What is Total Dynamic Head and Why Does It Matter?
Total Dynamic Head (TDH) represents the total work a pump must perform to move water from a suction source to a discharge destination under operating conditions. It is measured in metres of water column (m).
On site, plumbers often make the mistake of looking only at static height — for example, thinking a 5-metre lift only needs a pump rated for 5m head. In reality, water moving through long pipe runs and tight elbows encounters significant friction resistance. Add in the 200 kPa (20.4m head equivalent) needed to operate a modern shower head comfortably, and that 5-metre physical lift easily becomes a 30-metre Total Dynamic Head requirement.
Selecting a pump without calculating TDH leads to two major failures:
- Undersized Pump: The pump operates near its shut-off head, delivering low flow, poor pressure, and running continuously without switching off.
- Oversized Pump: Excess pressure causes water hammer, blows fitting seals, consumes unnecessary electricity, and leads to rapid motor cycling.
How Total Dynamic Head is Calculated
The complete TDH equation combines four distinct hydraulic head components:
TDH (m) = Static Lift + Suction Head + Friction Head + Pressure Head All components are expressed in metres of head. Convert pressure in kPa to metres of head by dividing by 9.81.
- Static Vertical Lift (Hstatic): The vertical distance from the pump discharge outlet up to the highest fixture in the building.
- Suction Head (Hsuction): The vertical lift required to pull water up from an underground tank or sump into the pump inlet.
- Friction Head (Hfriction): Hydraulic head lost due to friction between flowing water and internal pipe walls over straight pipe runs and fittings.
- Pressure Head (Hpressure): The remaining residual pressure required at the point of use (e.g. 200 kPa for residential fixtures = 20.4m head).
Hazen-Williams Friction Loss — The Pipe Resistance Factor
To calculate pipe friction head loss accurately, Australian plumbing hydraulics relies on the empirical Hazen-Williams equation:
h_f = (10.67 × L × Q^1.852) / (C^1.852 × d^4.87) Where L is total equivalent pipe length (m), Q is flow rate (m³/s), C is pipe smoothness factor, and d is internal pipe bore diameter (m).
The C-factor represents internal pipe roughness. Smoother pipes have higher C-factors and lower head loss:
| Pipe Material | Hazen-Williams C-Factor | Friction Resistance |
|---|---|---|
| PEX & Poly (HDPE) | 150 | Lowest friction loss |
| PVC Pressure (Class 12/18) | 150 | Lowest friction loss |
| Copper (Type A & B) | 140 | Low friction loss |
| Galvanised Iron | 120 | Higher friction loss |
Fitting Equivalent Lengths — The Hidden Head Loss
Elbows, tees, and check valves disrupt smooth laminar flow, creating localized turbulence and head loss. Plumbers account for this by converting each fitting into an "equivalent length" of straight pipe.
| Fitting Type | Equivalent Length (DN25 Baseline) | Hydraulic Impact |
|---|---|---|
| Check Valve (Non-Return) | 2.5 metres | High head loss (spring resistance) |
| Tee (Branch Flow) | 1.5 metres | Moderate-high loss |
| 90° Standard Elbow | 0.8 metres | Moderate loss |
| Tee (Line Run) | 0.5 metres | Low loss |
| 45° Elbow | 0.4 metres | Low loss |
| Gate / Ball Valve (Full Open) | 0.15 - 0.2 metres | Minimal loss |
Worked Example — Sizing a Rainwater Pump in Regional Victoria
Scenario: A licensed plumber is installing an automatic submersible rainwater pump for a 2-storey home in Ballarat. The pump sits inside an above-ground tank supplying 30 L/min to upstairs toilets and laundry.
- System Layout: Static vertical lift = 5.5 metres, 0m suction lift (submersible), 35 metres of DN25 Copper Type B pipe run.
- Fittings Installed: 4 × 90° elbows, 1 × check valve, 1 × ball valve. Total fitting equivalent length = (4 × 0.8) + 2.5 + 0.2 = 5.9 metres.
- Total Equivalent Length: 35m + 5.9m = 40.9 metres.
- Friction Head Loss: At 30 L/min through 23.03mm internal bore copper, Hazen-Williams yields 2.8 metres friction loss.
- Required Outlet Pressure: 200 kPa target pressure = 200 ÷ 9.81 = 20.4 metres.
- Total Dynamic Head (TDH): 5.5m (lift) + 2.8m (friction) + 20.4m (pressure) = 28.7 metres TDH.
- Pump Selection Result: The plumber selects a Davey RainBank or Grundfos SBA 3-45 rated for at least 30 L/min at 29 metres head.
Common Australian Pump Types and When to Use Each
- Submersible Rainwater Pumps: Installed directly inside the tank. Silent operation, no prime loss, ideal for urban rainwater reuse.
- Centrifugal Surface Pumps: Mounted externally beside tanks or bores. Reliable and easy to service for agricultural water transfer and garden irrigation.
- Multistage Pumps: Feature multiple impellers in series. Essential for high-head requirements, multi-storey commercial buildings, and long rural pipe runs.
- Variable Speed Drive (VSD) Pumps: Automatically adjust motor RPM to maintain constant pressure regardless of how many taps are open simultaneously.
Mistakes Plumbers Make When Sizing Pumps
- Ignoring Fitting Losses: Omitting check valves and elbows from head calculations, leading to 2–5 metres of uncounted head loss.
- Using Outside Diameter for Friction Calcs: Nominal size is not internal bore. Using 25mm instead of 23.03mm ID for Copper Type B undercounts friction loss.
- Excessive Flow Velocity: Sizing small pipes for high flow rates resulting in velocities > 2.5 m/s, causing pipe noise and premature erosion.
- Forgetting Pump Curves: Purchasing a pump based on maximum head or maximum flow specs alone, rather than checking the duty point intersection on the pump curve.