The System You Installed Is Failing and You're Not Sure Why: Decision Tree

Why this matters

A system you put in is now failing, and the cause is not obvious. Because it is your work, there is pressure to reach a fast answer that protects your reputation. Resist it. The right order is the same as any diagnosis: work from the simplest, most common cause to the complex, and let the evidence assign blame at the end. This tree walks that order for a system you know the history of.

Start here: is it safe to keep running

If the failure involves a hazard, stop and make it safe before you diagnose. De-energize and verify dead on an electrical fault, relieve pressure before opening a closed system, evacuate and call the utility on a gas or combustion smell, and shut down anything with a defeated safety device. A system that is actively unsafe gets secured first; the why comes after.

Step 1: confirm the failure is real and current

Reproduce the symptom before you chase it. A customer's description is a starting point, not a measurement.

• If you cannot reproduce it, treat it as intermittent and capture conditions rather than swapping parts. Note when it happens, in what weather, under what load.
• If you can reproduce it, measure the failure against the numbers you set at install. The gap between now and commissioning is your map.

Step 2: rule out the cheap, common causes first

Old installs fail most often for boring reasons. Clear these before you suspect your own design or a later modification.

• Maintenance neglect: a clogged filter or coil, a fouled strainer, low fluid, a dirty sensor, an unbalanced pool. Years of skipped upkeep fail a perfect install.
• A consumable at end of life: a capacitor, a contactor, a seal, an anode, a belt. These are wear items, not workmanship.
• Power or supply changes around the equipment: low voltage, water pressure changes, a supply valve throttled down. The equipment did not change; its feed did.

If one of these explains the symptom fully, you are done. Do not keep digging for a story about tampering when a dirty coil is the answer.

Step 3: check what changed since you left

If the common causes are clean, look for a delta. Has the load grown, has a component been swapped, has a setting drifted, has the space around the equipment changed. See related: Reading What Changed Since You Left the Job.

• If a later modification sits in the failure path, isolate it and prove it before you blame it. See related: A Later Modification Caused the Fault On Your Install.
• If nothing was changed and nothing is neglected, you are into the harder branch.

Step 4: reconsider your original design against today's conditions

Only now do you question your own install. This is not an admission of fault; it is the last branch because it is the least common.

• Was it right for the conditions then but not now? A load that was correctly sized for the original building can be undersized after an addition, a finished space, or a bigger appliance downstream. That is a changed-conditions finding, not a design error.
• Was it a genuine shortcoming from day one that took years to surface? Rare, but real. If the evidence says your original work is the cause, say so plainly and stand behind it.

Step 5: assign the cause on evidence, not on comfort

Write down what you measured and what it points to, then attribute. The finding might clear your original work, might land on a later change, or might land on you. All three are legitimate outcomes; only a guess is not.

Recap

1. Secure any hazard before diagnosing.
2. Reproduce and measure the failure against your commissioning numbers.
3. Clear neglect and worn consumables first.
4. Look for what changed since install; prove a modification before blaming it.
5. Question your original design last, against today's conditions, and own a real fault if it is one.

References

• Trade-standard fault-isolation and root-cause practice
• Manufacturer service documentation for wear-item life and operating ranges
• See related: Reading What Changed Since You Left the Job; A Later Modification Caused the Fault On Your Install; Who's Responsible When Your Work Was Altered