A brief cold burst between two stretches of hot water — the classic 'cold water sandwich' — is a known tankless quirk that shows up often in Burnaby homes with recirculation loops.
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Quick answer
A cold water sandwich is the brief blast of cold water that appears between two periods of hot water on a tankless system, and it happens because water left sitting in the pipe or in the unit's residual heat exchanger reaches the tap before the burner has fully ramped up again. It's most noticeable in Burnaby homes with recirculation systems or with plumbing runs to a laneway house or basement suite.
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A tank-style water heater keeps a large reserve of already-hot water ready to go, so there's no gap in supply when you turn on a tap. A tankless unit instead heats water on demand, and when you first open a hot tap after any pause, there's a moment where water that's been sitting cooler in the line reaches you before the unit's burner output has caught up to the flow. If you restart the flow quickly — someone else finishes a load partway through your shower, or a recirculation pump cycles off and on — that gap in already-heated water shows up as a distinct cold pocket between two hot stretches.
It's a known characteristic of on-demand systems rather than necessarily a sign of a fault, though how noticeable it is depends heavily on your plumbing layout and whether you have recirculation.
Notice whether the cold sandwich happens only right after another fixture in the house has just been used, or seemingly at random. If it correlates with someone else using hot water shortly before you, that's a strong sign a recirculation setting or timer adjustment could help, which is worth mentioning when you describe the issue.
There's no homeowner-side fix that involves opening the unit itself; this is really about understanding your plumbing layout and recirculation behaviour rather than a repair in the traditional sense.
CANRO reviews how your recirculation system (if you have one) is timed and configured, checks pipe layout and distance to the affected fixtures, and confirms the unit's minimum flow and ignition delay are within normal spec. In some cases, adjusting recirculation timing or adding a small buffer meaningfully reduces how noticeable the sandwich is.
If there's no recirculation involved and the gap is simply a function of plumbing distance, we'll explain realistically what can and can't be changed rather than promising a complete fix that isn't possible with the current layout.
If your Burnaby home has gone through a laneway house or basement suite conversion, the plumbing that resulted is sometimes a patchwork of the original layout plus whatever route was practical to add later, and that patchwork is often the real source of a pronounced cold water sandwich rather than anything about the tankless unit's settings. In those cases, a targeted plumbing change — shortening a run, relocating a crossover valve, or adjusting recirculation to actually match how the suite is used — tends to help more than any adjustment at the water heater itself.
It's reasonable to ask your technician to walk the actual pipe route with you once, especially if you're not sure how your home's hot water distribution was modified during a past renovation, since that visual context often explains a symptom that otherwise seems inconsistent.
Timing roughly how long the cold pocket lasts, and whether it's a few seconds or closer to half a minute, gives your technician a useful starting point before they've even looked at your plumbing. A brief one- or two-second dip is often just an inherent characteristic worth living with, while a longer, more disruptive gap is more likely to have a specific, fixable cause in your recirculation timing or pipe layout.
Even after adjustments, a small residual cold moment is a normal characteristic of most tankless and recirculation combinations, so the realistic goal is usually reducing how noticeable it is rather than eliminating it completely. Your technician can give you an honest sense of what's achievable for your specific plumbing layout before any work begins.
In a multi-storey Burnaby home with the mechanical room in the basement, an upstairs bathroom naturally has more pipe length to clear than a main-floor powder room, so a cold sandwich noticed only on the top floor while the main floor feels fine is a distance effect rather than a sign of an intermittent fault. Comparing how the symptom feels at different fixtures around the house is a genuinely useful diagnostic clue.
If you've only ever tested this in one bathroom, it's worth briefly checking a couple of others before your service visit, simply to know whether the issue is localized or affects the whole house evenly.
Safety note
If you ever smell gas (a rotten-egg odour) near your tankless unit, hear hissing at a gas fitting, or notice soot or a burning smell, leave the home right away and call FortisBC at 1-800-663-9911 from outside. Never try to relight or reset the unit yourself in that situation. A Technical Safety BC licensed gas fitter needs to inspect combustion, venting, and gas pressure before it runs again.
Usually not. It's a known characteristic of on-demand heating, especially with recirculation systems, rather than a defect in the unit itself.
If recirculation cycles off after their use and then restarts for you, the gap between the pump stopping and your tap opening can create the cold pocket.
It can often be reduced significantly by adjusting recirculation settings or plumbing layout, but eliminating it entirely depends on your specific home's setup.
Not directly; the cold sandwich is about timing and pipe distance, not the unit's overall capacity, so sizing isn't usually the relevant factor.
It's generally considered a characteristic of on-demand systems rather than a defect, so it's worth discussing with your technician in the context of your specific warranty terms.
That can reduce the sandwich effect but also removes the fast-hot-water benefit recirculation provides, so it's a trade-off worth discussing rather than a simple fix.
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