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Inverter Heat Pumps vs. Fixed Output Single Speed: The Finer Details

As the exclusive supplier of MasterTherm heat pumps in the United Kingdom, Thermal Earth is a huge advocate of the Inverter technology used in their range of air source and ground source heat pumps.

We’ve overseen the design and installation of thousands of heat pump systems in over a decade in the renewable heating industry. In our experience, the advantages that are offered by inverter drive air source and ground source heat pumps far outweigh those offered by a fixed output unit.

 

 

In a previous blog, we looked at what inverter units are and how they compare to fixed output heat pumps. In this article, we’re going to delve a little deeper into the finer details of the inverter vs. fixed output debate. We’ll explore 10 different focus points, covering: real-life performance, temperature control, reliability, and much more.

We begin with performance.

 

How do inverter units compare to fixed output in real world conditions?

Under testing, the inverter drive units yield significantly better results in terms of efficiency and cost-effectiveness. When compared against the EN 14825 standard, the inverter drive units – ground source, especially – consistently outperform fixed output units.

But how do interver units perform in real life conditions?

Taking an average over the course of a calendar year, performance data from an inverter drive unit shows that the heat pump is only working at a partial load for the majority of the time. On average, the heat pump is operating at a capacity of less than 50% of its peak output.

At less than 50% output, an inverter heat pump will significantly reduce the load placed on the ground loop, allowing for higher ground loop temperatures and increasing the efficiency of the system as a whole.

 

Efficient performance across the range

Inverter units are designed to work efficiently across a wide capacity range (measured in revolutions per second (RPS)) and pressure ratios.

The typical range of an inverter compressor is between 15rps and 100rps. The performance of the unit will start to reduce slightly above 65-70rps, but on average the unit operates between 30 – 70rps during the heating season. When looking at data for the year, it shows a far greater level of efficiency.

When looking at the hot water an inverter heat pump produces, the unit can modulate its output to reach the higher water temperatures. Up to 60°C of water temperature can be achieved with a compressor capacity between 40 and 80rps.

 

Do fixed output units have a longer product life?

The compressors used in fixed output and inverter drive units are very similar in how they are made and their functionality. The lifetime of a compressor is based not on how long they operate for, but in the amount of ON/OFF cycles they experience. A heat pump compressor typically has a lifespan of 200,000 ON/OFF cycles.

The Inverter drive is constantly modulating its output between two ranges, adjusting itself to the required heat demand. When there is a higher heat demand, the inverter unit will slowly raise its output to the amount needed. This approach exerts less force on the unit than the typical ON/OFF cycles in a fixed output unit. An Inverter drive unit has a much smoother start with a much lower load.

 

Are Inverter drive units more prone to fault than fixed output units?

The heat pump industry has been around a lot longer than people think. And with that experience, air source and ground source heat pumps have come on in leaps and bounds in terms of their functionality and their reliability. A typical heat pump, with good maintenance and regular servicing, has a lifespan of between 20 and 30 years.

But as with any electronic technology, some components do have a lifespan. An inverter heat pump does make use of more sophisticated technology than a fixed output unit, but this doesn’t make them less reliable. The microcontrollers and softstarters that can occasionally generate faults in inverters are also present in fixed output units.

With the heat pump industry rapidly developing, getting a hold of spare parts is no longer the issue it would have been 20 years ago. Most European heat pump manufacturers are well stocked with spare parts and are thorough in their servicing. Should an inverter heat pump develop a fault, fixing it needn’t be costly or troublesome.

 

Are the refrigerants that inverter drive units use environmentally friendly?

Inverter drive units use the R410a refrigerant. Some in the industry have claimed that this refrigerant is not environmentally friendly because it has a higher Global Warming Potential (GWP) per kg than other refrigerants. Delving deeper into the data, however, reveals a slightly different perspective.

The GWP of the R410a refrigerant sits at 2088 per kg. Compared to the R407c (1774) and the R134a (1430), it does sit at the higher end of the scale. However, the refrigerant charge used for a typical 10kW unit is 1.5kg for the R410a, whereas for R134a the typical charge is 2kg. The GWP total between the 2 units would be almost exactly the same.

The volume of refrigerant varies between different units and capacities. A 45kW inverter unit would typically have a unit charge of 3.5kg of R410a. To achieve the same output with an R134a refrigerant, approximately 8 – 10kg of charge would be needed.

More recently, inverter units are available with a new refrigerant R32, the GWP is 675 per kg. Although it is mildly flammable, is has 3-5% better performance and no installation restrictions up to 1.8kg, typically around 15kW capacity.

 

A deeper look into the mechanics of an inverter heat pump

For Inverter heat pump units up to 20kW peak capacity, the variable speed compressor found inside the unit is typically a twin rolling piston.

A twin rolling piston is preferred because of the efficiencies it offers compared to the scroll system. The twin rotary works with a continuous pressure ratio on the system, which produces excellent levels of efficiency at part load conditions (at which a ground source heat pump operates for the majority of the time). By deploying a twin rotary in the smaller units, the inefficiencies that can be found with the more complex scroll system in a more compact unit are negated.

 

There’s more to efficiency than electricity consumption

One of the clear and obvious benefits of an Inverter drive unit is that they modulate their output to meet the desired heat demand, using less electricity in the process. But there’s more going on beneath the surface that makes them so efficient.

The heat exchangers in the Inverter unit are designed to work at a maximum load. This is a necessity to functionality of the unit, because there will be times where the unit is going to have to work at maximum capacity to meet the heat demand.

Because this demand may be called upon, the heat exchangers of the unit (the evaporator and condenser) are designed to work comfortably and efficiently at maximum load. When the capacity is reduced and they are not working to maximum capacity, they offer significantly better performance thanks to their robust design.

 

Are inverter drive units more prone to fault than fixed output units?

The heat pump industry has been around a lot longer than people think. And with that experience, air source and ground source heat pumps have come on in leaps and bounds in terms what they are capable of and their reliability.

A typical heat pump, with good maintenance and regular servicing, has a lifespan of between 20 and 30 years.

But as with any electronic technology, some components do have a shelflife, and one of the common arguments held against Inverter units is that because they use more complex components that they are less reliable.

An Inverter heat pump does make use of more sophisticated technology than a fixed output unit, but this doesn’t make them less reliable than a fixed output unit. The microcontrollers and softstarters found in Inverters that can occasionally generate faults are also present in fixed output units.

With the heat pump industry rapidly developing, getting spare parts for units is no longer the issue it was 20 years ago. Most European heat pump manufacturers are thorough in their servicing and well stocked with spares. So should an inverter heat pump develop a fault, fixing it needn’t be troublesome or costly.

 

Do inverter units give you greater control over room temperature?

With an inverter drive unit, you can more accurately control the indoor temperature of the house without experiencing spikes in temperature.

A room terminal is used to provide the heat pump controller with realtime information about the requested room temperature and the actual room temperature. Using this information, the capacity is adjusted to reach the requested temperature.

 

Does an inverter heat pump require a larger ground loop?

The size of the ground loop is ultimately determined by the heating energy conmsuption of the house over the year. Whether the system is an inverter or a fixed speed unit has no bearing over the ground loop sizing. Due to the more linear heat extraction from the ground according to the building heat demand, the heat transfer efficiency of the ground loop is much higher, resulting in overall higher seasonal efficiency.

 

Do inverter units cause disruption to other electrical appliances?

One of the claims made against the installation of inverter heat pumps is that they generate harmonics, which cause disruption and interference across other electrical appliances connected locally.

To an extent, this is correct. But it is also correct for fixed speed compressors with a low power factor. The peak power loading caused by frequent ON/OFF cycling causes a flickering load on the power supply line.

With inverter drives, very strict standards are applied and must be met to minimise the effect of harmonics on the power supply. Complying fully to these standards means that the effect of an inverter compressor on a public power supply will be miniscule. To date, with the advanced controls and harmonics/flicker compliance we have not had a connection declined by a DNO.

 

Interested in converting to a heat pump system? You can find out more information on heat pumps by visiting our website. Or, if you’d like to speak to us directly, click here to get in touch with a member of our team.