Hot Water Recirculation Calculator

Inputs

Pipe length (m)

Total developed length of the recirc loop.

Inside diameter (mm)

Use internal diameter for hydraulics and volume.

Pipe material

Material sets an equivalent roughness for friction.

Insulation thickness (mm)

Zero is allowed, but increases heat loss.

Insulation k (W/m·K)

Typical elastomeric foam: 0.033–0.040.

Supply water temp (°C)

Loop average uses supply minus half ΔT.

Ambient temp (°C)

Mechanical room or ceiling plenum temperature.

Allowable ΔT (°C)

Supply-to-return drop you can tolerate.

Minor loss K (total)

Valves, elbows, checks, balancing devices.

Inside h (W/m²·K)

Typical internal convection for flowing water.

Outside h (W/m²·K)

Still air: 5–10; moving air: 10–25.

Pump efficiency (%)

Used for electrical power estimate.

Fixture flow (L/min)

For wait time and wasted volume without recirc.

Large screens use three columns Medium screens use two columns Mobile uses one column

Example Data Table

Scenario	Length (m)	ID (mm)	Insul (mm)	k (W/m·K)	Ts/Ta/ΔT (°C)	Result Flow (L/min)	Thermal Loss (W)
Apartment loop	60	25.4	13	0.035	55 / 24 / 5	≈ 0.7–1.2	≈ 200–350
Hotel corridor	120	32	19	0.034	60 / 24 / 5	≈ 1.5–2.6	≈ 500–900
Mechanical room loop	40	20	0	0.035	55 / 30 / 5	≈ 1.2–2.0	≈ 250–450

Example outputs are typical ranges; your actual inputs will drive the calculation.

Formula Used

1) Heat loss per meter is computed with a cylindrical resistance model:

R_in = 1 / (h_in · 2πr₁)
R_ins = ln(r₂/r₁) / (2πk)
R_out = 1 / (h_out · 2πr₂)
q′ = (T̄_w − T_a) / (R_in + R_ins + R_out)
Q = q′ · L

2) Required recirculation flow to limit the temperature drop ΔT is:

ṁ = Q / (c_p · ΔT)
Q_v = ṁ / ρ

3) Pressure drop uses Darcy–Weisbach with minor losses:

h_f = f(L/D)(v²/2g) + K(v²/2g)
Friction factor f uses laminar 64/Re or a Swamee–Jain turbulent approximation.

4) Pump power is estimated from hydraulic power and efficiency:

P = ρgQ_vh_f / η

How to Use This Calculator

Enter loop length, internal diameter, and pipe material.
Set insulation thickness and conductivity to match your specification.
Provide supply and ambient temperatures, then choose an allowable ΔT.
Add a total minor-loss K to represent fittings and control devices.
Click Calculate to view flow, heat loss, head loss, and energy values.
Export CSV or PDF for design notes and documentation.

Professional Design Notes

1) Why recirculation matters

Hot water recirculation reduces wait time, limits water waste, and improves comfort in multi‑fixture buildings. In construction, the goal is dependable delivery temperature at the furthest outlet without excessive pump power or heat loss. This calculator estimates the minimum loop flow required to offset heat losses and then checks head loss so you can select a realistic circulator.

2) Interpreting the flow result

The required recirculation flow is primarily driven by loop heat loss, allowable temperature drop (ΔT), and exposed pipe length. Tightening ΔT increases flow. Increasing insulation thickness or improving insulation conductivity reduces heat loss and lowers flow. Use the result as a starting point, then verify your balancing strategy can maintain the target flow during typical operating conditions.

Ambient temperature should reflect the pipe location: warm shafts reduce heat loss, while cool ceilings increase it. When using timers or temperature control, document setpoints and expected run hours to estimate daily energy use more accurately.

3) Reading head loss and pump power

Head loss includes straight‑pipe friction plus minor losses from fittings, check valves, balancing valves, strainers, and control devices. If the total head is high, consider a larger diameter, a shorter loop, or fewer high‑K components. Pump power is estimated from hydraulic power divided by efficiency, which supports quick electrical and energy comparisons across design options.

4) Example data and trends

Use this example set to validate inputs and understand trends. These values are typical of common building loops and are intended for quick checks:

Case	Length (m)	ID (mm)	Insul (mm)	Ts / Ta / ΔT (°C)	Minor K	Expected outcome
Short insulated loop	40	25.4	19	55 / 24 / 5	8	Lower heat loss; modest flow and head.
Long corridor loop	120	32	13	60 / 24 / 5	12	Higher heat loss; higher flow and energy.
No insulation	60	20	0	55 / 30 / 5	10	Heat loss rises; minimum flow increases noticeably.

5) Construction and commissioning tips

Confirm return routing on drawings, provide isolation valves for service, and insulate consistently through risers and plenums. During commissioning, record steady supply/return temperatures, verify intended loop flow at balancing points, and confirm check valves prevent reverse circulation. If performance is poor, reassess pipe diameters, missing insulation, or overly restrictive valves.

6) Reporting

Use the CSV for calculation logs and the PDF for submittals or handover packs. Clear documentation helps explain pipe sizing, insulation selection, and circulator choice to reviewers and site teams.

FAQs

1) What does ΔT represent in this tool?

ΔT is the allowable drop from supply to return temperature on the loop. A smaller ΔT improves comfort but increases required recirculation flow and may raise pump and energy demand.

2) How should I pick the minor loss K value?

Sum typical K values for elbows, tees, checks, balancing valves, strainers, and control valves along the loop. If details are unknown, start with 8–15 and refine using the final fitting schedule.

3) Why does insulation change the flow so much?

Insulation lowers heat loss to the surrounding air. Lower heat loss means the loop needs less energy replacement, so the minimum flow needed to limit temperature drop decreases.

4) Can I use this for demand-controlled recirculation?

Yes. Use the results as a baseline “full-loop” requirement, then apply your control duty cycle for energy planning. Confirm the control strategy still keeps the furthest fixture within your comfort limit.

5) What if my calculated velocity seems high?

High velocity increases noise, erosion risk, and head loss. Consider a larger diameter, split the loop, reduce K components, or relax ΔT if the project can tolerate a slightly larger temperature drop.

6) Does the calculator include heat loss from fittings and valves?

Heat loss is modeled from straight pipe length using an overall resistance approach. If fittings are extensive or uninsulated, add a safety factor by slightly increasing effective length or reducing insulation performance.

7) What field measurements confirm the system is working?

Check steady supply/return temperatures, verify no reverse flow across check valves, and confirm loop flow at commissioning points. Then test the furthest fixture for acceptable wait time and stable outlet temperature.