Quick Answer:
If the floor drain is overflowing while the installer tests the new heater, first stop any ongoing discharge and inspect the temperature-and-pressure (T&P) valve discharge pipe routing, look for temporary test hoses or open valves left by the installer, and confirm the heater’s drain pan is plumbed and sloped correctly to the drain. Then run a short controlled test of the T&P to see exactly where water goes. If the T&P keeps discharging or you see joint leaks, contact the installer or a licensed plumber right away.
Why This Happens
The T&P valve releases hot water when a heater gets too hot or pressure gets too high. During installation the installer may test the valve or refill the tank; if the discharge pipe is misrouted, blocked, or connected temporarily with a hose, the water can end up in the floor drain area and cause overflow. A misconnected or poorly sloped drain pan will also let that water run toward the floor drain instead of away from the house. Thermal expansion or an absence of an expansion tank can cause repeated discharges under pressure.
Step-by-Step What to Do
1. Stop the immediate overflow
- If water is actively overflowing, ask the installer to stop testing and close any test valves. If the installer is not present, shut off the water supply to the heater (the cold-water shutoff) and, if safe, shut off power or gas to stop heating until the issue is resolved.
- Contain water to limit damage: move rugs, towels, and any stored items away and mop or soak up standing water.
2. Inspect the T&P valve discharge pipe routing
- Trace the discharge pipe from the T&P valve to its outlet. It should lead to a proper safe outlet, not into a ceiling, an attic, or inside a cabinet. Confirm it terminates near an appropriate drain or outside where designed to do so.
- Check that the pipe is secured, not blocked, and that fittings are tight where it connects to the valve.
3. Check for temporary test hoses or open valves left by the installer
- Look for any garden hoses, flexible tubing, or open drains temporarily attached for testing. These are sometimes used during commissioning and can be left unsecured or routed improperly.
- Close any test valves or disconnect temporary hoses and make sure the system is returned to its permanent piping configuration.
4. Verify the heater’s drain pan routing and slope
- Confirm the drain pan under the heater is actually plumbed to the floor drain or to an exterior drain. The pan outlet should slope downward toward the drain with no sags or high spots that trap water.
- If the pan drains to a vertical pipe, check for clogs or trapped debris that could back up during a discharge.
5. Run a controlled test of the T&P to see where water discharges
- With the installer present (preferred) or after taking precautions, perform a short controlled lift of the T&P lever to let a small amount of water out. Hold the valve only briefly and release it; do not force it wide open or hold it for long periods.
- Watch where the water lands. If it drains into the pan and down the drain properly, routing is fine. If it heads to the floor drain area and overflows, note the exact path and whether a blocked drain or misrouted pipe is to blame.
- Document what you see (take photos) and share them with the installer or plumber if you need follow-up work.
What Not to Do
- Avoid capping the T&P outlet or otherwise blocking it to stop the discharge. That creates a dangerous pressure hazard.
- Do not redirect hot discharge into a sink, tub, or fixture that is not rated to accept hot, high-pressure water.
- Don’t delay contacting the installer or a licensed plumber if discharge continues or if you find leaking joints. Continuous discharge or visible leaks need professional attention.
When to Call a Professional
- Call the installer immediately if the unit is still being tested and water keeps discharging, or if you find temporary test hardware left connected.
- Call a licensed plumber if the discharge pipe is damaged, the drain pan isn’t plumbed correctly, the floor drain is blocked, or if you cannot safely stop the leak.
- Call a pro right away if you see continuous T&P discharge, leaking joints, or any sign that pressure or temperature are out of control.
Safety Notes
- Discharging water from a T&P can be very hot. Wear gloves and avoid skin contact with the flow.
- Do not attempt to modify or disable safety devices like the T&P valve. Those protect the tank from dangerous pressure or temperature conditions.
- If you must shut off the heater, use the cold-water shutoff valve to stop water flow to the tank, and turn off power or gas to the heater if it’s safe to do so.
For related troubleshooting see Overflow when water heater drains and Overflow after turning water back on for scenarios that look similar but have different causes and fixes.
Common Homeowner Questions
- Is it normal for the T&P to discharge during installation?
Short, controlled discharges during testing are normal; continuous discharge is not. - Can I stop the leak by putting a cap on the T&P?
No. Never cap or block a T&P outlet — that creates a safety hazard. - Will the installer fix any water damage?
Ask the installer about warranty and responsibility immediately; take photos and document the situation in case follow-up is needed.
More in this topic
For more related fixes and similar symptoms, see Floor Drain Overflows During Appliance Use.