Hydronic Balancing DIY – Step 7: Circulator Pump Sizing

In the seventh step of the “Hydronic Balancing DIY” series, we will calculate the parameters for our circulator pump and select the appropriate control parameters. In the first step, we determine the pump head and the required flow rate for the circulator pump, size a pump and then make the correct settings on the circulator pump.

Important: For the calculation, we refer to the basics from my article “Design of a central circulator pump with example calculation“.

Calculating the flow rate via the radiator volume flows

To calculate the maximum required flow rate (volume flow) of a circulator pump _QPU, we add the calculated radiator volume flows (Step 5: Volumetric Flow Calculation) and obtain the total flow rate for the circulator pump.

The sum of the radiator volume flows V_HK results in the flow rate Q_PU.

Calculating the pump head

In this section, we will roughly calculate the pump head (H) for our circulator pump (if you have forgotten what the pump head is and how it is calculated, just take another look at my articles “What is pump head and why it’s important?” and “Design of a circulator pump with example calculation“).

Note: In a new building, you will get the pump head from the pipework calculation of the hydronic heating system.

The following data is given for the example building:

• Detached house, built in 1935, renovated in 2005 (facade, roof, windows, heating)
• Total area (heated): approx. 100 m²
• Width: 6 m
• Length: 9 m
• Room height with ceiling ground floor: 2.5 m
• Room height with ceiling upstairs: 2.5 m
• ZF = 2.2 (fittings, valves and thermostatic valves available)
• Assumption of 120 Pa/m pressure loss

R = 120 Pa/m
L = (6 m 9 m 5 m) * 2 = 40 m
ZF = 2.2

The determined pump head of 1.06 mWs is an important setpoint value that can be stored in many modern circulator pumps and ensures optimised pump operation.

Setting options for circulator pumps

Before we look at the choice of circulator pump, I would like to introduce you to the various setting options for modern circulator pumps. In most electronic circulator pumps, you can set the control mode, the differential pressure setpoint (our calculated pump head H) and automation modes such as “AutoAdapt” (Grundfos) and “Dynamic Adapt” (Wilo). The volume flow is then calculated from the combination of the stored data, the resulting system characteristic curve and the valve positions.

For some pumps, a maximum flow rate (calculated Q-max) can also be stored as a cover, but you should only do this if you have carried out hydronic balancing. Below you will find a brief overview of the common setting options and control types as well as the appropriate setting recommendations.

Constant differential pressure Δp-c

With the “Constant differential pressure” setting, the pump keeps the set differential pressure (required pump head) constant within the pump characteristic curve. This means that the pressure loss between the flow and return remains stable, regardless of the current flow rate. The volume flow itself results from the valve positions and pipework.

When should the differential pressure Δp-c be set?

The “Constant differential pressure” setting is recommended if you have installed pressure-independent radiator valves (presettable radiator valves with internal differential pressure regulators) on your radiators in the secondary circuit (heat distribution and heat transfer – see step 6 for hydronic balancing) or, alternatively, if you have installed differential pressure regulators with conventional presettable radiator valves in larger buildings. This is necessary because pressure-independent radiator valves and differential pressure regulators require a minimum differential pressure to function correctly. If this were not constant, radiators could be undersupplied in the partial load range.

The “constant differential pressure” setting is also often useful in the primary circuit (area of the heat generator) if a minimum volume flow is required as cycle/overheating protection on the heat generator.

Differential pressure variable Δp-v:

With the “Variable differential pressure” setting, the pump dynamically adjusts the differential pressure to the current flow rate. If the flow rate drops due to closing thermostatic valves, for example, the pump head also drops linearly. This reduces flow noise, saves electricity and significantly improves partial load operation.

When should the differential pressure Δp-v be set?

The setting is recommended if you have installed conventional, pre-adjustable radiator valves without integrated differential pressure control in the secondary circuit (heat distribution and heat transfer) on your radiators(see step 6 for hydronic balancing) or, in larger buildings, if you have additionally installed line-wise pre-adjustable line regulating valves.

Note: Δp-c is usually used for underfloor heating systems. Δp-v is only worthwhile for very variable zone systems with many actuators.

Differential pressure temperature-controlled Δp-t:

The pump adjusts the differential pressure between the minimum and maximum pump head based on the flow or return temperature. There are two operating modes: positive and negative gradient.

When should the differential pressure Δp-t “positive gradient” be set?

For example, in the primary circuit of older standard boilers (which are virtually non-existent today): If the flow temperature rises (higher demand), the differential pressure also rises – more volume flow is available.

When should the differential pressure Δp-t “negative gradient” be set?

For example, in the primary circuit of condensing boilers: If the return temperature rises (unfavourable for the condensation of water vapour in the flue gases), the pump lowers the differential pressure. This reduces the volume flow, the dwell time in the heating surfaces increases and the return temperature falls again, so that the condensing effect can be better utilised.

Note: The flow or return temperature must be recorded. For pumps with an internal sensor, this means in practice: installation in the flow or return. With an external temperature sensor, the pump position is less critical and the flow is more likely to be selected. For condensing boilers and condensing boilers, the following guide values apply: favourable return temperatures for utilising the condensing effect: < ~52 °C (gas) or < ~45 °C (oil).

Auto modes: AutoAdapt (Grundfos) & Dynamic Adapt (Wilo)

Many smaller modern circulator pumps, especially for detached and semi-detached houses and small apartment blocks, have an automatic mode. The pump then automatically selects a suitable control characteristic or a differential pressure setpoint (pump head) within a specified range. The aim is to use automation to provide a sufficient volume flow with quiet operation and low power consumption. However, automatic mode is not always useful!

Important: Many pump manufacturers recommend setting their automatic mode for two-pipe hydronic heating systems. However, this also considers systems that are not hydronically balanced. This setting makes sense for a system that is not hydronically balanced. The automatic function does not replace hydronic balancing, but ensures more efficient operation of the circulator pump in a system that is not hydronically balanced. In a hydronically balanced system, you should therefore always store the control mode and the calculated setpoints in the pump.

If you have carried out hydronic balancing in a two-pipe hydronic heating system, the following recommendations are therefore available for the control type of the pump:

Circulator pump sizing with Wilo quick sizing function

If an electronically controlled pump is not yet installed in your building, you can now design a circulator pump using the available data. If you already have an electronically controlled circulator pump with energy efficiency class A, you can adjust the calculated parameters in the pump.

In the following, I will show you how you can roughly design a circulator pump with the Wilo quick sizing using our data. Other pump manufacturers such as Grundfos, KSB or Biral have similar sizing programmes, but they also appear somewhat more complicated and sometimes require a customer login.

We can use the Wilo design programme website to design a suitable pump using our calculated flow rate and pump head data. To do this, go to the website, enter your data and then click on“Search for suitable pumps“

Tip: If you do not have the data such as volume flow and pump head, you can also determine the volume flow and pump head in the programme using the building data. The result here differs slightly from our roughly calculated data, but the result should be similar. The calculation via the Wilo website produces a flow rate of 0.2 m³/h and a pump head of 1.31 mWs.

After you have clicked on“Search for suitable pumps“, you will receive a selection of possible circulator pumps. Based on the data entered, Wilo recommends various models, such as the Stratos PICO Plus, Yonos PICO Plus or Atmos PICO Plus.

Important: You should always consult your heating contractor to decide which pump to install! They can check the situation on site and make the best assessment.

Next, we have a closer look at two pumps that are suggested to you. I choose the Stratos PICO Plus and the Yonos PICO Plus, as these are among the smallest circulator pumps in the Wilo family, for which I can select Δp-v and Δp-c in the menu. Then take a closer look at the characteristic curves of the pumps. To do this, click on the pump and then select “Characteristic curve”. Here you can view the pump characteristics for variable differential pressure (Δp-v) and constant differential pressure (Δp-c).

Note as a reminder:
– For Δp-v (variable differential pressure), the operating point should be in the second third of the pump characteristic curve (shown in pink). This is suitable for both pumps.
– This is less relevant for Δp-c (constant differential pressure), as the operating point only shifts horizontally.

Based on the pump characteristic curve, I can see that I would have enough clearance with both pumps to increase the flow rate (if necessary). I can therefore select the pump based on the maximum electrical power consumption and the purchase price.

My choice would be the Wilo Yonos PICO Plus, as it is cheaper and has a lower maximum power consumption.

Once your pump has been installed, you can start with the settings. You can follow the settings in the Yonos PICO Plus menu (operating instructions – german version).

IMPORTANT: On the Yonos Pico Plus, the setting Δp-c (constant differential pressure) has a “floor heating symbol” and the setting Δp-v has a “radiator symbol”. Please do not let this confuse you.

In the Wilo Stratos PICO Plus menu, you should select “Manual setting” (see Wilo Stratos PICO Plus Quickguide on page 18 – german version) and enter all the data.

Regardless of which pump you have now selected: I would select the following default setting for both pumps:

• Control type: Δp-v for conventional, presettable radiator valves
• Control type: Δp-c for pressure-independent radiator valves (PIVC)
• Setpoint: 1.1 mWs (setting with one decimal place – 1.06 mWs ≈ 1.1 mWs)
• Optional Stratos only: Night setback – ON/OFF (depending on what you prefer, I would set to ON)
• Optional Stratos only: Pressure-independent valves (PICV) – ON/OFF (depending on what is installed)

For our example building, I would choose Yonos PICO Plus. Since conventional, presettable radiator valves are installed, I select differential pressure variable (radiator symbol) as the control type and set the pump head to 1.1 mWs (1.06 mWs ≈ 1.1 mWs) – done.

Setting the existing Grundfos pump

Our example building already has a Grundfos Alpha2 25-60 180 circulator pump. This means that we are trying to find the optimum settings here.

Always refer to the operating instructions for your pump for the setting options; you will find all the necessary information there. The following setting options are available for this Grundfos Alpha2:

• I – constant speed (stage 1)
• II – constant speed (stage 2)
• III – constant speed (level 3)
• AutoAdapt – automatic mode
• PP1 – lower proportional pressure characteristic (variable differential pressure)
• PP2 – upper proportional pressure characteristic curve (variable differential pressure)
• CP1 – lower constant pressure characteristic curve (constant differential pressure)
• CP2 – upper constant pressure characteristic curve (constant differential pressure)

According to the Grundfos instructions, the following default settings are recommended for a two-pipe hydronic heating system. I assume that these apply to all two-pipe hydronic heating systems, i.e. also to systems that have not been hydronically balanced.

Important: As our system is hydronically balanced and the pump dates back to a time when there were no pressure-independent radiator valves (PICV), I have added to the table in the instructions and adapted it slightly. Here are my general recommendations for the pump setting.

A look at the operating instructions provides us with information on which setting has a corresponding effect in the pump diagram. As a reminder: Our pumpy head (H) was 1.06 mWs and our flow rate (Q) was 0.29 m³/h. If I plot this in the pump diagram, I come to the following conclusion:

Operating point 1 (AP1)

If I have a two-pipe hydronic heating system with conventional, presettable radiator valves, I can select the lower proportional pressure characteristic (PP1). The operating point is then almost directly on the characteristic curve with H = 1.06 mWs and Q = 0.29 m³/h and the maximum power consumption is around 9 W. If the least favourable radiator in the network does not heat up reliably, I can increase the characteristic curve to PP2.

Operating point 2 (AP2)

If I have a two-pipe hydronic heating system with pressure-independent radiator valves (PICV) and select the lower constant pressure characteristic curve (CP1), the pump head is around 3 mWs, i.e. around 180 % above the calculated value of 1.06 mWs. However, since pressure-independent radiator valves require a minimum differential pressure, there are still many arguments in favour of the setting Δp-c (constant pressure) = CP1. The power consumption would then be around 16 W.

Even if CP1 requires more pump current and delivers a higher pump head than necessary, in my estimation the overall saving through correct hydronic balancing with pressure-independent radiator valves in the heating range is usually significantly greater. You could therefore proceed as follows:

• Start with the lowest characteristic curve CP1 (Δp-c). If there are no noises and everything warms up, leave it as it is.
• If unpleasant noises occur, replace the pump and size it smaller (the existing pump was selected before the adjustment, a smaller one would better meet the required differential pressure).

Note: If the CP1 setting is audibly too high for pressure-independent valves, the clean solution is a smaller pump or a model with a freely adjustable pump head (you would then have to replace the pump). Even if the pump has the option of storing a constant speed (I/II/III), I would advise against this for continuous operation. Although the constant speed sometimes achieves the desired pump head better than the excessively high CP1, it does not guarantee it in all operating situations, especially in the partial load range.

Setting in the example building

As the building has a two-pipe hydronic heating system with conventional, presettable radiator valves, I have set the pump to variable differential pressure (Δp-v) with the lower proportional pressure characteristic (PP1).

Conclusion

Selecting a circulator pump and the optimum operating point is not always easy. Especially with pumps where you cannot directly specify the pump head, a compromise is sometimes necessary. This may be a higher pump head with a higher power consumption. Therefore, always keep an eye on the overall picture: Correct hydronic balancing with valve adjustment and the right control strategy will bring the greatest savings.

In the next step of the “Do-it-yourself hydronic balancing” series, we will make the final settings for the heating curve . If you have any questions, suggestions or criticism, please use the comments function.

Important: You can only make the settings on the circulator pump if it is an electronically controlled circulator pump. If you still have a stage-controlled pump, you should have it replaced with an electronically controlled pump (please dimension it correctly – do not simply replace it one for one). The following image shows the differences between stage-controlled and electronically controlled circulator pumps. Always have the circulator pump replaced by a specialist company.

Best regards! Martin

Further links and sources:
Wilo pump guide 2016
Wilo Germany
Wilo quick sizing
Wilo-Select Online
Grundfos Germany
Grundfos Select