In this article I will show you how to calculate and design a central heating pump (circulating pump) with only few data. This calculation method is mostly used in existing buildings, because technical data is hard to find or not available at all. In a small example calculation I show you that the calculation of a heating pump is easier than you think. Afterwards we will design a central heating pump using the example and choose the right model for our application.
For the design of a circulating pump you actually only need two values: the head of the pump H and the pump flow rate QPU. So let’s get straight into it.
Table of Contents
- 1 Calculation of the head
- 2 Calculation of the flow rate
- 3 Example calculation
- 4 Pump curve and working range of a pump
- 5 Design for Grundfos
- 6 Design for Wilo
- 7 Conclusion
Calculation of the head
To bring the facts back to the point, I will quote an important passage from my article “What is pump head?“.
A pump is designed to transport liquids from A to B. For this transport kinetic energy is necessary. The kinetic energy is produced by the rotation of the pump impeller inside the pump. This creates a pressure build-up by the pump, which is necessary to overcome the friction and loss along the pipeline inside the heating system at a certain flow rate.
The pump must therefore build up a certain delivery pressure to overcome the friction and loss in the heating system and supply the radiators with hot water.
Normally the head of a system is calculated on the basis of geodetic head Hgeo, static pressure Hstat, dynamic pressure Hdyn and losses in the system HV.
In a circulation system (i.e. a heating system), however, only the losses of the system HV are decisive for the calculation of the delivery head according to VDMA 24199 (Control requirements for hydraulics in planning and execution of heating, cooling, drinking water and air conditioning systems from the german Mechanical Engineering Industry Association). Therefore, the following simplified formula can be used to calculate the pump head of a heating system:
Incidentally, the factor 10,000 in the formula is the factor used to convert the head unit from pascals to “meters water column”, since this is the unit predominantly used. The following applies:
1 m H2O = 0,0980665 bar = 98,0665 mbar = 9.806,65 Pa
For simplicity, the following can be noted:
1 m H2O ≈ 0,1 bar ≈ 100 mbar ≈ 10.000 Pa
10 m H2O ≈ 1 bar ≈ 1000 mbar ≈ 100.000 Pa
In the following I will explain to you how to use the formula. This information I have taken from the WILO Pump Basics Guide, which I can recommend to everyone. Here all important information about pump technology is explained in a simple and understandable way. The contents of the following table can be found in the WILO Pump Basics Guide on page 41.
|R||R indicates the pipe friction loss in a straight pipe.|
According to Wilo, 50 Pa/m to 150 Pa/m can be assumed here for standard systems. The age of the building plays the decisive role here. Due to the larger nominal diameters, older buildings have a smaller pipe friction loss (50 Pa/m) and newer ones a higher one (150 Pa/m).
|L||L stands for the length of the least favorable heatingline of feed and return in meters . Simplified you can take the dimensions of the house. This is calculated as follows: length+width+Height x 2.|
|ZF||ZF stands for the addition factor for valves and fittings which represent a resistance in the heating system. According to Wilo, the following factors apply: |
|10.000||Factor to convert Pascal Pa into meter water column [mH2O]|
(1 mH2O = 9.806,65 Pa)
Calculation of the flow rate
The flow rate is the volume flow at the outlet of the pressure side of a pump, in other words the side in which the water is to be delivered. If you are not familiar with the term volume flow rate, I recommend my article about volume flow rate and flow velocity.
The flow rate of a pump is calculated with the following formula:
| indicates the heat flow or the heat output. In this case it is the heat demand of the building, which was determined by a heating load calculation or corresponds to the existing boiler capacity in the building.|
Note: If the installed boiler capacity is greater than the calculated heat demand by means of a heating load calculation, it is recommended to calculate with the value of the heating load calculation. The heat demand is given in Watt .
|indicates the density for water. In this case can be calculated.|
|cW is the specific heat capacity for water. It can be calculated with .|
|stands for the temperature difference between feed flow and return flow. The temperature difference is given in Kelvin .|
In this example calculation I would like to show you how to design a heating pump. For this purpose I have made a small sketch in which you will find the necessary data to calculate the head and the flow rate.
When looking at the sketch we can determine the following values:
- Width: 15 m
- Length: 20 m
- Hight: 12m
- Construction year: 1990
- ZF = 2,2 (Moduled parts/ Fittings and valves + thermostatic radiator valve existent)
- assumption of 120 Pa/m pressure loss
Heat demand: 80 kW
Heating System Temperature: 75/55
Calculation of the head H
Calculation of the flow rate Q
We have now successfully calculated the most important data for determining the circulating pump. In the next step we will determine the dimension of the circulation pump.
The two companies Grundfos and Wilo are currently market leaders. Therefore we will take a closer look at the two online programs for pump design and the smartphone apps of these two companies (these are free of charge). Further sizing programs can be found for example at Biral Pump Selector Online and KSB EasySelect.
Pump curve and working range of a pump
To choose a suitable pump, it is necessary to look at the pump curve. Each pump has its own pump curve, which is divided into flow (Q) on the x-axis and head (H) on the y-axis.
Furthermore, the working range of the pump is indicated and it is shown which delivery head is possible at which flow rate. The design duty point of a pump shows us the point at maximum calculated head and flow rate.
This calculated duty point should be within the specified working range (red area) of the respective pump. Based on Figure 2, the following can be said, as described in the WILO Pump Basics Guide:
- First range: If the duty point is in this range, a smaller pump should be selected.
- Second range: If the duty point is in this range, the pump mainly works in its optimal working range (according to Wilo approx. 98 %).
- Third range: If the operating point is in this range, the pump only works in its optimum operating range in the design case, i.e. on the hottest and coldest days of the year (according to Wilo approx. 2 %).
Design for Grundfos
To design a pump, Grundfos originally created the online software WebCAPS, which is now simply called “Grundfos Product Center“. This is also optimized for smartphones, so an additional app is no longer necessary. To design a pump, go to the Grundfos Product Center website.
Here you have the choice between “Quick Design”, “Advanced Design” and “Guided Design”. We decide for the “Quick design”. If you want to have it a bit more precise, you can enter more details in the extended design. As data, we store our calculated values for the flow rate (Q) and the head (H), and then select: Select design type -> Size by application -> heating.
As installation we choose “Main Circulator”. Then the pump can be designed.
After you click on “Start sizing”, a list of potential pumps will be suggested. We select the Magna 3 25-40 and take a closer look at the data, because it is important that the calculated duty point is within the working range of the pump.
As we can see in the pump curve below, the duty point is within the operating range of the Magna 3 25-40 pump.
Done: Grundfos proposes a Magna 3 25-40 based on our data. Here, of course, the existing pipe connection size in the boiler room is also important, since the proposed Magna 3 fits a pipe with a nominal diameter of DN 25.
These and other specifications can be entered in the extended design. For further product information you can click on product details and get a detailed description of all pump functions.
Grundfos Pumps at Amazon:
Last updated prices on 2021-10-12 / Picture source: Amazon affiliate program
Design for Wilo
Pump design with Wilo-Select Online
Wilo has also launched an online software for its pumps. This is the program Wilo-Select Online. Here you also go first to the website of Wilo-Select Online and choose your language and the home market in the first step.
In the second step you choose “Hydraulic selection” -> “Start Now” and then you get to the input mask for Wilo Select.
In the input mask for Wilo Select you first select the area of application for the pump. In our example, the application area is “heating, air conditioning, cooling”. Then we select a pump for the respective requirement. I decide heating -> glandless pump -> high-efficiency pump. Wilo Select now automatically selects a pump group that could fit. In our case it is the Wilo Stratos Series. In the next step, enter the calculated flow rate and the calculated head in the corresponding field and click on “Next”.
The following picture shows a list of possible pumps and the corresponding pump performance curve. I select the first pump in the list, a Stratos MAXO 25/0.5-4 PN10. Here the duty point is located in the second third of the pump curve. It can therefore be assumed that the pump operates predominantly in its optimum working range.
We made it! With these few steps it is possible to design a pump. Here again, the existing pipe dimensions in the boiler room are important, as the Wilo Stratos is suitable for a pipe with a nominal diameter of DN 25. Like Grundfoss, Wilo also offers a large selection of product details in Wilo-Select Online. Here you can find, among other things, specification texts, an operating manual, a terminal diagram or CAD data.
Wilo Pumps at Amazon:
Last updated prices on 2021-10-12 / Picture source: Amazon affiliate program
Design of the pump with the Wilo Assistant for Smartphones
In figure 10 you can follow the pump design with the Wilo assistant. To start the pump sizing, open the installed app on your smartphone. In the first step you can directly enter the calculated values of flow rate and head into the Wilo Assistant and then press “Heating”.
Different to the online program Wilo Select, the steps of research work are omitted. A heating pump is suggested to you directly on the basis of the calculated data. The Wilo Assistant suggests a Stratos MAXO 25/0.5-4 PN10, as we have already determined in Wilo Select. Alternatively, a smaller pump with the Yonos PICO plus 25 1-8-130 PN10 is suggested.
If no pump sizing program is available, it is often possible to determine the head and flow rate with little data and then to design a pump using the pump manufacturers’ websites or smartphone apps.
However, before installing or purchasing a pump, the calculated values should always be checked by a specialist company or engineering office.
IMPORTANT: The working methods described here are based on my personal experience and thought processes. Trying out and implementing the described procedures is solely at your own responsibility and risk. I take no responsibility. Furthermore I recommend to always have the calculated values checked by a specialized company or an engineering office. Because even if the way described here seems to be simple, calculation errors can always creep in.
I hope I could give you with this article a small insight into the design and calculation of heating pumps. If you have questions, suggestions or criticism, I am looking forward to your comments.
Best regards! Martin
Further links and sources:
KSB – Centrifugal Pump Lexicon – Head
Lean and Green Business – The design process of a centrifugal pump
Pumpfundamentals.com – Tutorial Centrifugal Pumps [pdf]
Wilo Pump Basics Guide
Wilo Assistent – Google Playstore
Wilo Assistent – Apple
Grundfos Product Center
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