A tankless water heater that struggles when other appliances run at the same time often points to a gas pressure drop problem rather than a fault in the unit itself.
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Quick answer
Gas pressure drop is a reduction in gas pressure at an appliance, usually caused by undersized piping, a long run with too many fittings, or too much total demand pulling from the same line at once, and it shows up in a tankless water heater as reduced hot water temperature, fluctuating output, or the unit derating or shutting down when other gas appliances run simultaneously. Diagnosing it involves checking pressure at the appliance under load, not just at rest.
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Gas pressure isn't perfectly constant throughout a piping system; it naturally decreases somewhat as gas flows through pipe, fittings, and valves, especially under higher flow demand. A properly sized system keeps that drop within an acceptable range so every appliance still receives adequate pressure even when several are calling for gas at once. A pressure drop problem is when that drop becomes large enough to affect appliance performance.
This is fundamentally different from a gas leak, which loses gas volume entirely. Pressure drop is about the system's ability to deliver adequate pressure under the actual demand placed on it, which is why it often only shows up under load rather than when testing a single appliance in isolation.
A tankless water heater modulates its burner to match the flow of hot water being drawn, and it needs adequate gas pressure to do that across its full operating range, especially at higher firing rates during heavy draw. Because tankless units draw more instantaneous demand than a comparable storage tank heater, they're often the appliance in the house most likely to reveal a marginal gas supply that a lower-demand appliance never stressed.
This is why a pressure drop problem sometimes only becomes noticeable after a tank water heater is replaced with a tankless unit, even though the rest of the house's gas piping didn't change.
A licensed fitter diagnoses pressure drop by measuring gas pressure at the appliance both at rest and while it's actively firing, sometimes while other appliances are deliberately run at the same time to reproduce the reported symptom.
Typical symptoms include hot water temperature that dips or fluctuates when a furnace or another gas appliance kicks in, a tankless unit that seems to underperform compared to its rated capacity, or intermittent error conditions that only occur during periods of high combined household demand rather than consistently. These patterns point a technician toward the gas supply side of the system rather than the water heater's internal components.
If you notice this kind of pattern, it's useful information to relay to your service technician, since it narrows the diagnostic path considerably compared to a vague complaint about inconsistent hot water.
Depending on what the diagnosis finds, correction can mean upsizing the pipe on the tankless unit's branch line, re-routing to reduce equivalent length, or in cases where total household demand exceeds meter capacity, working with the gas utility on a meter upgrade. None of these are guesswork fixes; they follow from the specific measurement and calculation the fitter performs.
This is also a good example of why a load calculation matters at the time a tankless unit is installed, since catching a marginal gas supply before installation avoids a callback and rework after the fact.
Safety note
If you ever smell gas, hear a hissing sound near a gas line or appliance, or suspect a leak, leave the home immediately and call from outside. Do not flip switches, light anything, or use a phone inside the house. Call FortisBC at 1-800-663-9911 to report a suspected gas leak or carbon monoxide concern before doing anything else. Gas piping, venting, and combustion appliance work in BC must be done by a Technical Safety BC licensed gas fitter; CANRO holds gas licence LGA0043870 and boiler licence LBP0046450.
Pattern matters: if hot water performance dips specifically when another gas appliance is running, that points toward pressure drop. Consistent problems regardless of other appliance use point elsewhere, and a technician's on-site pressure test confirms which it is.
Gas pressure testing requires proper equipment and an understanding of safe procedure, and it's part of a licensed gas fitter's diagnostic process rather than a homeowner task.
No, pressure drop and leaks are different issues. Pressure drop is about supply capacity under demand; a leak is an escape of gas from the system. Both are diagnosed differently.
In many cases yes, if undersized pipe is the cause, but the fitter's diagnosis determines whether the fix is pipe size, routing, or a meter capacity issue, since the correct fix depends on the actual cause found.
It's more likely to be noticed with tankless units because of their higher instantaneous demand, but the underlying piping issue could affect any appliance on a marginal system, tankless or not.
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