It Leaks Only Sometimes Decision Tree
Why this matters
The hardest leak to find is the one that is dry when you arrive. The customer swears it leaks; the floor is wet; the joint is bone-dry under your light. An intermittent leak is not random, it is conditional, and the whole job is finding the condition that triggers it. Get systematic and you find it on the first visit; get lucky-based and you make three trips. This tree applies to plumbing, HVAC condensate, appliance lines, roofing and flashing, and pool equipment. The unifying idea: a leak that comes and goes is tied to something that comes and goes.
Start here: what was happening when it leaked
Before you touch anything, interview the timeline. The trigger is usually in the customer's answer.
- When did you see water? Time of day, and what was running.
- What else was on? A specific fixture, appliance, the heat, the AC, a pump, rain.
- Does it track with use, weather, or temperature? This single answer often names the family.
Sort the answer into one of four conditional families: use-driven, pressure/load-driven, temperature-driven, or environment-driven. Each has a different hunt.
If it leaks only when something runs (use-driven)
The leak appears when a particular fixture, appliance, or cycle operates.
1. Identify the exact thing. Which fixture, which appliance, which cycle. The water source is on that line or that drain, not the whole system.
2. Reproduce it. Run that fixture or cycle and watch. A drain leak shows only while water is draining; a supply leak on a sometimes-used line shows only when that line is opened.
3. Watch the drain path specifically. A huge share of "sometimes" leaks are drain, not supply: the line is dry until that appliance or fixture sends water through, then it weeps at a bad joint or a clog backing up. Static pressure tests miss these entirely because the drain is empty at rest.
4. If reproducing the use makes it leak, you found it. Trace to the wet joint while it is actively running.
If it leaks only under pressure or load (pressure-driven)
The joint holds at rest but weeps when pressure rises.
1. Note when pressure is highest: a pump kicking on, thermal expansion, a surge, peak demand, a pressure spike at shutoff (water hammer).
2. Pressurize and hold. Bring the system to its working or test pressure and watch the suspect joint over time. A joint that is dry static but weeps under pressure is a marginal seal that only opens under load.
3. Watch for the spike, not just steady pressure. Some leaks only appear on the transient, the moment a valve slams or a pump starts, when pressure briefly jumps. If steady pressure holds dry, cycle the trigger and watch for the brief weep.
4. A joint that passes static but fails under working pressure is the leak. Remake it; do not pass it just because it was dry when you walked in.
If it leaks only hot or cold (temperature-driven)
Temperature changes dimensions and reveals condensation.
1. Rule out condensation first (see Reading Leak Stains and Mineral Trails). A "leak" that appears on a cold surface in humid weather and dries up otherwise is sweating, not a leak. Wrap it dry, run it, and see if water returns on the outside of the cold surface.
2. Check for thermal-cycling leaks. A joint that is tight cold and weeps hot (or the reverse) is moving with temperature: expansion and contraction working a marginal seal loose. It leaks only at one end of the temperature range.
3. Reproduce the temperature. Run the system hot, or let it sit cold, to match the reported condition, then watch.
If it leaks only in certain weather (environment-driven)
Mostly roofing, flashing, and outdoor equipment.
1. Tie it to the weather event: rain (and from which direction), wind, freeze/thaw, snow load. The trigger is the weather, so the leak path opens only under that condition.
2. Reproduce what you can. A controlled water test on a roof or flashing, from the low side working up, can trigger a wind-driven or angle-specific intrusion that a calm rain does not.
3. Remember the water travels (see the leak-stains article). The intrusion point is uphill and upstream of the interior stain, often well away from it.
The shared method
| Reported trigger | The leak is tied to | How to find it |
|---|---|---|
| Only when X runs | That line or drain | Run X and watch |
| Only under pressure | A marginal seal | Pressurize and hold |
| Only the pressure spike | The transient | Cycle the trigger, watch the surge |
| Only hot or only cold | Thermal movement or condensation | Reproduce the temperature |
| Only in weather | An intrusion path | Controlled water test, low to high |
In every case: identify the condition, reproduce the condition, watch while it is happening, and trace to the active drip, not the collection point. Photograph the trail before you reproduce so you have the as-found evidence. An intermittent leak only stays a mystery until you find what flips the switch.
References
- Trade-standard leak-detection practice (static vs working pressure, controlled water testing)
- Manufacturer documentation on condensate management and pressure ratings
- See related: Reading Leak Stains and Mineral Trails; Reading Rust and Corrosion Patterns
- See related: It Smells Only When Running (decision tree)