Last Updated 1 year by Lukas
In this article I would like to cover ways to improve an existing heat pump system. I’ll be doing this using a system installed in my own home. This means that I can give you tested and practical experience of tuning the system. You can then judge for yourself if it makes sense to try to replicate similar changes. The original description of Passive House systems was covered in: Introduction, Solar System description, Recuperation and heat pump system description and finally inside real consumption numbers posts.
What are we going to discuss?
- What are the basic terms you should understand to properly fine-tune heat pump system
- What has been changed from default settings inside service menu
- We will also evaluate these changes together and try to assess if it makes any sense
What are the basic terms good to know?
In the following section, I’ll provide you with references to nice explanations of basic terms that are good to know before trying to fine-tune the heat pump. These will be principles on which heat pump works, how to read and what are p-h diagrams for cooling/heating medium inside heat pump. And a bit about controllers, in our case it will be PI type. The last bit of information will be about params/terms that we can read on the heat pump itself.
How heat pump works?
What are p-h diagrams and how to read them?
What is PID regulator?
What params can we see on the heat pump unit itself?
- We can see
- Manufacturer – Sinclair
- Unit Type – ASGE-12BI
- Rated voltage – 220-240 V
- Rated current – 6 A
- Climate Type – T1 (Describes type of environment this unit is supposed to work)
- Refrigerant Type – R-32
- Maximum operating pressure on Discharge / Suction side – 4.6 and 2.5 MPa
- Maximum operating pressure – 4.6 MPa (This corresponds with critical point of refrigerant)
- Usually technicians also mention how much they added of regrigerant (It’s good practice)
You can find more parameters inside manufacturer page.
In general, this heat pump outdoor unit has a maximum power that it can draw from the grid. In the case of the ASGE-12BI it’s 1 kW max. And based on the power of the compressor and the size of the evaporator, it has some maximum heating/cooling power. In the case of this unit, it’s maximum cooling 3.5 kW and heating 4 kW. Very simply said, you have to provide heat pump some energy from outside (from grid up to max 1 kW), heat pump will absorb energy from environment and multiply like this output power. This is usually called COP (coefficient of performance). So (in an ideal situation) you give 1 kW of energy (electricity), but on the other side you get 4 kW of energy (heat).
As many modern units include an ‘inverter’, you can usually regulate the output between 20% and 100%. The COP of a heat pump is highly dependent on the temperature difference between the evaporator and the condenser. In other words, if it is 20 degrees outside, you can absorb more energy than if it is -5 degrees. The energy that can’t be absorbed from the outside is provided by the compressor. If you want to heat water to 50 degrees and it’s cold outside, the heat pump can’t take much energy from the outside. The remaining work to heat the medium to 50 degrees is done by the compressor.
Changes inside system defaults from Atrea
In this part of the article we will describe the parameters we need to play with. I’ll also mention a reference to a nice article (Czech) about tuning the regulator from the graph. Let’s start with the description of the service menu that we will change, followed by the description of the menu for monitoring the behaviour of the system. There we will observe how good our regulation is and based on the graph we can fine tune it.
Menu Heat. Water Storage -> Heat Pump set.
- 9.3.1 and 9.3.2 indicate whether the temperature is lower than bivalence 1 (using the heat pump and electric heater for heating) or bivalence 2 (using only the electric heater for heating). Please note that if you change these parameters without fine-tuning 9.3.8 and 9.3.9, you will force the compressor to work more -> it will reduce efficiency and wear the compressor more.
- 9.3.3 – Is used to tell the system if it should stop ‘cooling’ and start heating with heat pump if there is a request for this. The Comfort option will ignore the request for heating if the heat pump is already cooling and will use the electric cartrige instead.
- 9.3.5 – How much time the system waits before checking if the heat pump has reached the expected performance. This parameter must be fine-tuned if you want to change the control constants. Otherwise, your installation will give false alarms about the low performance of the heat pump.
- 9.3.7 – It’s the minimum temperature for the heat pump to start. Because the 425 litre water tank works as a split unit for the heat pump outside the unit.
- 9.3.8 – Maximum temperature at which the heat pump is allowed to work (heating water).
- 9.3.9 – What is the temperature of the medium (R-32) used to heat the water.
- 9.3.8 + 9.3.9 – are together number for which to regulate temperature of heating medium (R-32).
- 9.3.10 to 9.3.12 – are constants used within the control equation
- 9.3.21 – Is control voltage in case the outside unit reports that it is doing defrost. Note that if you send 0V, it will cancel the defrost. Make sure it is ideally 5V for Sinclair outdoor unit.
Important parameters we will be tuning
- 9.3.1 and 9.3.2 can be fine tuned in two ways
- You set them around -6 and -10 and that’s end of fine tuning
- You set them around -10 and -15 and you will continue with fine tuning maximum temperature of refrigerant
- Be aware: If you opt for point two and keep all remaining values the same -> You are forcing compressor to work more -> It will break very likely sooner
- Be aware: Optimal numbers are based on used refrigerant (Here is used R-32)
- 9.3.5 – In case you change regulator parameters (described later) you may have to change this. It set time until when control unit of Atrea regulation will wait until evaluating if heat pump refrigerant reached sufficient temperature. If no it will raise alert and it may also try to use heating cartrige to support heat pump.
- 9.3.8 and 9.3.9 – are providing in total temperature onto which to regulate temperature of refrigerant
- 9.3.10 and 9.3.11 – are constants of regulation equation. They are used to increase influence of Proportional and Integral part of regulator.
- 9.3.12 – Says how often regulator re-evaluates it’s output
The output of the regulator is a voltage between 0 and 10 V. This is transferred to the SCMI 1.04 that controls the external heat pump unit. In simple terms, 0V means 0% output and 10V means 100% output. Atrea’s PI controller takes as input the measured temperature of the refrigerant. It compares it with the value obtained by adding 9.3.8 and 9.3.9 together. And based on the difference between the measured value and the expected value, it provides an output in the form of voltage. Simply put, if the temperature is too low, it increases the heat pump’s output and vice versa.
How can we see the regulation proces and tune it?
Without good monitoring tools and the ability to graph their changes over time. It would be very difficult to fine tune anything. Fortunately, the Atrea control system provides the perfect solution and it’s called Data Log. It stores all the important parameters in memory and allows you to plot them on the graph over time. You can even select only the parameters you are interested in.
- On the picture above we are selecting these inputs (values going into the system from outside)
- TEa Input – Air from outside temperature
- TEb Input – Air from inside temperature
- T1 – temperature in 2/3 of 425 l water tank (from bottom)
- T2 – temperature in 1/3 of 425 l water tank (from bottom)
- T5 – temperature of refrigerant (Vapor phase)
- On the picture above we are selecting also these outputs (values going out of the system)
- SA2 – Output of regulator in [V] for mixing valve. It’s used when system is heating inside through recuperation unit
- DA1 – Output of regulator for heat pump in [V]. We can influence this number by tuning regulator params
- R1no – Output that will turn on electric, heating cartrige
- On the picture above in upper part we see temperatures for input values
- On the picture above in lower part we see voltage regulator voltage output of heating and heat pump compressor power
- Two most important things on the picture are temperature T5 on the upper part and voltage output D1 on the lower part.
- In general we want PI regulator to reach expected temperature of refrigerant and hold it.
- On the picture above is detail when regulator reached expected state. The output at point is 3,25 V (Heat pump inverter goes around 32% of maximum power approx. 400 W). And temperature of refrigerant is around 46 degrees
- Of course this is real system. Hence heat pump can for example start freezing. And regulator would have to handle that and return system again into equlibrium ideally precisely at 46 degrees (9.3.8 + 9.3.9)
How can I tune regulator from graph?
If you read throught this article (in Czech). There you will find pictures of what the output of the controller looks like when it is set correctly or incorrectly. Based on these pictures, we can either increase/decrease the P or I constants of the regulator and by trial and error reach a satisfactory result after a while. You can use the parameters from the picture above and fine tune them for your current system. Please note that in my case it’s for an ASGE-12BI unit with R-32 refrigerant.
Why should I tune regulator of heat pump?
When your regulator is fine-tuned and can reach and maintain the expected refrigerant temperature. It will save energy and prolong the life of your compressor as it won’t just turn on and off. This is because an improperly set regulator can reach its limit pressure (temperature) and the heat pump will stop itself to prevent damage. And this is repeated over and over again until the heat pump is heating water. It’s much better if, for example, your heat pump can start, reach the expected temperature and maintain it until the water is heated to the expected temperature (refrigerant transfers energy to the water in the 425 litre water tank).
Here is very nice tool that can graph p-h diagram based on params you put into it. How can we get these params? Let’s measure working heat pump in steady state (When it reaches expected temperature and it’s holding it)
In the picture above we measure a larger pipe, inside which we have refrigerant in vapour phase with a temperature of 46 degrees. On the picture below we will measure the same inside a smaller pipe, where the refrigerant is in liquid phase and we will see 29 degrees.
OK, so we know that the vapour phase is 46 degrees and the liquid phase comes back into the heat pump at 29 degrees. What is this good for? We can plot this on a p-h diagram and calculate the approximate COP. And if we also know the maximum power of the heat pump (1 kW) and the control voltage (i.e. how much compressor is running). Then we can also calculate the heating/cooling capacity.
For example, if the compressor is running at 45%, it will consume about 500 W of energy from the outside. And if the COP is 4, it means that the heat pump provides in water heating power of 4 * 500 = 2 000 W. For our calculation to be possible, we need to know two values Condesation temperature (46 degrees, we measured it). And we need to know the temperature of the outside air (which is the temperature of the evaporator. It will tell us how much the heat pump can take from the outside environment).
And what is the 29 degree temperature we measured good for? Refrigerant vapour of 46 degrees enters the condenser inside the 425 litre water tank (inside is just a long pipe that spirals through the tank from top to bottom). As the refrigerant passes through, it loses it’s energy until it exits and returns to the heat pump. This returning liquid has a temperature of 29 degrees (it is actually a mixture of liquid and vapour).
This is called subcooling. We’ve cooled the refrigerant more than is needed to turn it into liquid. This sub-cooling helps us to get a higher quality liquid (actually it’s a mixture of vapour and liquid, so we want to get as much liquid as possible. As the liquid evaporates it takes energy from the outside environment). On the graph below this is represented by blue lines within a grey area. These represent the so-called quality. In other words, how much percentage of liquid and vapour is in the mixture.
Of course, we can’t go too low with the subcooling, because we would start cooling our water tank instead of heating it. So we try to find the sweet spot to just maximise the COP. The picture below shows an example of a p-h diagram based on measured values.
We see a nice COP of about 6.6 for heating and 5.6 for cooling. In reality, the heat pump would work with a consumption of 300-400 W. The real heating power would be at most around 2,600 W. To get better results, it would be necessary to use a more powerful heat pump that has a bigger evaporator and can extract more energy from the environment. We assume a compressor efficiency of 80% (how much energy it converts into pressure, the rest are losses in the form of heat).
It would be nice if Atrea could also measure subcooling and automatically fine tune the PI controller parameters. Unfortunately, this feature is not yet supported. Let’s hope it will be one day. It’s not so much a problem of missing inputs. You can connect an extra temperature sensor. But the Atrea controller won’t be able to use it for self-tuning (as of 04/04/2023).
Can we anyhow verify if our heat pump really delivers expected heating power?
Yes, we can. If we know how long our heat pump has been running. And we know the temperature change, and we also know how much water is in the water tank. We can simply work backwards to calculate how much power is needed to raise the temperature of that amount of water. And if the results are the same, we know that our heat pump has indeed delivered the expected heating output. Let me show you on the picture below.
It’s important to select the part of the graph where only the heat pump was running. Do not select parts where the system is heating and the heat pump is also running. This won’t allow you to base the measurement on known conditions. This is because if the system is also heating air, it is removing energy in between when the heat pump is adding it. This makes it impossible for you to measure the heat pump’s performance in isolation.
I used calculator available here. In other words, we are asking. When the water temperature rises from 30 degrees to 40 degrees. There are 425 litres of water in the tank. Heating is done by electricity (so no real losses). And the heat pump was able to heat it in two hours. It’s calculated that the heat pump should deliver 2.5 kW per hour. And it delivered a total of 5 kWh of heating power.