The Fault Only Happens in Certain Weather: Decision Tree

Why this matters

Some faults only show up when the weather cooperates: rain, a hard freeze, a heat wave, high humidity, a storm, a strong wind. On a clear day the system tests perfect, so the tech leaves, and the customer's problem comes back with the next front. The weather is not a coincidence, it is the trigger, and naming which weather it is points you straight at the mechanism. This tree sorts a weather-driven fault by the condition that brings it, so you look where that condition does its damage instead of guessing on a dry, calm afternoon.

Safety first

Weather-triggered faults sit next to weather hazards. Do not diagnose in the hazard to catch the fault. In an active electrical storm, stay off roofs, out of standing water, and away from anything the storm could energize; wait it out. Where the fault involves water reaching electrical equipment, treat it as a shock hazard first, de-energize and verify dead before you probe a wet circuit. On ice or in high wind at height, the footing and fall risk outrank the diagnosis. Recreate a condition only when you can do it without standing in the danger it creates.

Start here: match the fault to the weather

Get the customer to tie the fault to a specific condition, not just "bad weather." Which weather, how soon after it starts, and does it clear when the weather passes. The condition they name selects the branch.

If it happens in rain or wet weather

Suspect water intrusion. Water is getting somewhere it should not, and only rain delivers enough of it.

• If the fault is electrical or control-related, look for water reaching a connection, enclosure, or conductor: a failed seal, a gap at a penetration, a filled conduit, a wind-driven path under flashing. De-energize before probing anything wet.
• If it is a flow, drainage, or backup fault, suspect that rain is overwhelming a drain, gutter, sump, or grade, or that groundwater is rising into the system. Look at where the extra water lands and whether the system can move it.

If it happens in a hard freeze

Suspect ice and cold contraction.

• If flow stops or a line bursts, water froze somewhere exposed, uninsulated, or on an outside wall. Trace the coldest run.
• If something binds, cracks, or a seal leaks only when cold, materials contracted and pulled a joint or clearance out of range. Freeze-thaw cycling loosens what a single cold snap only stresses, so ask whether it started after repeated cycles.

If it happens in heat or strong sun

Suspect thermal load and expansion.

• If capacity falls short or a system cuts out only on the hottest part of the day, the heat load exceeded what it can reject, or a component drifted or tripped as its own temperature climbed past a limit. Measure under the heat, not after it passes.
• If the fault tracks direct sun rather than air temperature, suspect solar gain on one exposed spot: a sun-baked sensor reading high, a component in a hotbox, expansion in a sunlit run.

If it happens in high humidity

Suspect condensation and moisture, distinct from liquid rain.

• If it is electrical or control-related, moist air is condensing on a cold surface inside an enclosure, bridging or corroding a connection without any visible leak.
• If it is a comfort or performance fault, the system may simply be unable to handle the latent load the humid air brings, which reads as a fault but is a capacity limit.

If it happens during a storm or right after

Suspect an electrical transient. A nearby surge or a grid disturbance can damage or upset electronics and controls with no visible cause the next day. Look for a component that failed or a control that latched an error timed to the storm, and check surge protection and grounding.

If it happens in high wind

Suspect pressure and movement. Wind changes building pressure, which can reverse a draft, pull a drain seal, or move air the wrong way, and it physically moves loose mounts, panels, and runs. Check draft and pressure with the wind up, and look for anything working loose.

Recap

1. Clear the weather hazard before diagnosing; never stand in the danger to catch the fault.
2. Get the specific condition, not "bad weather," and let it pick the branch.
3. Rain, water intrusion. Freeze, ice and contraction. Heat and sun, thermal load. Humidity, condensation and latent load. Storm, electrical transient. Wind, pressure and movement.
4. Measure under the condition where you safely can, because the fault is not there when the weather is not.

References

• Trade-standard practice for environmental and weather-related fault diagnosis
• NFPA and manufacturer guidance on surge protection, grounding, and freeze protection
• See related: You Can't Reproduce a Weather-Driven Fault On Site (decision tree); The Difference Between an Intermittent and a Condition-Dependent Fault