Hydronic Balancing DIY – Step 6: Presetting Radiator Valves

In the sixth step of the “Hydronic Balancing DIY” series, we will determine the presettings for the radiator valves and have almost reached the end of the hydronic balancing process. I will show you how to determine the presetting radiator valves for the valves already installed from the manufacturers Danfoss and Oventrop from our example house as well as for pressure-independent control valves (PICV). You can then determine the presetting radiator valves for other valve manufacturers such as TA Heimeier, Herz, Honeywell and others.

Important update 19.08.2025: I have now also included the determination for the default setting of pressure-independent radiator valves, as these correspond to the state of the art, the determination of the default setting is much simpler, and the hydraulic balancing with these valves is also covered in the partial load case.

If you don’t yet know what the presetting of a radiator valve is supposed to achieve, I recommend my two articles on the function of hydronic balancing and the function of a thermostatic valve. Before we get to determining the presetting values, here are a few basics that are important when choosing a radiator valve.

Presettable radiator valves vs. pressure-independent control valves (PICV)

By presetting radiator valves, we limit the amount of water for a radiator. As water is the energy source for heat in our hydronic heating system, each radiator receives exactly the amount of heat it needs to cover the room heating load (if there are several radiators in a room, this is divided between the number of radiators).

In this way, we optimise the water and therefore also the heat distribution in the building. Presetting is carried out using a presetting key (see figure 1) or by hand if possible. Each presetting level has a different water flow rate, as can be seen in the following illustration. Level 1 has a very low water flow rate and level 6 a very high one.

Like every component in a heating network, a radiator valve also represents a resistance for the heating water, which must be overcome. As the water passes through the valve, it loses some pressure. This pressure loss is specified as the differential pressure (Delta P) across the valve and is calculated from the pressure upstream of the valve minus the pressure downstream of the valve.

In a rigid system, these differential pressures can be easily calculated. However, as a hydronic heating system is not a rigid system, fluctuating differential pressures across a radiator valve are a problem. Fluctuating differential pressures occur, for example, when thermostats open or close, which changes the volume flows in other parts of the network. Once a volume flow rate has been calculated for a radiator and the presetting determined with it is then only valid for the design case. This means that the hydronic heating system would only be hydronically balanced in the full load case.

As a solution, valve manufacturers have developed Pressure Independent Control Valves (PICV), which have an internal differential pressure regulator in addition to the presetting and keep the flow rate and the differential pressure across the valve constant at all times. The presetting then applies not only in the design case (full load), but also in the partial load case, such as the transitional periods in autumn and spring. The following illustration shows two pressure-independent control valves as examples.

Conventional pre-settable radiator valves cannot compensate for the differential pressure, meaning that the pre-setting determined is only optimally suited for the design case. The following video provides an excellent demonstration of the influence of a fluctuating differential pressure on the volume flow and how this can be prevented by using pressure-independent control valves. pressure-independent control valves are offered by all major valve manufacturers such as Danfoss, Oventrop, Herz and IMI Heimeier.

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Summary of the differences

Pressure-independent control valves keep the differential pressure across the radiator valve constant and can cover hydronic balancing at full and partial load, which leads to greater energy savings and better heat distribution all year round. It is also much easier to determine the pre-setting values. A few pressure-independent valves from different manufacturers are shown below.

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Conventional pre-settable radiator valves, on the other hand, cannot compensate for differential pressures, which means that the pre-setting values determined are only suitable under the design conditions (full load).

Conventional presettable radiator valves have one Kvs value and several Kv values, which means that you must assume a differential pressure above the radiator valve when determining the presetting values. The basis for the Kv and Kvs values is explained in the following section. Below you will find the most important properties of classic presettable and pressure-independent valves.

Criterion	Classic (presettable)	Pressure-independent (PICV)
Control principle	Fixed presetting stage, volume flow (V̇) depends on the differential pressure (Δp)	Integrated differential pressure regulator keeps the volume flow (V̇) constant
Balancing input	Q̇, ΔT → V̇, assumed Δp, Kv value, xp value → Presetting stage	Q̇, ΔT → V̇ → presetting level
Behaviour with Δp fluctuations	Over- or undersupply possible, noise risk increases	V̇ remains stable within the specification, lower noise tendency
Thermostat operation and partial load	Control quality depends on valve authority, partial load often critical	Good control quality even under partial load due to pressure-independent flow rate
Noise risk	Increased at high Δp and partial opening	Reduced by differential pressure control in the valve
Differential pressure regulator in the line	Useful to necessary in larger buildings for stable Δp ratios in the individual lines	Not necessary as Δp controllers are installed in the individual valves
Limits	Sensitive to Δp fluctuations, adjustment changes with system changes	Observe the operating range for Δp and V̇, correct dimensioning is crucial

Tip: If you have the choice, you should opt for pressure-independent pre-settable radiator valves, as it is much easier to determine the pre-setting values and hydronic balancing can be guaranteed all year round.

If you want to skip the basics and get straight to determining the pre-setting values, you can jump straight here:

• Determine presetting values for pressure-independent control valves
• Determine presetting values for conventional presettable radiator valves

Basics: Kv and Kvs value

If you are involved in hydronic balancing and use conventional pre-settable radiator valves, sooner or later you will be confronted with the terms Kv (flow factor) and Kvs value. These values are product values that are measured by the manufacturer and are used to compare valves with each other, to size a valve correctly and to select it for the respective application.

The Kv value indicates the volume flow in m³/h at a pressure loss of 1 bar across the radiator valve (see Figure 2). The Kv value is also known as the flow factor.

If we have a conventional pre-settable radiator valve (example: Oventrop AV6) with six pre-setting stages, the Kv value is different for each pre-setting stage, as the maximum flow rate is also different (see Figure 2).

The Kvs value indicates the value when the valve is fully open. There are therefore five different Kv values for our example valve, but only one Kvs value (see Figure 2.1). These can be found in the AV6 data sheet from Oventrop on page 19.

Why is the Kv value so important?

As already described, valves in a hydronic heating system represent a resistance for the heating water. Therefore, there is a pressure loss across the valve, which must be assumed with conventional pre-settable radiator valves.

As the pressure loss across the radiator valve is lower on radiators that are far away (e.g. 30 mbar) than on radiators that are close to the heat generator (e.g. 70 mbar), different pressure losses in larger buildings can be included in the calculation and therefore provide more accurate results for the presetting. The valve manufacturers’ recommendations vary in terms of the pressure loss to be assumed, but are generally between 50 – 100 mbar.

Example calculation of Kv value with different differential pressure

In the following example, I would like to show you what influence the differential pressure has on the pre-setting values of a radiator valve. We assume the following values for our example calculation:

• Radiator output: = 700 W
• Temperature spread at 75/55: = 20 K
• Differential pressure 1: = 30 mbar = 0.03 bar
• Differential pressure 2: = 50 mbar = 0.05 bar
• Differential pressure 3: = 70 mbar = 0.07 bar
• Design proportional range (explanation follows in the next paragraph): xp = 1 K

Calculation of the volume flow (if you have forgotten how to calculate the volume flow, simply look again in the fifth step of the series “Hydronic Balancing DIY” ):

   

To calculate the Kv value, we use the following formula:

   

Now we enter our values into the above formula.

• Volume flow: = 30.1 l/h = 0.0301 m³/h
• Differential pressure 1: = 0.03 bar
• Differential pressure 2: = 0.05 bar
• Differential pressure 3: = 0.07 bar

Kv value with differential pressure 1:

   

Kv value with differential pressure 2:

   

Kv value with differential pressure 3:

   

Now we go to the Oventrop AV6 data sheet (german version) on page 19 and look at the Kv values. The Kv values specified for each presetting correspond to the maximum Kv value of a presetting level. Our determined Kv values therefore correspond to the following presetting levels:

• = 0.174 m³/h = level 3
• = 0.135 m³/h = level 2
• = 0.114 m³/h = level 2

Furthermore, we can read the presetting in the pressure loss diagram of the radiator valve based on the volume flow and the respective differential pressure (see Figure 3).

This clearly shows the influence of the pressure loss with the same volume flow over the valve in the calculation for the presetting:

• A radiator with a low differential pressure across the valve and positioned relatively far away from the heat generator can lead to a higher presetting.
• A radiator with a high differential pressure across the valve and relatively close to the heat generator can lead to a lower presetting.

What is the design proportional range (xp)?

Furthermore, the design proportional range (xp) plays an important role for the Kv value. The design proportional range indicates the response speed of the thermostat head (sensing element), i.e. the time-delayed opening and closing of the thermostat (hysteresis). In Figure 2, I have chosen xp = 1 K.

Note: The xp value is not a product property, but rather an assumption that must be made. For high energy savings, a design proportional range of xp = 1 Kelvin is therefore recommended.

Example of the design proportional range of a thermostatic head: We have a desired room temperature of 20 °C in our room and a thermostatic head with xp = 1 K.

• If the room temperature is 20 °C, the position of the thermostatic head corresponds to the desired flow rate required to reach the room temperature of 20 °C. When the room temperature reaches 21 °C, the thermostatic head closes.
• When the room temperature reaches 21 °C, the thermostatic head closes because the room temperature is 1 Kelvin higher than the desired room temperature.
• The difference between the desired room temperature and the deviation temperature is therefore a maximum of 1 Kelvin.

Our thermostatic head is therefore able to respond to a deviation of 1 Kelvin in the room temperature. This corresponds to xp = 1 K. A thermostatic head with xp = 2 K would therefore only respond when the room temperature reaches 22 °C.

Brief summary of the selection of presettable radiator valves

Tip: If you have not yet installed presettable radiator valves on your radiators, it makes sense to choose pressure-independent control valves. Determining the pre-setting values is much easier and hydronic balancing is guaranteed all year round.

Pressure-independent valves have an internal differential pressure regulator, which makes the body of the valve appear somewhat “thicker”.

• It is much easier to determine preset values for pressure-independent valves.
• Pressure-independent valves work optimally at full and partial load.

Determining the presetting values for “conventional” presettable valves is comparatively complex and is based on the assumption of a pressure loss. In addition, conventional presettable valves only work optimally at full load (very few days a year). Below I will show you how to determine the presetting values for pressure-independent and conventional presettable radiator valves.

Determine preset values for pressure-independent control valves

The special thing about pressure-independent control valves is that you can set the flow rate directly on the valve. Figure 4 shows an example of a pre-setting gate of a pressure-independent radiator valve (based on HeimeierEclipse). You can set the calculated volume flow in litres per hour (l/h) directly via a scale on the valves. The scale values can be found in Table 2.

Stage	1	2	3	4	5	6	7	8	9	10
l/h	10	20	30	40	50	60	70	80	90	100

You can now use the table to determine the pre-setting values for your pressure-independent control valves, as shown in the example below.

Example calculation radiator no. 1 in the anteroom

• Volume flow: 27 l/h
• Greater than 20 l/h (level 2)
• Less than 30 l/h (level 3)
• Preset level: 3

As the volume flow is greater than 20 l/h (level 2) and less than 30 l/h (level 3), the preset value for level 3 must be selected. Now do this for all radiators and you have your preset values – done. You can now proceed to the next step 7, determining the pump output.

Room	HK no.	Volume flow	Preset value
Anteroom (ground floor)	1	27 l/h	3
Toilet (ground floor)	2	4 l/h	1
Washroom (ground floor)	3	6 l/h	1
Kitchen (ground floor)	4	26 l/h	3
Living room (ground floor)	5	39 l/h	4
	6	39 l/h	4
Corridor (ground floor)	7	16 l/h	2
Bedroom (upper floor)	8	24 l/h	3
Bathroom (upper floor)	9	18 l/h	2
Office (upper floor)	10	35 l/h	4
Guest (upper floor)	11	29 l/h	3
Corridor (upper floor) – NEW	12	29 l/h	3

Determine presetting values for conventional presettable radiator valves

Determining the presetting values for conventional presettable radiator valves is more difficult. To do this, we need to apply our knowledge of the Kv values. For our example building, we calculate with a flat-rate pressure loss of 50 mbar across each radiator valve – this is an assumption that must be made. In the following example, I calculate the Kv value for the vestibule in our example building.

Example calculation radiator no. 1 in the anteroom

• Volume flow = 27 l/h = 0.027 m³/h
• Differential pressure: = 0.05 bar

   

The Kv value for the radiator valve on the radiator in the anteroom is 0.121 m³/h.

You must now carry out this process for all radiators. The best way to do this is to create a small Excel spreadsheet that automatically calculates the Kv value for you.

Overview of the Kv values for the example building

Table 4 shows the Kv values determined for our example building.

Room	HK No.	p	Volume flow	Kv value
Anteroom (ground floor)	1	50 mbar	27 l/h	0,121
Toilet (ground floor)	2	50 mbar	4 l/h	0,017
Washroom (ground floor)	3	50 mbar	6 l/h	0,025
Kitchen (ground floor)	4	50 mbar	26 l/h	0,117
Living room (ground floor)	5	50 mbar	39 l/h	0,176
	6	50 mbar	39 l/h	0,176
Corridor (ground floor)	7	50 mbar	16 l/h	0,073
Bedroom (upper floor)	8	50 mbar	24 l/h	0,106
Bathroom (upper floor)	9	50 mbar	18 l/h	0,079
Office (upper floor)	10	50 mbar	35 l/h	0,178
Guest (upper floor)	11	50 mbar	29 l/h	0,129
Corridor (upper floor) – NEW	12	50 mbar	29 l/h	0,129

Presetting Danfoss – built-in valve RA Series 3 with presetting

Now that we have calculated the Kv values for all radiators, we can determine the preset values from the data sheets of the respective radiator valves.

IMPORTANT: Not all RA-N valves are the same! It also depends on the nominal connection size and whether it is an angle valve or a straight-way valve. There are countless variants of one type and each variant has its own Kv and pre-setting values. You need to research this carefully.

As our example building mainly contains radiator valves from Danfoss, we will start with this manufacturer. As an example, we will take a radiator with a pre-settable radiator valve from the living room (see Figure 5). This is a valve radiator with integrated radiator valve RA series “3” N with a red presetting crown. In some radiators, the radiator valve RA of series “3” U with yellow presetting crown (for low volume flows) is also installed.

To determine the setting values, we work with the following data sheet from Danfoss for the Danfoss valve – series “3” with presetting. You can find the data sheet here: Data sheet: Series 3 built-in valve with presetting (german version).

Firstly, we open the data sheet for our valve and find the Kv values for the respective presettings on page 1. The following Kv values result for the built-in valve RA-N with xp = 1 K:

AP	Stage 1	Level 2	Level 3	Level 4	Level 5	Level 6	Level 7	Level N
Xp = 1K	0,11	0,15	0,20	0,25	0,31	0,37	0,44	0,57
Xp = 2K	0,14	0,21	0,26	0,32	0,46	0,59	0,73	0,87

The Kv value for our example radiator from the living room is Kv = 0.176 and lies between 0.15 (level 2) and 0.20 (level 3). This gives the radiator valve in the living room a default setting of 3.

With our built-in valve series “3” RA-N and RA-U from Danfoss, the setting values can be adjusted without special tools. To do this, simply turn the preset to the setting mark on the radiator valve to obtain your preset (see figure 6).

Presetting Danfoss – presettable valve body type RA-N

The bathroom radiator on the upper floor has a presettable angle valve type RA-N from Danfoss. As the bathroom radiator is a compact radiator, the valve is not integrated into the radiator. It is therefore necessary to know the nominal connection diameter (DN) of the radiator. In our case, the nominal connection diameter is DN10 = 3/8″ (inch) = 17.2 mm (outside diameter).

According to the data sheet, the bathroom radiator therefore has a pre-settable valve body “RA-N 10”. To determine the presetting, we go to the data sheet for the radiator valve type RA-N on page 2 and look for the Kv value for the RA-N 10. Here we see that our Kv value of 0.079 for the RA-N 10 corresponds to xp = 1 K greater than stage 1 (0.04) and less than stage 2 (0.09). The default setting 2 is therefore selected.

AP	Level 1	Level 2	Level 3	Level 4	Level 5	Level 6	Level 7	Level N
Xp = 1K	0,04	0,09	0,14	0,21	0,23	0,27	0,28	0,34
Xp = 2K	0,04	0,09	0,16	0,25	0,32	0,38	0,42	0,56

The presettable radiator valve type RA-N 10 from Danfoss also requires no special tools for presetting. Instructions for presetting can be found in the data sheet on page 3.

Presetting Oventrop – presettable radiator valve AV6

The floor heating system with the Oventrop individual room control and return temperature limitation (“Unibox vario” with the thermostat “Uni RTLH”) has the presettable radiator valve AV6. To determine the pre-setting value for the radiator valve, please refer to the data sheet of the “Oventrop “Unibox” individual room control and return temperature limitation in panel heating systems“ on page 4.

We can now read the default setting from the pressure loss diagram with our calculated volume flow (see Figure 7). Our volume flow for underfloor heating is = 26.23 l/h (see Figure 4).

The presetting for the Oventrop AV6 radiator valve at = 50 mbar, xp = 4 K and a volume flow of = 26.23 l/h corresponds to level 2. A special spanner is required for presetting the Oventrop AV6.

Overview of the presettings for the example building

Below you will find an overview of the installed radiator valves and their default settings for our example building. The following valve types are installed:

• Danfoss built-in valve series 3 RA-N (D E3 RA-N)
• Danfoss built-in valve series 3 RA-U (D E3 RA-U)
• Danfoss pre-settable valve body type RA-N (D VV RA-N)
• Oventrop AV6 in AHU Unibox (OV AV6)

Room	HK No.	Valve type	Kv value	Presetting
Anteroom (ground floor)	1	D E3 RA-N	0,121	2
Toilet (ground floor)	2	D E3 RA-U	0,017	1
Washroom (ground floor)	3	D E3 RA-U	0,025	1
Kitchen (ground floor)	4	OV AV6	0,117	2
Living room (ground floor)	5	D E3 RA-N	0,176	3
	6	D E3 RA-N	0,176	3
Corridor (ground floor)	7	D E3 RA-U	0,073	4
Bedroom (upper floor)	8	D E3 RA-N	0,106	1
Bathroom (upper floor)	9	D VV RA-N	0,079	2
Office (upper floor)	10	D E3 RA-N	0,178	3
Guest (upper floor)	11	D E3 RA-U	0,129	2
Corridor (upper floor) – NEW	12	D E3 RA-U	0,129	2

Conclusion

You have now learnt how to determine the pre-setting values for pressure-independent and conventional pre-settable radiator valves. It is clear that the determination for pressure-independent control valves is much simpler and more accurate.

With the data now available, we can preset the radiator valves. By presetting radiator valves, we have taken an important step towards optimising the flow behaviour in our hydronic heating system. However, hydronic balancing is not yet complete.

In the first step of the series “Hydronic Balancing DIY”, I wrote that the use of differential pressure regulators is recommended in larger buildings. However, as we have a relatively small building and an electronically controlled pump with energy efficiency class A, we do not use differential pressure regulators.

Important: If pressure-independent control valves are used, differential pressure regulators are not required, even in large buildings with a complex heating network.

In the next step of the “Hydronic Balancing DIY” series, we can now determine the parameters for our circulator pump. If you have any questions, suggestions or criticism, please use the comments function.

Important: You can only carry out the presetting step if presettable radiator valves are installed in your radiators. If this is not the case, you will need to call in a specialist company to install pre-settable radiator valves. It may be that only the insert needs to be replaced or the entire valve.

Best regards! Martin

Further links and sources:
Wikipedia – Flow Coefficient
Danfoss – hydronic balancing made easy
Wikipedia – Nominal size
The KV value – hydraulischer-abgleich.de
The valve authority – hydraulischer-abgleich.de