Why is my tankless water heater giving inconsistent temperatures after installation?

Inconsistent temperatures from a newly installed tankless water heater typically indicate flow rate issues, incorrect sizing, or installation problems that need professional attention.

The most common culprit is insufficient flow rate to activate the unit consistently. Tankless heaters require a minimum flow rate (usually 0.5-0.75 gallons per minute) to fire up. If your water pressure is low or you're using low-flow fixtures, the unit may cycle on and off, creating temperature swings. This is particularly common in older Vancouver homes with galvanized plumbing or properties with pressure-reducing valves required by some Metro Vancouver municipalities.

Improper sizing is another frequent issue. Your contractor should have calculated your home's peak demand (simultaneous hot water usage) and selected a unit accordingly. In Vancouver's cold groundwater conditions (around 50°F in winter), a unit that works fine in warmer climates may struggle. For example, a typical Vancouver home needs 150,000-200,000 BTU capacity to handle multiple simultaneous uses, but many homeowners end up with undersized 120,000 BTU units.

Installation-specific problems include incorrect gas line sizing (must be sized for the unit's full BTU input), inadequate venting (especially in Vancouver's wet climate), or electrical issues with modulating units. Gas tankless heaters require proper combustion air and venting that meets both the manufacturer's specs and BC Building Code requirements. The gas line work must be done by a Technical Safety BC (TSBC) licensed gas contractor and inspected - this is non-negotiable in BC.

Vancouver climate considerations make proper installation critical. Our cold, wet winters mean the unit works harder, and condensation management becomes important. Many installations fail because contractors don't account for the temperature differential between our cold groundwater and desired output temperatures.

What you should do: Contact your installing contractor immediately - this should be covered under their workmanship warranty. Have them verify proper sizing calculations, check gas pressure at the unit, test flow rates at various fixtures, and ensure all venting meets code. If they can't resolve it, get a second opinion from another TSBC-licensed contractor. Don't attempt any gas-related troubleshooting yourself - that's illegal and dangerous in BC.

The fix usually involves adjusting the unit's settings, upgrading gas lines, or in worst cases, replacing with a properly sized unit.