Wells & water/ CALCULATOR

Hot-water wait time and wasted water

Estimate how long cooled water must run before hot water arrives, plus the daily and annual water sent to the drain.

Internal pipe diametermmUse the actual internal bore of the hot-water pipe, not its nominal or outside size.

Heater-to-outlet pipe lengthmMeasure the developed route, including vertical and horizontal sections.

Flow while waitingL/minMeasure with a container and timer at the usual tap setting.

Cold-start hot-water drawsdraws/dayCount only draws after the branch has cooled; closely spaced uses may share one purge.

Occupied days per yeardays/yearUse fewer days for a seasonal or intermittently occupied home.

Combined water and wastewater pricecurrency/m³Add both volumetric charges when wastewater is billed from metered water use.

Water held in the branch

2.41 L

Estimated wait from a cold branch

24.1 s

Water sent to drain per occupied day

14.5 L/day

Annual water sent to drain

5.28 m³/year

Annual water and wastewater cost

13.21 currency/year

A full straight circular pipe of one internal diameter, constant waiting flow and one complete purge for every counted cold-start draw. Fittings, mixed diameters, branch dead legs, recirculation, the volume inside fixtures and the user’s chosen acceptable arrival temperature are excluded.

FROM ESTIMATE TO ACTION

Use the result, not just the number.

Describe the branch, not the whole house

Use the actual internal bore in millimetres and the developed route from heater to outlet in metres, including vertical runs. Nominal or outside diameter is not the internal bore. The example branch is 16 mm × 12 m and holds about 2.41 L. For sections of different bores, calculate each volume separately and sum it; this single-diameter model does not do that automatically.

Measure flow and count genuine cold starts

Safely measure volume and elapsed time at the usual waiting setting: 1 L in 10 s gives 6 L/min. Count only draws after this branch has cooled, not every closely spaced use or draws on unrelated branches. At 6 L/min the example purge takes about 24.1 s. Halving flow doubles modeled waiting time but does not reduce the pipe volume purged per cold start.

Compare observations and charges, not promised savings

Record actual arrival time using a consistent temperature criterion; the pipe-volume estimate does not simulate heating, fixture volumes, fittings or recirculation. Use occupied days, not automatically 365 for a seasonal home. Apply only relevant volumetric water/wastewater charges: fixed fees, heating energy and recirculation costs are excluded. The estimate is not a bill or guaranteed savings.

Inputs to verify

Internal pipe diameter (mm): Use the actual internal bore of the hot-water pipe, not its nominal or outside size.
Heater-to-outlet pipe length (m): Measure the developed route, including vertical and horizontal sections.
Flow while waiting (L/min): Measure with a container and timer at the usual tap setting.
Cold-start hot-water draws (draws/day): Count only draws after the branch has cooled; closely spaced uses may share one purge.
Occupied days per year (days/year): Use fewer days for a seasonal or intermittently occupied home.
Combined water and wastewater price (currency/m³): Add both volumetric charges when wastewater is billed from metered water use.

Continue your calculation

Independent energy

Hot-water recirculation energy and cost

Estimate annual pump electricity and the energy needed to replace heat lost from a hot recirculation loop.