Monobloc heat pumps are becoming increasingly popular, and for good reason: their compact design and ease of installation make them an attractive alternative for many builders and renovators. However, there is one question that comes up in the design of almost every monobloc system, especially as the colder months approach: can the monobloc heat pump freeze up and, if so, should glycol or water be used in the system?
The answer is not black and white. It can't be done in a single sentence, because in reality it is always the specific system, the specific building and the operating conditions that determine the level of risk and the protection that is warranted. In this article, we will go through what the risk of freezing means in practice, the difference between glycol and water systems, and how SMARTAIR thinks about it from a professional perspective.
What exactly is a monobloc heat pump?
The essence of the monobloc heat pump, that the entire cooling circuit - including compressor, heat exchangers, controls - is housed in a single, enclosed outdoor unit. The building is supplied with water or a water-glycol mixture which supplies the heating system with thermal energy.
This design has many advantages, but also one important consequence: there is a water-filled pipe section running between the outdoor unit and the building, which can be exposed to frost under certain conditions. While a split system where a refrigerant circulates between the outdoor and indoor units, whereas in a monoblock the heating medium is in direct contact with the cold outside.
This is not a problem in itself, but it explains why the issue of freezing occurs again and again.
Can a monobloc heat pump really freeze up?
In short: yes, in some cases it can freeze, but not because the monobloc heat pump is a „bad solution”, but because the system is not designed, built or operated properly.
Freezing does not typically occur during normal operation. A well-adjusted heat pump will continue to circulate water throughout the winter, and will have its own protection features. If a frost protection valve is professionally installed in the system, it will always protect the equipment from freezing. It does not require a power supply to operate, and it works even in the event of a power failure. Problems can occur if the antifreeze valve is not installed correctly, for example:
- in the event of a permanent power failure
- for extended periods of downtime in extreme cold
- in the event of a faulty control or a faulty circulating pump
- inadequately insulated outdoor pipe sections
When water freezes, its volume increases, which can lead to cracks in pipes, heat exchangers and valves. This results in serious material damage and repairs can be time-consuming.
This brings us to the fundamental question that arises in the design of any monoblock system: water or glycol?
Water in the system - simple, effective but conditional
There are strong arguments for water-filled monoblock systems. Water is an excellent heat transfer medium and has low viscosity, so it circulates heat with lower pumping losses. This results in better efficiency and lower energy consumption.
In a well-designed water-fired system, the performance of the heat pump is optimal, the need for maintenance is minimal. It is no coincidence that many manufacturers use water as a default.
However, the use of water is subject to conditions. It is considered safe if the system is designed and operated in such a way as to ensure that a frost situation does not occur. This is typically the case if:
- outdoor pipe sections are short and well insulated
- the system has active frost protection features
- the electricity network is stable or there is a back-up power supply
- the heat pump does not stop for long periods in winter
If several of these conditions are not met, the water could pose a risk.
Glycolic mixture - a safety reserve for extreme situations
Glycol is essentially an antifreeze additive used in heating systems when mixed with water. Its function is simple: it lowers the freezing point so that the medium remains liquid even when the temperature drops below zero. This can be particularly useful in places where the risk of frost cannot be completely eliminated. This could be the case, for example, in a weekend house that is not permanently occupied or in a building with frequent power cuts.
However, it is important to understand that glycol also means trade-offs in return for frost protection. If glycol is used, an extra heat exchanger must be installed, which leads to a loss of efficiency. In addition, the investment and running costs of a glycol system are significantly higher compared to water. In addition, glycol ages over time, requiring regular inspection and occasional replacement.
Too high a glycol content can be extremely harmful, so it should always be used on the basis of a precise calculation and the manufacturer's recommendation. It is important to know that For refrigerants classified A3 (explosive) (e.g. R290), the use of glycol is prohibited, because if the heat exchanger is ruptured, explosive material can easily enter indoors.
What decides whether glycol is needed?
In SMARTAIR's experience, there is no universal answer. Every system is different and every decision is determined by a combination of factors.
The geographical location of the building and the minimum winter temperatures in the area are important, but the length and routing of the pipes are just as important. A short section of pipe running indoors poses a completely different risk from a long section running partly outdoors.
The quality of the insulation, the control of the system, and the built-in frost protection features of the heat pump are also important. Last but not least, the manufacturer's specifications are a guide that should not be ignored.
How does a well-designed monoblock system protect itself?
Modern monobloc heat pumps are not passive systems. They use built-in sensors to monitor the temperature and automatically start the circulation when necessary to prevent the water from freezing. In many cases, defrost cycles and emergency operation logic are also available.
However, these solutions only work if there is an electricity supply and the system is in technically sound condition. That is why we always stress that frost protection is not a question of a single element, but of the whole system.
Delusion or reality? - what is often misunderstood about glycol
Many people believe that without glycol, a monobloc heat pump will surely run out of steam. This is not true. With proper design, a water system can be completely safe. It is also a misconception that glycol will „ruin” the heat pump. In fact, most problems are caused by improper concentration or neglectful maintenance.
SMARTAIR's professional position
At SMARTAIR, we always look at the system as a whole. Our aim is to choose the safest and most efficient solution possible, without unnecessary compromises. The decision is always based on site conditions, operating habits and technical parameters.
What happens if the system does freeze?
When a monobloc heat pump fails, it is not necessarily the end of the system, but it does require serious intervention. The most common damage is to pipes, heat exchangers or valves, which require specialist skills and equipment to repair.
In such cases, it is particularly important to have the system serviced by a service technician who is familiar with the design and characteristics of monobloc heat pumps. A poorly chosen repair solution can do more harm than good.
Conclusion: it's not the material, but the mindset that counts
So the question is not whether it is glycol or water, but how well thought out the design and operation of the system is. A well-designed monobloc heat pump with adequate protection will operate safely over the long term, with or without glycol.
If you're unsure or want to avoid unnecessary risks, it's a good idea to consult an expert at the planning stage. A good decision will give you peace of mind and will determine the lifetime and efficiency of your system in the long term.