High gas bill and the water heater

A gas bill is measured in gigajoules, and the water heater's share is roughly its input times how long it runs, which is why no honest percentage exists without usage data. The useful first checks are seasonal: winter bills rise because incoming water is colder and every litre needs more heat, and a home that uses more hot water in winter will see it on the gas bill. The second check is the other loads: furnace, fireplace, dryer and range share the same meter.

FortisBC bills show daily consumption in gigajoules; comparing a summer week with the heater running against a period it was off (a holiday, or the unit shut down) brackets the heater's contribution better than any rule of thumb.

Several faults burn gas without delivering hot water. A recirculation loop running continuously, or a crossover valve stuck open, keeps the burner firing against pipe losses around the clock and is the single most common hidden waste on tankless and tank systems alike. A relief valve stuck open, a dripping hot tap, or a tankless unit that short-cycles because it is oversized for the draw all add gas without useful water.

On a storage tank, a failed thermostat that overshoots makes the tank reheat from hotter-than-needed, and a tank with heavy sediment takes longer to heat the same water. Each of these is found by testing rather than assumed.

Upgrading from an old atmospheric tank to a condensing unit reduces the gas used per litre of hot water, and this site documents the efficiency measures honestly: UEF figures at their draw patterns, never as a savings percentage. WaterHeating.ca publishes no savings claims, because the bill depends on usage, fuel price and the other loads on the meter. The estimate and the demand assessment are where the real number for a specific home gets calculated.