What flow rate and pressure do I need for my flanged pump
Flow and head of a flanged pump: how to calculate and select in practice
When a customer focuses only on the flange diameter or the nominal voltage when selecting a flanged circulation pump, they almost always run into a problem – either the pump cannot push the water through the entire system, or it is oversized, runs inefficiently and unnecessarily consumes energy. Two key parameters that determine the correct operation of the entire heating circuit are flow (Q) and head, or pumping height (H). Without their correct determination, the selection of the pump is just guessing.
In this article, we will look at how to calculate these values from scratch, what affects their size in real installations, how to cope without design documentation, and where the most common mistakes are that most installers and building technical managers run into. We will work with specific numbers and examples from practice – not with general phrases.
What is flow Q and why it cannot be estimated by eye
Flow expresses the volume of liquid that the pump pushes through per unit of time. In heating, it is most often given in m³/h (cubic meters per hour) or in l/s (liters per second). Both units are commonly found in pump technical sheets, with a simple conversion: 1 m³/h = 0.278 l/s.
In practice, flow depends on how much thermal power needs to be delivered to the heat emitters (radiators, floor heating, heat exchangers) and what temperature difference between the supply and return is designed. The basic formula is as follows:
Let's take a concrete example: a family house with a total boiler thermal power of 18 kW and a classic temperature drop of 70/50 °C, i.e. ΔT = 20 K.
Q = 18 000 / (4 186 × 1 000 × 20) × 3 600 = 0,215 × 3 600 / 1 000 ≈ 0,774 m³/h
Rounded up, we therefore need a flow of about 0.8 m³/h. For modern low-energy houses with floor heating and a temperature drop of 40/30 °C, ΔT = 10 K, which doubles the required flow to 1.55 m³/h at the same power – it is important to realize this when the system is renovated without replacing the pump.
Effect of temperature drop on flow in practice
This is one of the most underestimated aspects. Many installers select a pump once for the original system with radiators and a high-temperature drop, then the customer adds floor heating or replaces the boiler with a heat pump – and the old system with the flanged pump suddenly does not suffice. The temperature drop for heat pumps is typically 5 to 10 K, which at the same thermal power means two- to four times higher required flow compared to a gas boiler with ΔT 20 K.
In practice, for more complex projects – industrial halls, administrative buildings, apartment buildings – the system power grows into tens, sometimes even hundreds of kilowatts, and the flow ranges from 5 m³/h up to 100 m³/h and more. Precisely for these volumes are flanged pumps intended: threaded pumps are not sufficient for such flows either structurally or hydraulically.
What is head H (pumping height) and how to calculate it
While flow speaks about the amount of water pushed through, head (pumping height) expresses what hydraulic resistance the pump must overcome. It is given in meters of water column (m w.c.) or in Pa, kPa (pascals, kilopascals). Conversion: 1 m w.c. ≈ 9 810 Pa ≈ 9.81 kPa, or simplified 1 m w.c. ≈ 0.1 bar.
The pump head must cover the sum of all pressure losses in the hydraulic circuit. The longest, most hydraulically demanding branch (so-called index circuit) determines the required head of the entire pump.
Pressure losses in the pipe are calculated as:
ΔP = R × L × (1 + ξ)
where R is the specific pressure loss per meter of pipe length [Pa/m], L is the length of the straight pipe [m] and ξ is the sum of the local resistance coefficients (bends, T-pieces, valves, heat exchangers). In practice, installers use approximate values R = 100–150 Pa/m for heating pipes with average velocities of 0.3–0.8 m/s.
Approximate step-by-step calculation of the pump head
From practice, I recommend the following procedure, which works even without sophisticated hydraulic software:
1. Determining the index circuit: Find the longest and most hydraulically demanding branch of the system – the one where the sum of pipe lengths is the highest and where there are the hardest fitting components (control valves, heat exchangers).
2. Measure or estimate the pipe length: Include both the supply and return pipe of the index circuit. For a single-family house, it is usually 30–80 m, for an apartment building 100–300 m, for an industrial building even 500 m and more.
3. Calculate losses in the pipe: Use the value R = 100 Pa/m for standard distribution. Multiply length × R. For a house with 50 m of pipe: 50 × 100 = 5 000 Pa = 5 kPa ≈ 0.5 m w.c.
4. Include local resistances: Add 30–50 % for elbows, tees, valves, or more if there are many fittings or a ball valve in the defined area. For our example: 5 000 × 1.5 = 7 500 Pa.
5. Include device resistances: Boiler, heat exchanger, floor heating distributor, filter – each of them has a pressure loss at a given flow rate listed in the technical data sheet. Add these values together.
6. The result is the required pump head: In most single-family houses, H = 2–5 m w.c., for larger buildings 5–15 m w.c., for industrial systems even 15–25 m w.c.
Pump characteristic curve and operating point
Every manufacturer provides a H-Q curve (pump characteristic) for a flanged pump – a graphical dependence of head on flow rate. As the flow rate increases, the head the pump can develop decreases. This is a basic property of centrifugal pumps.
On the pump curve, the system curve is superimposed – the dependence of pressure losses in the system on the flow rate. Pressure losses increase with the square of the flow rate: if the flow rate is doubled, pressure losses increase fourfold. The intersection of the two curves is the operating point – the actual operating parameters of the pump in the given system.
A properly selected pump has its operating point within its optimal efficiency range, typically in the middle part of the H-Q curve. If the operating point lies too far to the right (low head, high flow), the pump operates inefficiently and may be noisy. If it lies too far to the left (high head, low flow), the pump is unnecessarily operating near the closed valve condition and may overheat.
Why looking only at maximum values is not enough
A very common mistake: the customer looks at the catalog, finds a pump with Qmax = 30 m³/h and Hmax = 15 m w.c. and thinks it can simultaneously deliver 30 m³/h at 15 m w.c. This is a fundamental error. Maximum flow occurs at zero head (open valve condition), maximum head at zero flow (closed valve condition). In real operation, the pump operates somewhere between these extremes – and that is why it is important to have the H-Q curve and find the actual operating point.
Calculations for typical practical situations
Example 1: Apartment building with 24 flats
An apartment building with 24 flats, each with an average thermal power of 6 kW, total power thus 144 kW. Temperature difference 70/50 °C (ΔT = 20 K). Length of the longest circuit from the boiler to the furthest apartment and back: 120 m. The system includes two ball valves, four 90° elbows, a distributor and a boiler with pressure losses of 0.8 m w.c. each.
Flow calculation:
Q = 144 000 / (4 186 × 1 000 × 20) × 3 600 = 6.22 m³/h
Head calculation:
Pipe losses: 120 m × 120 Pa/m = 14 400 Pa (using R = 120 Pa/m for a larger diameter)
Local resistances (+40 %): 14 400 × 1.4 = 20 160 Pa
Boiler: 8 000 Pa
Distributor: 5 000 Pa
Total: 33 160 Pa ≈ 3.4 m w.c.
For this building, we are looking for a flanged pump with parameters Q ≈ 6.5 m³/h, H ≈ 4 m w.c. (adding a reserve of 15–20 %).
Example 2: Industrial hall with warm air units
Production hall with 5 warm air units at 40 kW each, total power 200 kW. Temperature difference 80/60 °C (ΔT = 20 K). Length of the index circuit 200 m, distribution DN 80, closing valves, filters, heat exchangers in the warm air units with pressure losses of 0.5 bar each.
Flow:
Q = 200 000 / (4 186 × 1 000 × 20) × 3 600 = 8.6 m³/h
Head:
Pipe losses: 200 m × 80 Pa/m = 16 000 Pa (DN80, lower R)
Local resistances (+35 %): 21 600 Pa
Heat exchangers: 5 × 5 000 Pa = 25 000 Pa (in parallel, only one branch is calculated)
Filter + valves: 8 000 Pa
Total: ≈ 54 600 Pa ≈ 5.6 m w.c. → with reserve: H = 7 m w.c.
Result: flanged pump Q ≈ 9–10 m³/h, H ≈ 7 m w.c. with a motor suitable for continuous operation.
Example 3: System reconstruction when switching to a heat pump
Reconstruction of a single-family house: original boiler 25 kW with ΔT = 20 K → new heat pump 12 kW with ΔT = 7 K (low-temperature system). The original flanged pump was dimensioned for Q = 1.1 m³/h.
New required flow:
Q = 12 000 / (4 186 × 1 000 × 7) × 3 600 = 1.47 m³/h
Flow increased by 34 %, which in combination with a new circulation through a hydraulic balancing unit and a new heat pump heat exchanger means a change in the system curve – the original pump operates outside the optimal point. This is a typical situation we see very often in practice, where a new flanged pump with electronic speed control solves the problem much more elegantly than an old single-stage pump.
Reserve capacity – how much to add and when not to
Every calculation carries a certain degree of inaccuracy. Pipe lengths are estimated, valve resistances are approximated, and actual operating conditions differ from the design. Therefore, a design reserve of 10–20 % is typically included in the pump selection for both flow and head.
However, be cautious about overdimensioning: a pump with too large a reserve (30–50 % or more) operates in a non-optimal point, causing hydraulic noise, unnecessary electricity consumption, and shortening the lifespan of valves due to higher flow velocities. In systems with electronically controlled flanged pumps, this can be partially compensated by regulation, but the selection is still better done as accurately as possible.
For larger buildings (industrial, administrative), two pumps are sometimes installed in parallel connection: one runs during normal operation, and the second starts automatically in case of peak load or failure. This configuration is particularly advantageous for flanged pumps, where replacement is simple thanks to standardized flange dimensions.
Hydraulic connection and its impact on parameters
How the pump is connected in the system significantly changes the actual operating conditions.
Series connection: The heads of the pumps add up, the flow remains the same. Used when one pump is not sufficient to overcome high pressure losses in a long network. Rare in practice, mostly for special applications.
Parallel connection: The flows of the pumps add up, the head remains the same. A common solution for large heating networks where one pump is not sufficient for the required flow. Important: parallel pumps should be identical, otherwise hydraulic imbalance occurs and one of the pumps may be forced out of operation.
For more information on the correct selection of size, pump series and its parameters, read the article How to choose a flanged circulation pump for a heating system or Flanged pumps for large heating systems – what to watch out for.
Fluid flow velocity and correct pipe diameter selection
Flow rate Q and pipe diameter DN are closely related through flow velocity. The recommended water velocity in heating pipe systems is:
- Supply systems in apartments and single-family homes: 0.3–0.5 m/s
- Riser and distribution pipes in apartment buildings: 0.5–0.8 m/s
- Main pipes in industrial boiler rooms: 0.8–1.5 m/s
- Special applications (district heating networks): up to 2–3 m/s
Velocities above 1.5 m/s in most standard installations cause noise, increased wear on valves, and a sharp increase in pressure losses. Therefore: if your flow calculation gives, for example, Q = 5 m³/h, choose a pipe diameter DN so that the velocity remains within the recommended range. DN 40 gives a velocity of ≈ 1.1 m/s at Q = 5 m³/h – acceptable for a main line. DN 32 would be tight (≈ 1.7 m/s), DN 50 would be comfortable (≈ 0.7 m/s).
This relationship is also key when connecting the pump to the pipe: the pump flanges must match the pipe diameter, otherwise local losses occur at the reduction transitions. More about dimensional standards can be found in the article Dimensions and types of flanges – what DN and PN standards mean for pumps.
Software tools and simplified tables for selection
Several helpful tools exist for everyday practice. Most pump manufacturers (Grundfos, Wilo, DAB, Ebara) provide online selection tools where you enter Q and H and the program suggests a specific pump type with an H-Q curve and consumption. These tools are very good, but they assume you know the input values.
For approximate dimensioning without a project, you can also use these simplified recommendations (they do not replace calculations, but help with quick estimates):
| Type of building | Power [kW] | Q at ΔT=20K [m³/h] | Typical H [m w.s.] |
|---|---|---|---|
| Single-family home (radiators) | 10–25 | 0.4–1.1 | 2–4 |
| Single-family home (floor heating) | 10–20 | 0.9–1.7 | 2–5 |
| Apartment building (12–24 apartments) | 60–150 | 2.6–6.5 | 3–7 |
| Industrial hall / commercial building | 100–500 | 4.3–21.5 | 5–15 |
| Large industrial boiler room | 500–2 000 | 21–86 | 10–25 |
These values are only approximate and apply to a system with water, ΔT = 20 K and medium-length piping. For other temperature differences, glycol mixtures or specific applications, an accurate calculation is necessary.
Glycol mixtures and their impact on flow and head
If the system contains an antifreeze mixture (ethylene glycol or propylene glycol mixture), the pump parameters change. Glycol mixtures have higher viscosity and density than pure water, which means:
- Higher pressure losses at the same flow (higher viscosity = more friction resistance)
- Lower pump head for a given H-Q curve (measurements in catalogs are for water)
- Higher pump power to achieve the same parameters
Correction factors depend on the concentration and temperature of the mixture. For a 30 % ethylene glycol mixture at 20 °C, the viscosity increase compared to water is about 2.5 times, pressure losses increase by 15–25 %. For a 40 % propylene glycol mixture at 10 °C, pressure losses can be 40–60 % higher. These corrections are particularly important in geothermal heat pumps or solar collectors, where glycol mixture circulates in the primary circuit.
Most frequently asked questions (FAQ)
How do I determine the required flow if I don't have the project documentation?
Start with the thermal power of the source – this is usually indicated on the boiler, heat pump or technical data sheet of the equipment. Then estimate the temperature difference of the system (for an older system with cast iron radiators, calculate ΔT = 20 K, for floor heating ΔT = 8–10 K, for a heat pump ΔT = 5–7 K). By plugging into the formula Q = P / (c × ρ × ΔT), you get the required flow. For the head, measure or estimate the length of the longest circuit and use an approximate value of R = 100–150 Pa/m, adjust by 40–50 % for local losses and add the pressure losses of the boiler and other equipment.
Can I use a pump with a higher flow than my calculation shows?
Yes, but with caution. A reserve of 15–20 % is acceptable and accounts for calculation inaccuracies. A pump with double the required flow is problematic – it operates in an unsuitable part of the H-Q curve, causes noise, excessive flow velocity in the pipes and unnecessary energy consumption. Modern flanged pumps with frequency inverters (EC motors) can adjust performance according to actual needs, which partially compensates for this problem, but correct dimensioning is always a better basis.
What if the flow is 2.5 m³/h and the head is 6 m w.c., but I can't find a pump in the catalog that matches these values exactly?
This is a common situation. Pumps are manufactured in specific ranges with discrete performance levels. Choose the next higher model whose operating point (the intersection of the H-Q curve and the system curve) lies within the zone of optimal efficiency. If the pump has speed control, you can set it to the desired point – more about this can be found in the article Setting the speed and regulating a flanged pump in practice.
I have an old pump without a label – how can I determine its parameters?
If there is a serial number or product type on the pump, try to find the technical data sheet from the manufacturer or distributor. Otherwise, measure: pressure gauges before and after the pump indicate the differential pressure (head), and a flow meter or calorimeter in the circuit provides the flow rate. These measurements will also help you verify whether the pump is operating at the correct point. If the pump draws more power than its rated parameters or overheats, there is a high probability that it is operating outside the optimal point.
Do the calculations for a flanged pump differ from those for a threaded pump?
The hydraulic calculations – the calculation of Q and H – are the same regardless of the pump connection type. The difference lies in the area of application: flanged pumps are used where higher flows (usually above 4–6 m³/h) are required or where the flanged construction provides advantages in terms of installation, sealing, and maintenance. The head/flow relationship follows the same physical laws. More about the differences between types can be found in the article Flanged vs. threaded circulation pumps – which is more suitable.
How much does the altitude of the installation affect the head?
For closed heating systems, the effect of altitude on the pump head calculation is negligible – the pump overcomes the hydraulic losses in the circuit, not the static height of the liquid column, since the system is closed and pressurized. Static height (e.g., how many meters the building is high) is relevant only for the expansion tank and system overpressure, not for pump sizing. An exception are open systems (e.g., open tanks, gravity circulation systems without pressure closure), where static height plays a role.
Conclusion: accurate calculation saves money and problems
Calculating the flow and head of a flanged pump is not rocket science, but it does require a systematic approach and respect for physical laws. Every skipped step – whether ignoring the temperature drop, underestimating pressure losses from fittings, or neglecting changes in operating conditions during renovation – will manifest either as insufficient system performance or unnecessary costs for an oversized and inefficient pump.
Flanged circulation pumps are designed for demanding applications with higher flows and long operating cycles. Therefore, it pays to give the calculation proper attention. If you have the project documentation, use it. If not, follow the procedure described in this article and, in case of doubts, have the selection verified by a professional – a correctly dimensioned pump will pay for itself many times over in energy savings and trouble-free system operation in the long run.
The selected category of flanged circulation pumps can be found on atria.sk, where technical data sheets with H-Q curves for individual models are available. We recommend comparing the calculated operating point with the curves of specific pumps before ordering.
Do you have a question about this topic?
Not sure or dealing with a specific situation in your home? Write to us – we are happy to help.