Heating Curve

Heating and cooling curve explained simply

This block automatically calculates the correct flow temperature (the water temperature sent to the mixer) – matched to the current outside temperature. The basic idea: the colder it is outside, the more heating is needed; the hotter it is outside, the more cooling. Nobody has to readjust by hand.

• Heating: cold outside → flow warmer than the room setpoint.
• Cooling: hot outside → flow colder than the room setpoint. The cooling curve is the mirror image of the heating curve.

At around 20 °C outside temperature the flow is exactly at the room setpoint (neutral point). Only below it (heating) or above it (cooling) does it deviate.

1. Slope (input „Heating curve“ or „Cooling curve“)

The slope determines how strongly the flow reacts to a change of the outside temperature – comparable to a sensitivity or gain. A larger value makes the curve steeper, a smaller one flatter.

Heating example – room setpoint 22 °C, at +5 °C outside temperature:

Same outside temperature, but the larger the slope, the warmer the flow. For cooling it is the other way round: a larger slope → colder flow at the same outside temperature (more cooling).

Important: The slope always has to be entered as a positive value – for the cooling curve as well. The block itself takes care of reversing heating/cooling. A negative value would mirror the curve and cause exactly the opposite. The controller therefore does not permit negative values: it continues to calculate with the absolute value and issues an error message with block and value in the diagnostics (program). The value itself is not changed – it has to be corrected at the block.

Tip: Usual values are small (heating approx. 0.2–1.5, cooling approx. 0.1–0.3). A very large value (e.g. 5) makes the flow immediately hit the min/max limit, so it then seems as if „nothing changes anymore“.

2. Parallel shift (inputs „Heating/Cooling parallel shift“)

The parallel shift raises or lowers the entire curve by a fixed amount – independent of the outside temperature. It is simply added to the calculated flow temperature.

Important: it works the same in both operating modes (it is not inverted):

• Positive value (e.g. +2) → flow always 2 °C higher → warmer rooms (more heating when heating, less cooling when cooling).
• Negative value (e.g. -2) → flow always 2 °C lower → cooler rooms.

So consistently: „+ = warmer, - = colder“. This corresponds to the „level“ setting of common heat pump controllers.

Use: Are the rooms uniformly – i.e. at every outside temperature – a little too cool? Then increase the parallel shift instead of changing the slope. Rule of thumb: approx. 3 °C of flow produce roughly 1 °C in the room.

3. Flow minimum / maximum

These parameters limit the result at the bottom and top (protective limits):

• Heating flow maximum: protects e.g. underfloor heating from water that is too hot (typically 35–40 °C).
• Cooling flow minimum: protects against condensation (dew point). If the cooling water gets too cold, pipes, floor or walls start to „sweat“. Surface-cooling example: set the minimum to approx. 18 °C so nothing fogs up. Depending on humidity the safe value can be higher – for full protection additionally evaluate the „Dew point“ block.

Cooling calculation example

In hot weather the cooling curve lowers the flow below the room setpoint. It would be wrong if the flow were above the room setpoint in the heat – that would heat instead of cool. The following example shows the calculation step by step.

Given values:

Calculation:

With room sensor influence: If the cooling room sensor influence were additionally set to 50 % and the room were 2 K too warm at 24 °C, the deviation setpoint − actual = 22 − 24 = −2 results in a correction of −2 / 100 × 50 = −1 K. The flow therefore drops to 17.6 °C and is limited to 18.0 °C by the cooling minimum – so the dew point protection takes effect before the control does.

The same calculation for other slopes (room setpoint 22 °C, without parallel shift, cooling minimum 18 °C):

Important: The cooling curve has to be set considerably flatter than the heating curve – usual values are approx. 0.1–0.3. The usable range between room setpoint and dew point is only a few kelvin; with larger slopes the flow immediately hits the cooling minimum and the curve then acts like a fixed temperature.

Which values apply for unconnected inputs?

Unconnected inputs are not automatically treated as 0 – the block uses reasonable substitute values. The following overview applies to both blocks (heating curve and heating/cooling curve).

Room setpoint (RS) at 0 or not connected

• Not connected: the calculation uses 22 °C.
• Connected, value 0 or negative: the calculation uses 20 °C. With a setpoint of 0 °C the curve could not be calculated sensibly, therefore the value is replaced.

The room setpoint has a double effect: it is the starting point of the curve (flow at 20 °C outside temperature) and it also enters the curvature factor. A typical case from practice: after a restart the setpoint (e.g. a KNX address) has no value yet and is therefore 0 – the calculation then uses 20 °C instead of the desired 22 °C, so the flow is a few kelvin too low until the setpoint arrives. Have the setpoint sent actively after the start.

Room temperature (RT) at 0 or not connected

• There is no substitute value here: the calculation uses 0 °C – for an unconnected input just as for a connected value of 0 (e.g. a failed sensor).
• As long as the parameter room sensor influence is 0 % (factory setting), the room temperature is not evaluated at all – a missing value then has no effect.
• If the room sensor influence is greater than 0, the value 0 °C is interpreted as an extremely cold room. With a room setpoint of 22 °C this gives a deviation of 22 K, i.e. a correction of +11 K on the flow at 50 % influence:
  • Heating: the flow runs into the flow maximum – it stays permanently too warm.
  • Cooling: the correction raises the flow, so there is practically no cooling any more (the flow runs into the cooling flow maximum).

Rule: Only set a room sensor influence greater than 0 if the room temperature input is actually connected and supplies a plausible value. Otherwise leave the room sensor influence at 0 and additionally cover a sensor failure via the enable input.

The flow does not get colder as the outside temperature rises

If the flow stays constant in cooling mode or even rises, it is almost always due to one of the following points:

1. Slope entered as a negative value. The most frequent cause: a negative value is entered for cooling mode. However, the block reverses the curve itself. Current controllers therefore calculate with the absolute value and report the value as an error in the diagnostics (program) – older versions mirrored the curve, so there the flow rose with the outside temperature and got stuck at the cooling flow maximum. Always enter the cooling curve as a positive value (e.g. 0.2).
2. Limit reached. If the result is at the cooling flow minimum (dew point protection) or at the cooling flow maximum, the output no longer changes despite a rising outside temperature. If the flow reaches the minimum too early, reduce the slope.
3. Cooling enable missing. Without a signal at input FK the heating curve is calculated. It is limited at the bottom by the heating flow minimum and stays there.
4. Room sensor influence without room temperature. If the room sensor influence is greater than 0 but the input RT is not connected (i.e. 0 °C), the correction raises the flow considerably – it stays at the cooling flow maximum.
5. Outside temperature does not change. Check the value at input AT. If no new measured value arrives, the block keeps calculating with the last value received.

Try out the curve

The representation below uses the same formula as the controller. The sliders show how slope, parallel shift, limits and room sensor influence take effect. The solid line is the value at output VS, the dashed line shows the calculation before the limitation by minimum and maximum. The other operating mode is drawn faintly alongside.

Heating curve block

Heating/Cooling Curve block