Standing water after water heater install

Quick Answer:

If you find standing water in the mechanical room after a new water heater was fitted, don’t panic. Start with a quick visual check: the heater’s drain pan, any temporary plugs left from the install, and the relief valve piping. Run a short hot-water cycle to see if the water changes, and check the expansion tank and system pressure. These checks often reveal the cause; if water keeps appearing or the relief valve is active, call a plumber.

Why This Happens

After a water heater installation, a few common things can cause water to collect on the floor:

• Installer left a temporary shipping or test plug in a pan or drain line.
• The drain pan discharge was routed incorrectly or tied into a floor drain line.
• The temperature-and-pressure relief (T&P) valve was piped in a way that sends discharge to the room.
• Thermal expansion raised system pressure and pushed water out of a valve or weak joint.
• Loose fittings or a small leak at new connections.

These situations are similar to other post-work issues such as Standing water after plumbing repair, so the same basic checks apply.

Step-by-Step What to Do

1. Visually inspect the drain pan and its discharge routing

• Look under the heater at the pan. The pan should have a dedicated drain line leading to a safe outlet (floor drain, utility drain, or outside). Confirm the pan drain line doesn’t simply feed the floor drain in a way that traps water.
• Follow the pan’s discharge piping as far as you can see. If it appears connected to another drain incorrectly, stop and call a pro.

2. Check for temporary plugs used during install

• Installers sometimes use rubber or plastic test plugs in drain lines or the heater drain valve. Inspect the drain, fittings, and any visible flexible hoses for leftover plugs or caps and remove them if present.
• If you’re unsure whether a fitting is a temporary plug, don’t cut or force it—document it (photo) and contact the installer or a plumber.

3. Run a short hot-water cycle

• Open a hot faucet for 1–2 minutes and watch the area. If water appears only during or immediately after the cycle, the source may be the relief valve or a heating expansion event.
• Note whether the standing water level changes or if water is actively flowing from a particular pipe or valve.

4. Examine new relief valve piping for misrouting

• The T&P valve must have a discharge line that points downward and ends at a proper location. Ensure it isn’t routed into the pan or feeding other drains inappropriately.
• If the relief valve is discharging, that indicates pressure or temperature issues. Capture where that discharge goes and do not block it.

5. Verify the expansion tank and pressure settings

• If an expansion tank was added or replaced, check that it’s installed and that its air charge roughly matches the household water pressure (often near 40–60 psi). A soft-tired tank or incorrect pre-charge can let pressure spike and push water out.
• Look for pressure irregularities at a faucet or on a visible pressure gauge if you have one. If pressure seems unusually high or fluctuates when the heater cycles, call a pro.

What Not to Do

• Do not assume the water heater pan can be used as a trap for other drains. Pans are sized and intended only to catch relief or small leaks and must be routed correctly.
• Do not cut unknown pipes or remove piping without confirming what they are. Cutting can make leaks worse or create safety hazards.
• Do not block, cap, or reroute a T&P valve discharge yourself. That valve must remain free to discharge for safety reasons.
• Do not keep working on gas or electrical heater connections unless you are qualified and comfortable—those systems can be dangerous.
• Call a pro when installation coincides with persistent standing water, an actively leaking relief valve, or pressure irregularities.

When to Call a Professional

• Water keeps returning after you’ve checked the pan, plugs, and piping.
• The T&P valve is actively discharging or you see water coming from relief piping.
• You notice large or rapidly increasing pressure, noisy cycling, or the expansion tank appears failed.
• The source is unclear, the routing looks incorrect, or work is needed on gas or electrical connections.

Safety Notes

• If you must touch any connections, shut off the water supply to the heater first. For electrical heaters, switch off the breaker before reaching into the area near wiring.
• If you smell gas or suspect a gas leak, leave the building immediately and call your gas company or emergency services. Do not operate electrical switches or use open flames.
• Wear gloves and eye protection when inspecting under the heater—standing water may hide sharp edges or contaminated fluids.
• Take photos of any suspicious fittings or leaks before touching them. That helps the professional you call diagnose the issue faster.

Common Homeowner Questions

• Why is there water only after the new heater was installed?
  Often it’s a temporary plug, misrouted pan or relief piping, or thermal expansion pushing water out of a weak joint.
• Can I just mop it up and ignore it?
  No. Mopping hides the symptom but not the cause. Locate the source or call a plumber if you can’t find it.
• Is a leaking relief valve an emergency?
  Yes—if the relief valve is actively discharging, contact a professional promptly because it indicates pressure or temperature problems that affect safety.

More in this topic

For more related fixes and similar symptoms, see Standing Water in Floor Drain.