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Advantages of Variable Frequency Heat Pumps over Fixed Output Single Speed

Advantages of Variable Frequency Heat Pumps over Fixed Output Single Speed

May 7,2022
Deciding to install a heat pump is a big decision for a homeowner. Replacing traditional fossil fuel heating systems (like gas boilers) with renewable alternatives is a system that people spend a lot of time researching before committing to it.

People will want to know things like what it's like to use a regenerative heating system, how much it will cost to install and run, and whether to install an air source heat pump or a ground source heat pump.

When conducting research, the question of installing a heat pump with an inverter drive unit or fixed output may come up. And there is a lot of data to confirm that the variable frequency heat pump has significant advantages in the following aspects:

Higher annual overall energy efficiency

Less likely to have problems with grid connection

Space requirements

The service life of the heat pump

Overall comfort

But what is it about inverter heat pumps that make them the heat pump of choice? In this article, we will explain in detail how they differ from the two units of fixed output heat pumps and why they are the units of our choice.
What is the difference between the two heat pumps?

The difference between fixed output and inverter heat pumps is how they provide the required energy from the heat pump to meet the heating needs of the property.

Fixed output heat pumps work by turning on or off continuously. When switched on, the fixed output heat pump operates at 100% capacity to meet the heating needs of the property. It will continue to do so until the heat demand is met, and will then cycle between heating the large buffer on and off to keep the desired temperature in balance.

However, an inverter heat pump uses a variable speed compressor whose output can be adjusted to increase or decrease its speed to precisely match the building's heat needs as the outdoor air temperature changes.

When demand is low, the heat pump reduces its output, limiting power usage and consumption of heat pump components, thereby limiting start-up cycles. 

The importance of correctly sizing your heat pump

Essentially, the output of the heat pump system and how it delivers its capacity are at the heart of the inverter vs. fixed output debate. To understand and appreciate the performance benefits offered by an inverter heat pump, it is important to understand the size of the heat pump.

To determine the size of the heat pump needed, the heat pump system designer calculates how much heat is lost from the property and how much energy the heat pump needs to replace the heat lost through fabrics in the building or through ventilation losses. Using measurements taken from the property, engineers can determine the property's heat needs at an outside temperature of -3℃. This value is calculated in kilowatts, and it is this calculation that determines the size of the heat pump.

For example, if the calculation determines the heat demand to be 15kW, it will be nominally -3℃according to BS EN 12831 and the expected minimum temperature for the area.

The size of the heat pump is important in the inverter vs. fixed output heat pump debate because when a fixed output unit is installed, it will operate at maximum capacity when turned on, regardless of the outside temperature. This is an inefficient use of energy, as 15kW at -3℃ may only require 10 kW at 2℃. There will be more start-stop cycles.

However, the inverter drive unit regulates its output between 30% and 100% of its maximum capacity. If heat losses from the property determine that a 15kW heat pump is required, install a 5kW to 15kW inverter heat pump. This means that when the property's heat demand is at its lowest level, the heat pump will operate at 30% of its maximum capacity (5kW), rather than the 15kW used by the fixed output unit. 
The inverter drive unit provides higher efficiency

Both fixed output heat pumps and variable frequency heat pumps offer higher levels of energy efficiency than conventional fossil fuel-burning heating systems.

A well-designed heat pump system will provide a coefficient of performance (CoP) between 3 and 5 (depending on ASHP or GSHP). For every 1kW of electrical energy used to power the heat pump, it will return 3-5kW of thermal energy. The average efficiency of a natural gas boiler is around 90-95%. Compared to burning fossil fuels for heating, heat pumps will provide approximately 300% more efficiency.

To get the most efficiency out of a heat pump, homeowners are advised to keep the heat pump running continuously in the background. Leaving the heat pump on will maintain a stable continuous temperature within the property, reducing "peak" heating demand, which is best for inverter units. 

The inverter heat pump will continuously adjust its output in the background to provide a consistent temperature. It reacts to changes in heat demand to ensure temperature fluctuations are kept to a minimum. Whereas a fixed output heat pump will cycle continuously between maximum capacity and zero, finding the right balance to provide the temperature required for more frequent cycles.

Reduce wear and tear with inverter units

With a fixed output unit, cycling between on and off and running at maximum capacity will not only stress the heat pump unit but also the power supply network. A surge is created at each startup cycle. This can be reduced by using soft-start but is prone to failure after only a few years of operation.

When the fixed output heat pump cycle starts, the heat pump will absorb the current surge to make it start. This puts the mechanical components of the power supply and heat pump under stress - and the process of cycling on/off occurs multiple times a day to meet the property's heat loss needs.

The inverter unit, on the other hand, uses a brushless DC compressor and has no real startup spikes during the startup cycle. The heat pump starts with a zero amp starting current and continues to build until it reaches the capacity needed to meet the building's needs. This puts less stress on the heat pump unit and power supply while being easier and smoother to control than an on/off unit. Often, multiple start/stop units are connected to the grid, which can cause problems and grid providers may refuse connections without network upgrades.
Save money and space

One of the other attractive aspects of installing an inverter drive unit is that it saves money and space requirements by eliminating the need to install a buffer tank, or if using floor heating full zone control, the buffer tank can be much smaller.

When a fixed output unit is installed into a property, space is required to install a buffer tank next to it, approximately 15 liters per 1kW of heat pump capacity. The purpose of the buffer tank is to store preheated water in the system to circulate around the central heating system as needed, limiting on/off cycles.

For example, let's say you have a spare room in your home that you rarely use, and that room's temperature is set to be lower than other rooms in the house. But now you want to use that room and decide to turn on the thermostat. You adjust the temperature, but now the heating system has to meet the new heat demand for that room.

We know that a fixed output heat pump can only run at maximum capacity, so it will start working at maximum capacity to meet a fraction of the actual maximum heat demand-wasting a lot of electricity. To bypass this, the buffer tank sends the preheated water to the radiator or floor heating in the spare room and uses the maximum output of the heat pump to reheat the buffer tank which may cause the buffer to overheat the tank for the next call in the process. It's ready.

With an inverter drive unit installed, the heat pump will adjust itself to lower output in the background, recognizing changes in demand and adjusting its output based on low changes in water temperature. Therefore, this feature allows owners to save money and the space required to install large buffer tanks.

The above describes in detail the advantages of variable frequency heat pumps over fixed output single speed. If you plan to buy a DC variable frequency heat pump, please contact us.

SUOHER is a professional custom heat pump manufacturer and supplier, we have a team with rich experience in heat pump design, processing, quality control, and controller function development. Our target is global, including Europe, South America, South Africa, and Asia.

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