs of failure

  • Locate the PRV near the main shutoff or where the supply enters the home. Look for leaks, heavy corrosion, or a loose adjusting screw or cap.
  • Check the expansion tank (usually near the water heater): look for wetness, rust, or a tank that feels full of water rather than having an air cushion. Tap gently—an air-charged tank sounds hollow, a waterlogged tank sounds dull.
  • Do not try to recharge or disassemble the expansion tank unless you are familiar with the process and have the right tools.

4. Look for new leaks downstream from pressure spikes

  • Inspect under sinks, around the water heater, behind appliances, and along exposed pipes for new drips or wetness.
  • Check ceilings and walls below bathrooms and supply lines for stains or active leaks that may have started when pressure spiked.

5. Slowly cycle indoor valves to relieve trapped pressure

  • Open and close a few indoor faucets slowly—kitchen and a couple of bathrooms—one at a time. Do this gently to avoid creating new pressure surges.
  • Watch the floor drain while you do this to see if flow into the drain begins or stops when you operate valves. This helps identify whether trapped pressure or a particular branch of piping is involved.
  • If operating a single fixture causes the drain to overflow, note which fixture and report that to the plumber if you call one.

What Not to Do

  • Avoid adjusting the PRV without a proper pressure gauge or experience; incorrect adjustment can make pressure problems worse.
  • Do not cap or block relief devices (pressure relief valves or temperature/pressure valves). These are safety devices and must remain functional.
  • If pressure spikes persist, plumbing fixtures leak, or the PRV appears damaged, call a professional rather than attempting major repairs yourself.

When to Call a Professional

  • If the static pressure stays high (consistently above recommended levels) or pressure spikes continue after basic checks.
  • If you find visible damage, leaking at the PRV, a waterlogged expansion tank, or persistent leaks downstream.
  • If you cannot safely stop the overflow, or if water has damaged ceilings, electrical systems, or appliances.

Safety Notes

  • Turn off the main water supply before doing any hands-on repairs that involve opening pipes or removing fittings.
  • Avoid working near electrical panels or outlets that have been exposed to water; shut power off to affected areas if water has reached them and call an electrician if needed.
  • If you suspect the water heater relief valve discharged, do not cap it—this can cause dangerous pressure build-up. Leave it accessible and call a plumber.

Common Homeowner Questions

  • Can I adjust the PRV myself? Short answer: Not unless you have a proper pressure gauge and know how; incorrect adjustments can harm the system.
  • Will a high pressure spike damage appliances? Short answer: Yes—repeated high pressure can stress fittings, valves, and appliance connections and cause leaks.
  • Is the pressure relief valve the same as the PRV? Short answer: No—the PRV lowers supply pressure to the house; relief valves are safety devices that discharge when pressure or temperature gets too high.

More in this topic

For more related fixes and similar symptoms, see Toilet Overflows with Clear Drain.

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