The useful baseline is stored pipe volume, not distance alone. Measure the actual route and flow first, then compare solutions by water saved, heat loss, electrical use and installation complexity.
1. Measure the route that actually fills with hot water
Trace the developed pipe length from the active hot-water source to the outlet. With a recirculation loop, start at the branch connection nearest the outlet rather than at the heater. Record each internal diameter separately if the route changes size.
A compact route stores less cooled water. DOE building guidance describes compact core plumbing as a way to shorten delivery paths and reduce both water and energy waste.
- Identify the source that is hot before the draw starts.
- Measure horizontal and vertical pipe sections along the route.
- Look up actual internal diameters from product data.
- Calculate mixed-diameter sections separately and add their volumes.
2. Compare calculated purge time with a timed draw
Measure flow at the normal tap position with a container and timer. Divide stored pipe volume by this flow to estimate a complete purge, then time how long the outlet takes to reach your chosen acceptable temperature.
A longer observed wait can come from a larger hidden volume, cooled heater outlet piping, mixing, low flow, a failed recirculation control or the time needed to warm the pipe wall. A shorter wait may mean the branch had not fully cooled. Record the starting condition instead of forcing the observation to match the model.
3. Compare solutions as systems, not promises
For new work, a shorter route and the smallest code-compliant pipe that still carries required peak flow can reduce stored volume. In existing homes, options may include changing habits, relocating the heater, adding a suitable point-of-use heater or installing controlled recirculation.
Recirculation can reduce water sent to drain but adds pipe heat loss and pump electricity. DOE guidance favors demand-initiated operation over continuous circulation. Check local plumbing, electrical, scald-protection and water-quality requirements before selecting equipment.
DOE Building Science Education — Hot Water Recirc on Demand ↗4. Verify the result under repeatable conditions
Repeat the cold-start timing at the same outlet, flow and starting temperature after any change. Measure pump electricity separately if recirculation is added, and check that return-line heat does not create unwanted warming of nearby cold-water piping.
Do not reduce pipe diameter below the size required for simultaneous demand, pressure loss and local code. Do not raise storage temperature simply to mask a distribution delay; temperature control and scald protection require their own safety assessment.
Options and the trade-offs to record
| Option | Potential benefit | Trade-off to check |
|---|---|---|
| Shorter, compact route | Less stored water and less pipe surface | Remodelling access and fixture layout |
| Smaller permitted pipe | Lower purge volume | Peak flow and pressure loss |
| Point-of-use heater | Short local delivery path | Power, maintenance and equipment cost |
| Demand-controlled recirculation | Faster delivery with less drain waste | Return piping, heat loss and pump energy |
Worked example
Turn one branch into a measurable baseline
A 12 m branch has a 16 mm internal diameter. Flow while waiting is 6 L/min and the branch cools before six draws on each of 365 occupied days.
Stored water = π × (0.016 / 2)² × 12 × 1000 = 2.41 L Complete purge time = 2.41 / 6 × 60 = 24.1 s Annual drain volume = 2.41 × 6 × 365 / 1000 = 5.28 m³
Use the measured wait and this annual baseline to compare a layout change or controlled recirculation. Water savings alone do not prove an energy saving.
What to have ready
- Developed pipe length and actual internal diameter by section
- Measured waiting flow at the outlet
- Timed cold-start delivery to a defined acceptable temperature
- Number of genuinely cooled starts per occupied day
- Peak-flow, pressure-loss and local-code constraints
- Heat loss and pump electricity for any recirculation option
Common mistakes
- Using straight-line room distance instead of the developed pipe route.
- Assuming every hot-water use starts with a fully cooled branch.
- Reducing pipe size without checking required flow and pressure loss.
- Treating continuous recirculation as free water savings.
- Changing storage temperature to solve a distribution problem.
Reference check: 5 September 2026. Sources include US public agencies. Principles are general; regulations and design values must be checked where you live.Worked examples use stated hypothetical inputs.