The Fault Started After Another Trade Did Work (Decision Tree)
Why this matters
The customer tells you it started right after the electrician, the plumber, the remodeler, or the solar crew was here. That timeline is the best lead you will get all day, and also the easiest to misread. Another trade's work can trigger your fault without their work being defective, because they changed a background condition your equipment quietly depended on. It can also be pure coincidence that happened to land the same week. Sorting the causal from the coincidental, without either whitewashing or blaming, is the job. Get it wrong and you either replace good parts on your side or accuse a colleague based on timing alone.
Start here: safety if the changed condition is a hazard
If the condition another trade may have altered is one that carries a hazard, address that before diagnosis. A change to grounding or bonding, to a gas appliance's venting or draft, or to anything that puts water near energized parts is a life-safety item, not just a fault. De-energize and verify, or shut off and confirm the hazard is controlled, before you chase the cause. A backdrafting appliance or a lost equipment ground gets handled first, then diagnosed.
Establish the timeline before anything else
Pin the onset down harder than "after they were here."
- When exactly did the symptom first appear, and when exactly was the other trade on site? A gap of days weakens the link; a same-day onset strengthens it.
- Was there a period of normal operation in between? If your equipment ran fine for a while after their visit, a slow-developing coincidence is more plausible.
- What did they actually do, in the customer's words and, if you can see it, in the physical evidence?
A tight timeline is a strong lead. It is still only a lead.
Correlation is not proof: what could their work have changed
Their work does not have to touch your equipment to break it. It only has to change a shared background condition. Ask which of these their scope could have moved.
| Their work | The shared condition it can change | How your equipment reacts |
|---|---|---|
| Panel or service change, new grounding | Equipment ground, bonding, neutral path | Controls misbehave, protection trips, a pool or spa bond is lost |
| New ductwork, insulation, air sealing | Static pressure, airflow, combustion air | Airflow faults, short cycling, a starved burner |
| Repipe, new pressure regulator, main work | Water pressure and flow to your equipment | Weak output, a device that will not fill or fire, water hammer |
| Drain or vent change | Drainage and venting your equipment shares | Backups, trap siphoning, slow drainage |
| Any work near your controls or shutoffs | A valve, damper, breaker, or setting left changed | A system that is simply switched or set wrong |
| Structural work | Support, level, alignment, vibration path | New noise, a component knocked out of alignment |
| Cutting into a shared line | Debris or air introduced into the line | Blockage, airlock, a fouled valve or nozzle |
Collateral, or coincidence
Two branches, and you test them differently.
If the changed condition is real and present, confirm it directly. Measure the ground, the static pressure, the water pressure, the draft, whatever the table pointed you to, and compare it against what your equipment needs. If the condition is out of range and restoring it clears your symptom, you have found a collateral effect of their work, correct in their scope, breaking in yours.
If the shared conditions all check out in range, the timeline is likely coincidence. A part on your side reached the end of its life the same week. Do not force a causal story onto a normal wear failure just because the dates line up, and do not point at the other trade without the condition measurement to back it.
Handle it as coordination
- Lead with the measured condition, not the accusation. "Your equipment needs this much draft and it is getting less since the vent changed" is a finding; "the other guys messed it up" is a fight.
- Stay in your scope. You diagnosed the interaction; correcting their installed work, if it needs it, is their call or a coordinated one.
- Document the onset, what changed, and the measurement so the customer and the other trade can act on facts.
The judgment to bank: a fault that starts after another trade's work is a timeline to verify, not a verdict to announce. Find the shared condition they changed and measure it. If it is out of range, that is your cause. If it is fine, the calendar lied.
Recap
- Handle any hazardous changed condition (ground, gas draft, water near power) before diagnosing.
- Pin the exact onset against the exact visit; a tight gap is a strong lead, not proof.
- List the shared background conditions their scope could have changed, and measure the suspect one.
- Out-of-range condition that restores your symptom means collateral effect; all-in-range means the timing was coincidence.
References
- NFPA 70 (National Electrical Code) on grounding and bonding; NFPA 54 on venting and combustion air
- Trade-standard practice on measuring background conditions (pressure, airflow, draft, grounding) against equipment requirements
- See related: A Newer System Next to Yours Might Be the Cause; A Recent Repair May Have Caused the New Fault