Why I Chose an Air-to-Air Heat Pump
Earlier this year, a friend asked me to help with their heating problem. Their system was expensive, inefficient, and irritating. They were relying on old electric resistive radiators, which heat a room by passing electricity through a wire that gets hot and warms the surrounding air. While this method is technically 100% efficient—since all the electricity converts to heat—it is costly and an impractical way to heat a space, at about three times the cost of a mains gas heating system.
Since they weren’t on the gas grid, gas was not an option. Many people in this situation might consider a propane or oil-based system, but she had no traditional water-based radiators, so it would have been absurd to fit a wet heating system just to heat her home with expensive propane or add an oil boiler.
One option we considered was a wet heat pump system, which would qualify for the £7,500 government grant. However, after receiving a quote of £15,000 (after the grant) for installation, we decided a different approach was necessary—an approach that, unfortunately, receives no government funding and little attention.
My solution was an air-to-air heat pump. This type of system skips the step of transferring heat to radiators, which would require internal plumbing. Instead, it uses a direct link between the heat pump and a wall cassette to send heat into the home. A cassette is essentially a radiator with a fan, but instead of circulating water, it has refrigerant running through it.
The refrigerant for this heat pump is R32, Difluoromethane. I looked into getting an air-to-air heat pump that uses propane as the refrigerant (known as R290), but there are currently very few on the market. The main advantage of using propane is that you don’t need to be FGAS registered to commission the heat pump. Commissioning involves tightening four pipes, attaching a vacuum pump to check for leaks, and then releasing the refrigerant. Hopefully, we’ll see more propane units available soon.
Why is a Heat Pump Better?
A heat pump doesn’t generate heat directly. Instead, it transfers heat from the outside to the inside. For those unfamiliar with how this works, I’ve explained it below.
If you’re more interested in the installation details, scroll down.
The Aerosol Can Analogy
Think of an aerosol can, like deodorant. When you spray it, the can cools down—this happens due to pressure changes and rapid expansion.
Inside the can, there’s a liquid under high pressure, with tightly packed molecules. When you press the nozzle, a valve opens, allowing the liquid to escape. Since the pressure outside is much lower, the liquid rapidly expands and turns into a gas, similar to how air rushes out of a balloon from high to low pressure, but here the liquid undergoes an even greater change.
Turning from liquid to gas requires energy, known as the latent heat of vapourisation. When the liquid evaporates, it absorbs this energy from the can and surrounding air, causing the temperature of both to drop.
In brief:
- Energy Absorption: The liquid requires energy to become gas.
- Source of Energy: This energy is drawn from the can and surrounding air.
- Cooling Effect: As energy leaves these surroundings, they cool down.
Connecting the Analogy to Heat Pumps
A heat pump works on the same principles of pressure changes and phase transitions to transfer heat from outside to inside your home.
How a Heat Pump Works
A heat pump contains a refrigerant and four main components: an expansion valve, a compressor, and two heat exchangers (one outside and one inside). Here’s how the cycle functions:
Cooling and Absorbing Heat from Outside
- Expansion Valve: The refrigerant, initially a high-pressure liquid, passes through the expansion valve, where the pressure drops suddenly.
- Evaporation: This pressure drop allows the refrigerant to begin expanding and evaporating.
- Energy Absorption: Similar to the aerosol can, the refrigerant needs energy to vapourise, which it draws from the outside air as is passes through the outdoor heat exchanger.
Even in cold weather, this low-pressure refrigerant is designed to be colder than the outdoor air, allowing it to absorb heat.
Compression and Heating
- Compressor: The compressor draws the refrigerant through the evaporator by creating suction. As the vaporised refrigerant is pulled through the evaporator into the compressor, its pressure is increased, thereby raising its temperature significantly.
Releasing Heat Inside
- Indoor Heat Exchanger: The hot, high-pressure gas flows through coils in the indoor unit, releasing heat into your home’s air. As it cools, the refrigerant condenses back into a liquid.
Cycle Repeats
The high-pressure liquid returns to the expansion valve, and the cycle continues, ensuring a consistent flow of heat into your home.
Why This Process Is Efficient
- Energy Multiplication: For every unit of electricity used to operate the compressor, multiple units of heat are extracted from the outside air and pumped into your home.
- Heat Transfer vs. Heat Generation: Unlike resistive heating, which produces heat, a heat pump simply moves heat from the ambient air.
Which Heat Pump I Chose and Why
Initially, I sought quotes for professional installation by certified FGAS engineers. I received two quotes, one for £2,000 and another for £2,500. However, both quotes were for fairly standard heat pump models—one would have installed a lower-end Fujitsu at around £700, and the other a more expensive Daikin at £950. Notably, neither quote included electrical or structural work. In my view, these prices were excessive for what essentially involves drilling a few holes and bending some pipes.
After careful consideration, I decided to take on the installation myself. Following extensive research, I opted for the Mitsubishi Electric MSZ-LN35VG2, a 3.5 kW unit that matched the heat loss requirements of the main living space in the property where this heat pump would be installed.
MSZ-LN35VG2 Product Information
I should note here that Mitsubishi Electric is distinct from Mitsubishi Heavy Industries. According to many forum discussions, Mitsubishi Electric is both more reliable and offers reasonably priced, easily accessible replacement parts. I considered other brands, such as Panasonic, Daikin, and Toshiba, but none seemed to have quite the following that Mitsubishi Electric does. My thinking was, “buy once, buy right”—if I was going to invest in one, it was worth spending the extra £500 for peace of mind. This decision was reinforced when the FGAS engineer arrived to commission the heat pump; upon seeing the unit, he said, “Oh, you’ve got the best model,” and even gave it a polish before leaving. But more on that later.
The MSZ-LN35VG2 is Mitsubishi Electric’s top-of-the-range model, selected for its superior performance in colder temperatures. It maintains a 3.5 kW heat output even at 0°C. There is a “HyperCore” version of this model available in New Zealand, which sustains this performance down to -15°C, but this option isn’t available in the UK. While I initially considered a cheaper unit, I ultimately decided not to take the risk. The whole point of this unit was to provide adequate heat at all temperatures.
The unit cost £1,525, including VAT and delivery, and came with a 7.5 m flared fitting kit. This kit included pipe fixings and a bracket for the heat pump.
What the Installation Involved
The first task was to provide an electricity supply to the outdoor unit. I was fortunate here, as the house had been fitted with electric resistive radiators, each with its own RCB (Residual Circuit Breaker) in the consumer unit. All I had to do was remove the radiator in the main living space, which had an attached switch, and run a new cable from the switch to the heat pump’s outdoor unit. This arrangement meant the heat pump had its own isolated RCB, with a fused switch between it and the RCB. As you can see, I used a white cable; this is because there was white cable in the house, of the right size. This will be hidden in time.
Once the heat pump arrived, the next step was to fit the internal unit. This was very easy. There is a metal plate that it clips onto, so I took the metal plate off, marked up on the wall where I wanted it to go, put in the appropriate fixings, and screwed it in place. Once the plate was in place, it showed me where to drill the hole for the pipework and the size of the hole, in this case, 65mm. So I drilled the hole, then put my pipes through and mounted the unit on the plate. Since the house has a timber frame with timber cladding, drilling was easy; for stone or brick walls, a core drill would be necessary.
Next, I prepared the pipework. It’s a straightforward job but would have been easier with two people! The pipes are thin copper pipes and can be bent to conform to the direction required. You, of course, need to be gentle with the bending as there is a possibility that you could kink them. I chose pre-flared pipes for quick fitting, but you can easily flare them yourself with the right kit. I attached the white pipe connectors to the wall and then secured the black insulated pipes in place.
The outdoor unit was straightforward to install, although I had to make some minor adjustments to the bracket design. To ensure the bracket was level, I inserted a plank of wood between it and the wall. I then adjusted the feet of the bracket to keep it perfectly level. If possible, I’d recommend securing the unit to the ground rather than the wall to minimise vibration. However, in this case, ground mounting wasn’t an option, as the house is on a flood plain. Given that the area regularly floods, elevating the heat pump was essential.
Since I am not FGAS registered, I couldn’t legally commission the heat pump myself, so I found a certified engineer prior to purchasing the unit who agreed to do it for £280. While this might seem steep for about an hour’s work, the expertise and precision required in this step are crucial for the system’s performance and longevity.
Understanding the Importance of Proper Commissioning
Commissioning is a simple procedure; but it ensures the heat pump operates safely and efficiently. Here’s what it involves:
The engineer attaches a vacuum pump to the newly installed refrigerant lines. This step is vital for two main reasons:
- Leak Detection: By creating a vacuum, the engineer can verify the integrity of all connections. If the system maintains the vacuum without any pressure increase over a set period, it indicates there are no leaks in the pipework.
- Removal of Non-Condensable Gases: Air and moisture are non-condensable gases that can severely impact the performance of a heat pump. Moisture can react with the refrigerant and oils inside the system to form acids, leading to corrosion and potential compressor failure. Air can cause increased pressures, reduced efficiency, and higher energy consumption. The vacuum pump removes these gases, ensuring the refrigerant circulates in a clean, sealed environment.
Ensuring System Integrity and Efficiency
Proper evacuation of the system is essential for:
- Optimal Performance: A system free of contaminants operates more efficiently, providing better heating and cooling while consuming less energy.
- Longevity: Removing harmful gases prevents internal damage, significantly extending the life of the heat pump.
Releasing the Refrigerant
Once the system holds the vacuum and is free of non-condensable gases, the engineer carefully releases the refrigerant pre-charged in the outdoor unit into the system.
I’ve still got a few more clips to add, and I’ve got to box in some pipes and tidy up here and there, but the unit is fully functional and ticking away nicely.
How Does It Perform?
So far, it has performed wonderfully. The outside temperature is currently 6 degrees Celsius and it is heating the room using around 400 W of electricity – measured via the Octopus Home Mini with everything else turned off. I can’t be sure of this unit’s precise COP because it isn’t measured, but what I can be sure of is that at 22 degrees Celsius the room is very warm and the unit is very quiet and barely noticeable.
Was It a Success?
It has gone well so far, but I’d like to see how well it performs in sub-zero temperatures to give my full review on how good it is.
According to the energy ratings, it should operate down to -15°C and produce up to 3.2 kW of heat at that temperature. If it does, then I’d count this as more than a success.
Q&As
Q: Why did you choose a mini-split over a multi-split? (A mini-split connects one outdoor unit to one indoor unit, whereas a multi-split connects one outdoor unit to multiple indoor units.)
A: There were several reasons why I chose a mini-split.
- Firstly, a mini-split can modulate to a lower output than a multi-split, which means that the minimum heat output and electricity input are lower. In theory, this should result in less frequent on-and-off cycling.
- Secondly, a multi-split would have been more expensive, even compared to using two mini-splits. As such, there was no cost advantage to choosing a multi-split. I may install another heat pump in the main bedroom, but I’ll opt for a less expensive unit, as it’s a smaller room with lower heat loss.
- Lastly, due to the shape of the house, a multi-split would likely have required the heat pump to be centrally located, which would have placed it on the decking, causing obstruction.