Quick Answer:
When house pressure collapses right after adding a new appliance, start by isolating that new device. Bypass it or reconnect the supply directly, check flexible supply lines for kinks, confirm you didn’t tie into a restricted saddle valve, flush the new branch to clear debris, then re-test pressure at a hose bib with a gauge. These checks quickly separate an appliance problem from a house plumbing problem.
Why This Happens
New installations introduce extra fittings, valves and short runs of flexible hose. Any of those can reduce flow if they’re bent, partially closed, clogged with debris from installation, or tied into a small saddle valve. Pressure drop that happens instantly after install usually points to the new branch; slow fading or intermittent loss may be a broader supply issue such as mineral buildup or main valve problems. If you see a pattern like House water supply fades slowly then stops or Water only flows when main valve partially open, those suggest systemic supply or main-valve trouble rather than the appliance itself.
Step-by-Step What to Do
1. Stop using the new appliance and shut its supply
- Turn off the appliance’s inlet valve(s). If the unit has a built-in bypass, set it to bypass mode per the manufacturer so water flows around the unit.
- If you can’t set a built-in bypass, close the shutoff valves on the appliance lines and, if possible, reconnect the incoming and outgoing lines directly to restore the original path.
2. Bypass the new appliance or reconnect the line directly
- Bypassing isolates the device from the house system so you can see whether the pressure returns. If pressure returns to normal with the unit out of the loop, the appliance or its installation is the issue.
- When reconnecting directly, make sure connections are sealed and open valves slowly to avoid water hammer.
3. Check for kinked flex lines and tight fittings
- Inspect any flexible hoses, braided connectors or PEX drops used for the tap-in. A sharp bend or pinched flex line can drastically cut flow.
- Loosen and visually inspect fittings where practical—look for collapsed tubing or internal crush.
4. Confirm you didn’t tie into a restricted saddle valve
- Saddle valves and small-diameter “self-tapping” valves are cheap and restrictive. If the new branch uses one, that’s a common cause of poor flow.
- If you find a saddle valve, plan to replace it with a proper shutoff valve sized to the line by a professional if you’re not comfortable doing the swap yourself.
5. Flush the new branch
- Debris from cutting, soldering, or new hardware can clog fixtures. With the branch bypassed or isolated, open the new branch and flush it into a bucket or drain until water runs clear.
- Check any inline filters or strainers that came with the appliance—clean or remove them temporarily to see if flow improves.
6. Re-test pressure at a hose bib gauge
- Attach a pressure gauge to an outdoor hose bib or a threaded fixture and open it fully. Compare pressure with the appliance bypassed and then with it connected.
- Use this test to confirm whether the appliance installation is causing the collapse or if the issue is upstream.
What Not to Do
- Don’t leave the new appliance connected if it’s choking flow—bypass to isolate before you troubleshoot elsewhere.
- Don’t repeatedly open and close the main without checking for leaks—doing so can stress fittings or pumps.
- Avoid removing sealed components (like factory-mounted cartridges or electrical panels) unless the instructions explicitly say it’s safe for homeowner servicing.
When to Call a Professional
- If pressure does not return when the appliance is bypassed or the reconnection is unclear, call a plumber.
- Call if you find a leaking fitting, a damaged pipe, or a saddle valve you don’t know how to replace safely.
- If troubleshooting shows patterns like the house supply fading intermittently or only flowing under odd valve positions, get professional help to diagnose the main supply—these symptoms can point to bigger system problems.
Safety Notes
- Shut off the water at the appropriate valve before disconnecting fittings. Relieve pressure at a nearby tap first.
- Expect some water when you open connections—have towels and a bucket ready and protect nearby finishes.
- If the job involves soldering, cutting copper, or working near electrical components, hire a licensed pro unless you have the training and tools.
Common Homeowner Questions
- Why did pressure drop immediately after install? Most often a new shutoff, flex line or filter in the new branch is restrictive or closed; bypassing will show this quickly.
- Can flushing the branch fix it? Yes—debris or trapped air is a common cause and flushing often restores normal flow.
- How do I test if the appliance is the problem? Bypass the unit and check house pressure at a hose bib gauge; if pressure returns, the appliance or its fittings are the culprit.