Vaillant Heat Pump Controls: Part 2: Room Temperature Modulation.

Friends, this is another technical article about Vaillant heat pumps. Most users control their Vaillant heat pump using a so-called SensoComfort controller that looks like this.

Click on image for a larger version. My Vaillant SensoComfortSensoComfort controller. In normal use the display is black, but the lights up when a secret button is pressed. Notice that the actual temperature exceeds the desired temperature.

Notice that the controller is displaying room temperature. It can do this because within the controller is a thermometer. From menus within the controller, a user (or their installer) can navigate to:

Settings:Installer Level:Code 00:Installation Config: Circuit 1

…and be faced with these options:

Click on image for a larger version. A screen on the SensoComfort controller.

We have discussed Heat Curves in the previous article. In this article I want to explain how the thermometer within the SensoComfort affects the operation of the heat pump. The options available can be selected under the room temp. mod. (i.e. room temperature modulation) settings menu item: Three options are available: Inactive, Active, and Expanded, and each one offers useful functionality.

Let’s look at each one in turn

Room Temperature Modulation: Inactive Mode

When the inactive menu item is selected then the control system pays no attention at all to the reading of the thermometer inside the SensoComfort controller. This situation is known as pure weather compensation.

With this setting the flow temperature is determined by the heat curve selected. As the external temperature rises and falls, the flow temperature will fall and rise in order to keep the internal temperature constant. Here’s an example from my own house from January 2023.

Click on image for a larger version. Weather compensation in action. The graph shows that as the external temperature falls, the flow temperature rises and the internal temperature stays pretty much constant. The data were taken every 2 minutes and averaged hourly during the month of January 2023.

The graph shows that as the external temperature falls, the flow temperature rises and the internal temperature stays pretty much constant. So it’s clear that Weather Compensation as a concept works pretty well.

I sampled a few flow temperatures and external temperatures from the above graph and plotted them on the Vaillant Heat Curve Chart.

Click on image for a larger version. Data from the previous graph plotted on the Vaillant Heat Curve Chart

The data seem to cluster around a Heat Curve somewhere in between the 0.6 and 0.8. I can’t exactly remember the heat curve I used, but I think it was around 0.7, which would make sense.

Weather Compensation is not perfect. After the last article a reader Duncan wrote that Weather Compensation should really be called be call Outside Temperature Compensation. This is because this mode cannot respond to increased heat loads due to windy weather, or reduced heat loads due to solar gain. Also if you assemble 10 people for a party (adding another kilowatt of heating from people’s bodies), or do a lot of cooking in the kitchen, the system will still add the heat at the same rate based solely on the outside temperature.

Room Temperature Modulation: Active Mode

When the Active menu item is selected then the control system now considers the reading of the thermometer inside the SensoComfort controller. The basic control still uses the weather compensation determined by the heat curve, but if – for some reason – the standard weather compensation algorithm is not matching the heat loss, then this will be reflected in the internal temperature. This mode allows the system to respond in order to maintain room temperature.

Suppose the room temperature measured by the SensoComfort is (say) 2 °C below the set Target Room Temperature, with Active mode enabled, the manual says that the system will add 2 °C to the Target Room Temperature. If I understand this correctly, the New Target Room Temperature is not displayed on the SensoComfort unit: it is used internally in the algorithm  to increase the flow temperature to try to match measured room temperature with the original target temperature.

It is not clear from the manual if this function works in both directions i.e. it can increase flow temperatures when the target temperature is not being achieved, but I am not sure if it can reduce flow temperatures if the temperature measured by the SensoComfort is too high.

Room Temperature Modulation: Expanded Mode

When the expanded menu item is selected then the control system watches out for over-temperature situations at the SensoComfort control, and if it detects that the actual temperature is more than 0.125 °C above the target, it stops all heating until the temperature falls 0.188 °C below the target temperature, at which point heating is resumed.

Notice the key difference between Expanded Mode and Active Mode. 

• In Active Mode, the flow temperature is adjusted to try to achieve the target temperature.
• In Expanded Mode, the entire heating flow is stopped when the target temperature is exceeded.

Summary 

All modes of control offered by Vaillant are basically weather compensation, but the Active Mode and Expanded Mode, allow for some of the shortcomings of weather compensation, or as Duncan accurately calls it, Outside Temperature Compensation.

In particular, pure weather compensation takes no account of extra heating within the dwelling (e.g. from people or cooking) or changes in the heat loss from the property (e.g. due to windiness or solar gain). In response to these shortcomings:

• Active mode allows adjustments to the flow temperature based on the actual temperature in the room.
• Expanded mode adds classic “thermostat” functionality to prevent overheating.

Now that I understand what these settings mean, I cannot think of any reason not to try some experiments and see what works in my home!

What I have learned from writing these articles – and all the thinking involved in writing them – is why Outside Temperature Compensation is a good idea.

Outside Temperature Compensation

Most people are familiar with the use of a thermostat placed in a room” It switches a heating system on or off when a set point is exceeded. Since it’s the room temperature we are concerned with, it makes perfect sense to put a thermostat in control of the heating.

But thermostat-based systems used with over-powered gas boilers typically lead to oscillating room temperatures. This occurs because when the thermostat switches on, the heating system – having no knowledge of the outside temperature – applies full power until the set temperature is exceeded.

• Typically, a boiler might have a heating power of 20 kW but the equilibrium heat loss at the target temperature might be only 5 kW. This makes temperature oscillations all but inevitable.
• In contrast, heat pumps are typically slow to ramp up to full power, but this would also inevitably lead to temperature oscillations.

The classic next step in temperature control is so-called proportional control (the P in so-called PID). In this scheme the system measures the difference between the actual temperature and the target temperature and reduces the heating power gently in proportion to this difference. In this scheme the amount of heat applied when the target temperature is reached is zero! This is clearly the wrong answer! So a proportional control system either (a) oscillates around the target temperature or (b) settles at a temperature below the target temperature.

One can add Integral (I) and Differential (D) elements to the control system, but as anyone who has ever done this can attest, it is easy to get lost, and end up with temperatures that wander all over the place. And since the time constant of houses is somewhere between many hours and many days, it can be very difficult to achieve the correct settings.

The overwhelming advantage of Outside Temperature Compensation, is that it sets the flow temperature (which is linked to the heating power) based on the dominant requirement for heat – the heat losses through the fabric of the house. And these are nearly, but not quite constant.

Used alone, Outside Temperature Compensation cannot account for extra heat sources in a dwelling, or extra heat losses caused by increased windiness. This is reason for the Room Temperature Modulation options in the Vaillant Control System.

Outside Temperature Compensation is also well-suited to the way heat pumps operate. By tracking the relatively slow changes in outside temperature,  heat pumps can operate continuously, minimising the flow temperature required for delivering heat to a home, and this maximises their efficiency.

I am not the first person to have reflected on these issues. Indeed I was struck by this quotation which is not from Winston Churchill.

“Many forms of Temperature Control have been tried, and will be tried in this world of sin and woe. No one pretends that Outside-Temperature Control is perfect or all-wise. Indeed it has been said that Outside-Temperature Control is the worst form of Temperature Control, except for all those other forms that have been tried from time to time.…But Outside-Temperature Control with Room-Temperature Modulation options is not bad.’

NOT Winston S Churchill, 11 November 1947

 

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Edit: 18 Oct 2024: Unbelievably I got the name of the temperature controller wrong! It’s a SensoComfort not a SensoTherm!

Edit: 1 Dec 2024: Unbelievably – again – I got the name of the expanded mode wrong: I called it extended. Doh!