It Fails Only When Another Appliance Runs: A Decision Tree

Why this matters

A device that works fine alone and fails only when something else switches on is not broken by itself. Two loads are fighting over one shared resource, and the one you were called about is usually the victim, not the culprit. Replace the victim and the new one fails the same way the first time the other appliance runs. The fix is finding the shared resource and the aggressor, not condemning the device in front of you.

Safety first: the two hazard branches

Two of these branches are hazards. Handle them before any diagnosis.

• Fuel or combustion air. If the shared resource is a fuel line or combustion air (a furnace and a water heater competing for air, or one gas appliance starving another), incomplete combustion produces carbon monoxide. If anyone reports headaches, nausea, or a CO alarm sounds, get people out, shut off the fuel, ventilate, and do not relight until it is proven safe with a combustion analyzer and proper draft.
• Shared electrical circuit. A sag deep enough to drop or trip something is a real overload, not a nuisance. De-energize and verify dead before opening panels or making connections, and never jumper or defeat a device that trips when the aggressor starts.

Start here: name the aggressor and the shared resource

Pin two facts before you test anything. First, what is the aggressor, the appliance whose start-up or run coincides with the failure? Second, what do the two devices share that could be finite: a circuit, a supply line, a drain, a vent, a fuel line? The intersection of "what runs when it fails" and "what they share" is your diagnosis. Everything below is just confirming which shared resource is short.

Branch: shared electrical circuit or service

Large motor loads (compressors, well pumps, sump pumps, vehicle chargers) pull heavy starting current and sag the voltage on a shared circuit or an undersized service. The victim, anything voltage-sensitive, browns out, drops offline, or resets when the aggressor starts. Measure voltage at the victim while the aggressor starts. A small, brief dip on start-up is normal and a healthy service recovers fast; the fault is a dip deep or long enough to drop the victim, which points to an overloaded or undersized service, a loose or corroded shared connection, or too much on one circuit.

Branch: shared water supply

When one fixture draws, pressure falls everywhere on a shared line. A device that needs a minimum inlet pressure (a tankless heater, a pressure-driven valve, an irrigation zone) fails when a big draw opens elsewhere. Check static pressure versus flowing pressure at the victim while the other fixture runs. A large drop under flow points to undersized pipe, a partly closed valve, scaled piping, or a weak pressure source. Note that a small pressure drop under flow is normal, so compare against the victim's actual minimum, not against the static reading.

Branch: shared drain or vent

Here the tell is concurrency with a discharge, not a draw. A fixture that gurgles, backs up, or loses its trap seal only when another fixture drains is sharing an undersized or partly blocked drain, or a venting problem that siphons traps when flow surges through. Watch what discharges at the moment of failure. The victim is usually the lowest or furthest fixture on the shared line.

Branch: shared fuel or combustion air

Covered under safety above, and worth stating as a pattern: two gas appliances on one undersized line, or two drawing from one starved air source, leave the second to fire running lean. Observe both with them firing together, and treat any sign of poor combustion (sooting, flame rollout, a tripping safety) as the hazard it is, not a tuning issue.

The victim is not the fault

The habit to bank: when a device fails only in company, suspect the company. Prove the shared resource is adequate under combined load before you touch the device you were called about.

Recap

1. If fuel or combustion air is shared, treat CO as the first concern: people out, fuel off, ventilate, prove it safe.
2. If a circuit is shared, work de-energized and never defeat a device that trips.
3. Name the aggressor and the shared resource, then measure that resource under combined load.
4. Fix the shared resource, not the victim.

References

• Trade-standard practice for load interaction and shared-resource diagnosis
• NFPA 70E principles for de-energizing before electrical work
• Manufacturer minimum-supply specifications (voltage, inlet pressure, gas and combustion air)
• See related: The Interaction Between Two Loads That Creates a Fault; The Power Supply vs the Load (Decision Tree)