Blockage vs Supply Problem Decision Tree
Why this matters
When something downstream is starved - low water, weak airflow, no gas, short refrigerant - the cause is either a blockage in the path or a shortfall at the supply. They produce the same complaint ("not enough getting through") but demand opposite fixes: clear the path, or fix the source. Chasing the wrong one wastes a trip. This tree gives you a clean way to split blockage from supply across water, air, gas, refrigerant, and electrical-as-flow systems in any trade.
Start here: is the source delivering at all
Go to the source side and confirm it is producing before you touch the path.
- Source reads full output (pressure, voltage, level) with nothing drawing: the supply is fine. The problem is between the source and the point of use - a blockage. Move downstream.
- Source reads low or nothing even unloaded: the supply is the problem. Stop and trace the source. No point hunting for a clog in a line that has nothing feeding it.
This one test - measure at the source with no demand - settles most of these calls in a minute.
The "everything vs one thing" rule
Where the shortage shows up tells you which side it is on.
- Everything downstream is starved equally: points to the supply or a restriction in the single common line everything shares. One cause feeding all symptoms.
- One outlet, zone, or branch is starved while others are fine: points to a blockage specific to that branch. The proof: the working outlets show the source is good.
If it is a blockage: find the restriction
A blockage raises pressure (or backs up flow) on the upstream side and starves the downstream side. Use that.
- Walk the path from source to symptom and find where the loss happens. Pressure or flow good up to a point, then bad after it, brackets the blockage.
- Hit the usual suspects: filters, strainers, screens, traps, aerators, dampers, metering devices, and the lowest point where debris settles.
- Look for the upstream backup. A line plugging up shows high pressure, pooling, or backflow just ahead of the clog.
- Confirm with a pressure drop test. An abnormal pressure difference across a single component (filter, valve, coil) is that component plugged. Replace or clear it.
Partial blockages are the sneaky ones: enough gets through to look almost normal, but flow under load collapses. Test under real demand, not just at idle.
If it is a supply problem: trace the source
A supply shortfall means the source is not producing what it should.
- Confirm the source is on and operating - pump primed and running, valve open, breaker on, tank not empty, regulator set right.
- Check the source's own input. A pump with no prime, a regulator with low inlet, or a heater with no fuel cannot deliver. The source may be starved itself.
- Measure output at the source. A pump that will not build head, a supply line below spec, or a regulator stuck low is a source fault, not a downstream clog.
- Watch for a source that fails only under load - fine at idle, collapses on demand. That is a weak or undersized source, or air-bound (see the air tree).
Decision summary
| Source output (unloaded) | Symptom spread | Conclusion |
|---|---|---|
| Full | One branch starved | Blockage in that branch |
| Full | All branches starved | Blockage in common line |
| Low/none | Anything starved | Supply/source fault |
| Full idle, drops under load | Starved on demand | Weak source or partial blockage |
The trap to avoid
Do not condemn the source on a hunch. A "bad pump" or "low supply" that turns out to be a plugged strainer is a classic miss. Prove the supply is short by measuring it unloaded at the source. If the source delivers with no demand, the fault is downstream - every time.
References
- Pump and compressor manufacturer general troubleshooting practice
- HVAC and plumbing flow-and-pressure diagnostic practice
- See related: Low Flow vs Low Pressure Decision Tree
- See related: Air in the System Symptoms Decision Tree