Introduction

While not the most popular brand in the UK, Swedish company NIBE have a reputation for high quality but expensive Ground and Air Source Heat Pumps.

Any heat pump is at its most efficient when producing relatively low-temperature water so a lot of the control strategies aim to deliver that, which can be surprising for people used to fossil fuel boilers and heating systems which ‘call for heat’.

Weather Compensation and Degree Minutes

The key part of any Weather Compensation control system is the WC ‘curve’. Effectively, this a crude model for how much heat a building loses – expressed in terms of what Flow Temperature is required in a heating system to offset the building’s Heat Loss at different external temperatures.

The WC curve is used to look-up the Target Flow Temperature (what NIBE call the ‘Calculated’ temperature, with code S1) for a given external temperature – then the heat pump can try to generate a Flow Temperature equal to the Target Flow Temperature.

The “try to” is the important bit here – for various reasons it will never match exactly:

• On an older-style heat pump with a fixed-speed compressor (which is only ever On or Off) it will always overshoot the Target Flow Temperature, then undershoot, then overshoot etc.
• On a newer-style heat pump with a variable-speed compressor (which can vary its output – but only within certain limits) there’s a much better chance of matching the Target, but an ASHP needs to run periodic Defrost cycles (and most heat pumps need to run Hot Water cycles) and whenever it’s not actively suppling heat to the heating system the Flow Temperature (in the Heating Circuit) will drop below the Target Flow Temperature, which needs to be compensated for by running above the Target for a bit.

To address this, NIBE systems constantly compare the Actual Flow Temperature with the Target Flow Temperature. Let’s say the Target is 30C and the Actual is 28C; that’s a ‘deficit’ of 2 degrees, and if that situation continues for 3 minutes that counts as a deficit of 6 Degree Minutes (DMs). The NIBE control system aims for the DM value to be zero – i.e. for the Actual Flow Temperature to be the same as the Target Flow Temperature on average over time.

The control system behaviour is as follows:

• With the compressor off, the Actual Flow Temperature will typically be lower than the Target, so the DM value will become increasingly negative.
• When the DM value reaches the setting configured in Menu TBC, the compressor switches on.
• With the compressor on, the Actual Flow Temperature will be higher than the Target, so the DM value starts increasing (i.e. becoming less-negative).
• When the DM value gets to zero, the compressor switches off.
• There will be some residual heat in the heat exchangers so the DM value will continue increasing for a while, sending the DM value positive.
• The DM value is capped at +100. It is not allowed to increase above that, no matter what happens to the Actual and Target Flow Temperatures.

Setting a larger (i.e. more-negative) value in Menu TBC has the effect of making the heat pump wait longer before starting the compressor so that when the compressor does start it will tend to run for longer. This will lead to fewer compressor starts (which is better for compressor longevity) but also tends to lead to larger swings in internal temperature (depending on thermal mass within the building).

Indoor Temperature Sensor (BT50)

One problem with the WC ‘curve’ is that by default, it only responds to the external temperature. This means it is not able to take account of:

• Factors which might increase the building’s heat loss, such as strong winds
• Factors which might lead to internal heat gains from other sources than the heating system, such as passive solar gain or lots of baking

An enhancement is to add an internal temperature sensor which can indicate when the internal temperature is getting a bit high or a bit low and adjust the WC parameters accordingly.

NIBE Heatpump Control System Notes by Marsh Flatts Farm Self Build Diary is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.