Hydronic Balancing DIY – Step 8: Heating Curve Settings

In the eighth and final step of hydronic balancing, we carry out the necessary adjustment of the heating curve settings. To do this, we will first look at what a heating curve is, how it is structured and what recommendations there are for setting it.

What is a heating curve?

The heating curve or heating characteristic of a heating system is a control and regulation option and, if set correctly, ensures efficient and energy-saving operation. The heating curve establishes a relationship between the outside temperature and the flow temperature (supply) in the heating system. It works something like this:

• When outside temperatures fall, the flow temperature rises
• When outside temperatures rise, the flow temperature decreases.

This saves energy and the building is only provided with as much heat as necessary at different outside temperatures. To be able to set the heating curve, you need a weather-compensated control, which is now installed in almost all buildings with hot water heating systems.

Structure of a heating curve

A heating curve is set using three variables: the slope (gradient), the curve offset (parallel shift) and the room temperature setpoint. These are shown in the following illustration.

Slope of the heating curve (gradient)

The slope of the heating curve is also called the steepness of the heating curve and indicates how strongly the flow temperature (supply) reacts to falling outside temperatures.

• Steep slope: very high flow temperatures at low outside temperatures, typical for unrenovated old buildings with high heat losses
• Flat slope: low flow temperatures at low outside temperatures, typical for well-insulated new buildings and heating systems with low flow temperatures.

In Figure 1 in the slope area, you can see that the steep heating curve wants to provide a flow temperature of 55 °C at an outside temperature of around 12 °C, while the flat heating curve only wants to provide around 28 °C.

Curve offset (parallel shift) of the heating curve

The curve offset (parallel shift) shifts the entire heating curve up or down in parallel. The reference value is the reference outdoor temperature (usually +20 °C in practice). The standard setting is usually 0 Kelvin (0 K), with the base of the curve starting at 20 °C outside temperature.

If you set the curve offset to +15 K, all flow temperatures in the curve are 15 K higher, including at the base point. The base point is then at 35 °C (20 °C + 15 K = 35 °C).

Room temperature setpoint of the heating curve

You can specify the desired room temperature setpoint via a further parallel shift of the characteristic curve. In modern control systems, the room temperature setpoint can be entered directly. The control system then shifts the curve internally accordingly. As a rule, a room temperature of 20 °C is stored. If you increase or decrease the setpoint, the entire curve shifts in parallel. A temporary parallel shift downwards takes place during the night setback (approx. 16 °C).

Note: Changing the curve offset or the room temperature setpoint does not change the slope; it applies a parallel shift (offset) to the heating curve.

Example to understand the heating curve

Figure 2 shows a typical heating curve diagram with a slope spectrum from 2.6 = steep to 0.2 = flat.

If we now look at the heating curve with a slope of 1.2, a curve offset of 0 K and a room temperature setpoint of 20 °C, you can read off the flow temperatures at different outside temperatures:

• At an outside temperature of -15 °C, the heat generator supplies approx. 65 °C flow temperature.
• At an outside temperature of 5 °C, the heat generator supplies a flow temperature of approx. 55 °C.

Note: Such a heating characteristic diagram is usually included in every operating manual for your control system. This allows you to check at any time whether the temperatures are as expected.

Which heating curve settings are recommended?

You have now learnt about the principle of the heating curve. To ensure that your heating system can also be operated in an energy-efficient manner, the following guide values are available for setting the heating curve:

Important: There are no general recommendations for the optimum setting of a heating curve. Every building is different, so you need to find your “perfect heating curve”.

Guide values for setting the slope and curve offset

• 0.2 to 0.8: New build with excellent insulation, heat pump, underfloor heating, flow temperature below 50 °C
• 0.8 to 1.4: Insulated existing building, low-temperature heating, condensing boiler or heat pump, flow temperature approx. 50 to 75 °C
• 1.4 to 2.0: Old building with weak insulation, older heat generators, flow temperature above 75 °C
• Curve offset: Start at 0 K

These values are a starting point for you. You can then fine-tune them during operation. Make changes in small steps and observe how they work for one or two days. Make a note of your settings and document them so that you can compare them and find the best heating curve.

Correcting the heating curve during operation

You can use the following correction options to adjust the heating curve if it is too cold or too warm in your building.

• Too cold in winter, transitional period (autumn and spring) → Increase the slope to the next higher value
• Too warm in winter, transition period (autumn and spring) fits → Lower the slope to the next lower value
• Too cold in the transitional period (autumn and spring), winter fits
  → Step 1: Raise the curve offset to the next higher value
  → Step 2: Lower the slope to the next lower value
• Too warm in the transitional period (autumn and spring), winter fits
  → Step 1: Lower the curve offset to the next lower value
  → Step 2: Increase the slope to the next higher value

Room temperature setpoint

• Every degree less saves up to 6 % heating energy.
• For efficient operation, do not set the normal room temperature above 20 °C.

Where is the heating curve set?

The heating curve is set in the heating control unit. This is located either in an external controller or directly in the heat generator. Depending on how old your heating system is, the heating curve can also be set in different ways, either manually or digitally. Figure 3 shows two typical heating control systems. On the left you can see an old decentralised and manual control unit and on the right a digital control unit, which is located directly in the heat generator.

To make the correct settings for the heating curve, you will always need the operating instructions for your heat generator and the control unit. It is also advisable to carry out the setting once with a specialised company.

Connection between heating curve and hydronic balancing

In“Step 3: Data recording” and“Step 4: Calculate radiator output“, we defined the system/design temperatures for our heating system. This is 75/55/22 with a design outdoor temperature of – 14 °C. As we have installed a condensing boiler in the example building, we are aiming for low return temperatures ≤ 55 °C so that we can utilise the condensing effect.

Calculating the radiators with different system temperatures has shown that the majority of radiators deliver sufficient output even with lower system temperatures, for example 70/50/22. In some rooms, a room temperature of 22 °C is not even necessary.

And this is exactly where your energy saving potential lies: hydronic balancing allows you to reduce the flow temperature in the heating curve and all radiators still receive the heat required to heat all rooms sufficiently. This reduces heat loss and the system works more efficiently.

We can therefore summarise the following correlation:

• Hydronic balancing ensures that each radiator receives exactly the necessary volume flow so that it delivers the required output at the intended temperature differential (20 K in our example) and flow temperature.
• We use the slope, the curve offset and the room temperature setpoint of the heating curve to determine the flow temperature that the system provides at an outside temperature.
• The heating curve can also be changed without hydronic balancing, but the full potential for optimising the heating curve cannot be exploited.

Important: You should only carry out the heating curve optimisation AFTER a successful hydronic balancing.

Setting the heating curve in the example building

We now want to set the heating curve in our building. In theory, you can also do this on a mild day. However, we need a cold day with outside temperatures below freezing to fine-tune and measure whether everything is satisfactory.

In our example building, there is a condensing boiler with internal control. To adjust the heating characteristic, we open all thermostatic heads to the highest level for the adjustment phase to ensure that all rooms are heated to the required flow temperature.

• Target values for the building: 22 °C room temperature, approx. 75 °C flow temperature at -14 °C outside temperature.
• Starting values: slope 1.5 and curve offset +10 K.

I start with the setting on the controller with the following values:

• Slope = 1.5
• Parallel shift (curve offset): +10 K
• Daytime temperature (room setpoint): 22 °C

For the fine tuning, I select a cold winter day and set all thermostatic heads to the highest level before the setting (level 4 – 5 for manual thermostats, for electronic thermostats I select a high room temperature, for example 28 °C), as would also be the case in the design condition with an outside temperature of -14 °C.

Note: All thermostats should remain fully open during the entire optimisation phase until you have found your final heating characteristic.

For fine tuning, I go through all the rooms and measure the room temperature, the flow temperature at the radiator and the outside temperature. You can measure the outside and room temperature with a digital air thermometer and the flow temperature with an infrared thermometer.

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Then write down the values as shown in Table 2:

Table 2: Example measured values for optimising the heating curve

I am satisfied with the setting of the heating curve based on the measured values. There are only two areas that deviate slightly. It is not worth adjusting the heating curve for this. Instead, you could calculate the preset values for the two rooms and adjust them slightly.

If there are deviations in a large area of your building or you are dissatisfied with the room and flow temperature, you can make the following changes to the heating curve and note them in the table.

• Too cold/warm on very cold days: slope ± 0.1.
• Too warm/too cold in mild weather: curve offset ±1 K or room setpoint ±1 °C.

Important: Wait at least 48 hours for underfloor heating and 24 hours for radiators after each change to the heating characteristic before making any further adjustments.

Fine-tuning may take some time, but it will be worth it. After all, if you have adjusted your heating curve and preset values correctly, your system will run at a lower flow temperature and all rooms in your building will be sufficiently warm at all times.

Completion of the hydronic balancing

With the eighth and final step in the“Hydronic Balancing DIY” series and the associated adjustment of the heating curve, we have successfully completed the hydronic balancing. In the previous steps, we calculated the heating load of the example building, recorded the data in the building, and used it to determine the radiator outputs, volume flows, Kv values, and radiator valve presetting. We also optimised the design and setting of the circulation pump (circulator).

Review

One more question:“Are we finished with the hydronic balancing yet?” This question can be answered with a clear NO. From now on, it is important to observe whether the assumptions and calculations we have made are successful and correct.

This means: If there are still areas that are not getting enough heat, it is necessary to make a new assumption for these and adjust the settings or the heating curve. It is also necessary to adjust the settings if flow noises or excessively high temperatures are reached in individual rooms. Another significant step is to monitor success by observing energy consumption in the building.

Hydronic balancing is only successful when the desired room temperatures are achieved in every area of a building and the system is running smoothly and energy-efficiently.

Documentation

The documentation of hydronic balancing is also an essential factor for monitoring success. It can provide a quick and easy overview of the calculated values afterwards.

Detailed documentation therefore includes entering the recorded, calculated and assumed data in tables and drawings. I have already tried to implement this procedure throughout the series.

I wish you every success with your hydronic balancing! If you have any questions, suggestions or criticism, please use the comments function.

Best regards, Martin

Further links and sources:
Wikipedia
Wolf heating technology
Bosy-Online