Low water pressure can stop a tankless unit from firing at all, since most models need a minimum flow rate before the burner activates.
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Low water pressure affects a gas tankless water heater more noticeably than a storage tank because most tankless units require a minimum flow rate before the burner will activate at all, so weak pressure can mean no hot water rather than just a slower stream. Causes range from a partially closed shutoff valve, a clogged inlet filter, sediment buildup inside the unit, municipal supply pressure issues, or old, narrow plumbing feeding the fixture. A technician traces the cause by checking pressure and flow at several points to isolate whether the issue is at the unit, the plumbing, or the supply.
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A storage tank water heater keeps a supply of hot water ready regardless of how fast or slow water is drawn from it, so low pressure mostly just means a weaker stream at the tap. A tankless unit works differently: it only heats water while it senses adequate flow passing through it, and most models have a documented minimum flow rate below which the burner simply will not ignite. That means a pressure or flow problem that would be a minor annoyance with a tank can mean a complete loss of hot water with a tankless unit.
This is one of the most common reasons homeowners call after a tankless installation, and in most cases the tankless unit itself is not the problem, it is simply revealing a flow limitation elsewhere in the system that was always present but less noticeable.
| Cause | Where it typically shows up |
|---|---|
| Partially closed isolation valve | Right at the unit, after service or installation |
| Clogged inlet water filter | Inside the unit, more common on older or unfiltered water |
| Sediment or scale buildup | Inside the heat exchanger, on units overdue for descaling |
| Narrow or corroded old plumbing | Older Vancouver-special or character homes with original pipe |
| Municipal supply pressure dip | Neighbourhood-wide, not isolated to one home |
| Multiple fixtures running at once | Anywhere flow demand exceeds what's available |
Older character homes in Vancouver sometimes still have sections of original, narrower-bore piping that was adequate for the fixtures and appliances of decades ago but restricts flow more than modern plumbing would, which is worth flagging to a technician if your home predates recent renovations.
This step-by-step approach matters because low pressure at one fixture points toward a localized plumbing issue, while low pressure throughout the home points toward the main supply, the meter, or the unit itself.
Before calling for service, a homeowner can safely confirm that the shutoff valves near the water heater are fully open, check whether low pressure is happening at just one fixture or the whole home, and note whether it started after any recent plumbing or renovation work. This information helps a technician narrow down the cause more quickly once they arrive.
If the cause turns out to be aging, narrow plumbing throughout an older home rather than something at the tankless unit itself, the realistic fix is a plumbing upgrade rather than any adjustment to the water heater. In that situation, a technician can explain what a reasonable scope of repiping would look like, though that work typically falls to a plumber rather than the gas fitter servicing the tankless unit.
Safety note
Any inspection or adjustment involving the tankless unit's gas connection, venting, or combustion components must be performed by a Technical Safety BC licensed gas fitter. If you ever smell gas or suspect a carbon monoxide issue, leave the home immediately and call FortisBC at 1-800-663-9911.
Most tankless units require a minimum flow rate to activate the burner, so a pressure or flow drop below that threshold can mean the unit doesn't fire at all, rather than just producing a weaker stream.
Low pressure on its own typically doesn't damage the unit, but the underlying cause, such as sediment buildup, can affect the unit's internal components over time if left unaddressed.
It can be, particularly in character homes with sections of original, narrower plumbing that restricts flow more than modern piping would.
You can safely confirm the shutoff valves near the water heater are fully open and note whether the issue affects one fixture or the whole home, which helps speed up diagnosis.
If scale buildup inside the heat exchanger is restricting flow, descaling can resolve it, but a technician needs to confirm that's actually the cause before assuming it's the fix.
Yes, and it often is. Old or narrow plumbing, a partially closed valve, or a municipal supply dip can all cause low pressure that has nothing to do with the tankless unit itself.
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