Liquid thermal media - water or glycol?

New installation, why use glycol when we have water? Are there any other alternatives to water and glycol? 

Heat transfer agent, or alternatively heat carrier - an important term - is a substance that can transport heat from a heat source (such as a boiler, fireplace, heat accumulator, heat pump, solar panels, etc.) to a heat receiver, such as a radiator or underfloor heating loop. For central heating systems to work - heat is transported through pipes, loops, and other types of ducts by the heat transfer agent. It is essential and there is no debate about it. The most common is of course water, which has many advantages but also disadvantages. The main requirements for heat transfer agents in practical use in heating or domestic hot water systems include: availability (ease of purchase), price, and in more specialized installations, designers take into account more stringent requirements such as: impact on indoor climate parameters, maximum heat capacity when generated at the source (so-called heat accumulation capacity), and low resistance to flows in the installation. Water performs well in terms of these parameters, but sometimes various substances are added to it to improve certain parameters - for example, water is mixed with glycol in different concentrations to achieve specific properties of this mixture. In this article, we will only discuss liquid heat transfer agents, leaving aside gaseous ones such as air or water vapor, which basically do not occur in domestic installations in Poland. We will address their impact on the lifespan of hydraulic devices, such as accumulators, heat exchangers, pumps, water sterilizers, etc.

Water and its properties in heating installations

It is the most obvious solution because it is the fastest and easiest to obtain. It is also definitely one of the cheapest. However, it contains significant impurities in the form of organic and inorganic suspensions, which are not visible to the naked eye - but above all, dissolved in it are gases, colloids (in the form of silica and various other compounds). These have an adverse effect on the operation of heating devices. Therefore, it sometimes requires treatment to be safe for our heating system and efficient in heating. However, water also has other properties, which we forget about - even when designing the installation - such as: dry residue, oxidizability, and pH. Interestingly, water changes its specific heat depending on the temperature it reaches. It ranges from 4.226 kJ/kgK (for water at Celsius degrees), to 4.194 kJ/kgK for water at a temperature of 80 degrees Celsius. The same goes for the thermal conductivity coefficient, which ranges depending on the temperature from 0.558 W/mK to 0.681 W/mK (for temperatures that may occur in a typical domestic installation, i.e., from low temperatures to +80 degrees Celsius). However, let's be honest - which plumber (unless he is an engineering designer from a technical university) has knowledge of and uses this data as practical knowledge needed to design installations? None, because in practice, knowledge of basic principles in which installations make exceptions to the use of water - is sufficient to design efficient and effective heating. This is our own assessment.

The inconveniences of using water in some installations, more often industrial than domestic, also arise from the fact that it changes its state of aggregation around 0 degrees and starts boiling and intensely vaporizing around 90-100 degrees, significantly raising the pressure in the installation. Therefore, installations must be protected against significant cooling (although in practice, to freeze water in heating, you would have to leave the house for many days without turning it on) and against high pressure in the event of water evaporation.

Water properties affecting hydraulic devices also include the content of dissolved salts (provoking the possibility of scale formation in boilers) and pH, expressed by the hydrogen index - i.e., the so-called pH (related to the potential occurrence of corrosion on steel devices, e.g., tanks, distributors, or plate heat exchangers). Due to the harmful effect accelerating the corrosion of devices - both the chemist and the user should be interested in the oxygen and carbon dioxide content in the water. The content of these gases - changes with changes in water temperature, which is a natural phenomenon in any installation. This, in turn, causes changes in pressure in the installations and precipitation of the above gases, which manifests itself as air bubbles in the installation. They will always accumulate in it when we use water - and they must be removed from the installation, which determines how we should design the water installation and how we should anticipate the possibility of their removal.

The worst is yet to come - hard water in the heating system

Salts are dissolved in water, with their concentration depending on where the water comes from. Magnesium and calcium are considered the worst. They have relatively poor solubility but have the ability to decompose (dangerous for our installation). Their decomposition intensifies at high temperatures, starting to weaken at 30-40 degrees Celsius, so practically any installation will provide conditions for the decomposition of magnesium and calcium. In heating installations, due to the increase in temperature, water evaporation in boilers intensifies - while as a result of salt decomposition, sediment is precipitated, which settles on heat exchange surfaces (such as boiler walls, pipes, underfloor heating loops, hydraulic devices, etc.). This sediment is the known antagonist of water installations, namely boiler scale.

The salt content in water is referred to as water hardness. It can have different degrees. Temporary (carbonate) hardness and permanent (non-carbonate) hardness are distinguished. Temporary hardness occurs in water when it contains acidic calcium and magnesium carbonates, which break down into soluble salts at high temperatures. Therefore, they are not permanent. If the hardness is caused by neutral calcium and magnesium salts, then they will not break down as the temperature of the medium increases - and we are dealing with permanent, non-passing hardness.

Because we use water, installations eventually become scaled, as the vast majority of areas in Poland have hard or very hard water. This is an invisible enemy because hard water will not be recognized by us without tests, i.e., physicochemical analysis. However, we can know that over time, the presence of water (because it will be hard one way or another if not properly treated) will definitely shorten the lifespan of devices such as the heating boiler, heat pump, UV water sterilizer, plate heat exchanger, accumulator, underfloor heating manifold, and even the radiator. Over time, the tubes and ducts of our installation overgrown with scale cause our central heating system to have such poor thermal conductivity that our heating bills or fuel consumption can increase by as much as 30-40%. Fortunately, there are methods for neutralizing hard water and also devices such as water softeners.

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Other heat transfer agents - what is an alternative to using water in central heating systems? Is there a better option? Dowtherm, glycol, mineral oil? Are they a better solution?

Above all, when using them, there is no need to treat the water for the installation, nor the risk of scale formation in the heating pipes, scaling of devices, which leads to the need for more frequent replacements and a decrease in heating efficiency. But unfortunately, they also have a number of disadvantages.

In practice, the balance of pros and cons of these fluids will lead us to the conclusion that they are more specialized heat carriers, which are predisposed due to their properties for rooms with specific applications, e.g., industrial ones.

Diphenyl ether

An exotic-sounding but impractical heat transfer medium for domestic installations is dowtherm or diphenyl ether. Used as a heat transfer fluid practically only in highly specialized industrial installations, which is a pity, because its specific heat ranging between 1.55 kJ/kgK and 2.76 kJ/kgK would be a very interesting parameter. Similarly, its freezing point at even -55 degrees Celsius would allow its use in spaces heated only periodically. Unfortunately, it is a hazardous substance to human health and flammable. This excludes its use in domestic installations, but even in industrial buildings where spark-generating work such as welding is performed. Information about it is provided more as a curiosity. Although it was invented at the beginning of the 20th century - it is still used.

Mineral oil

Its main advantage is that it will not freeze in our climate. With a freezing point even down to -50 degrees Celsius, it can be used even in Scandinavia or northern regions of Russia. It remains in a liquid state over a very wide temperature range, as its boiling point ranges between 280 and 350 degrees Celsius, and its ignition temperature is basically unattainable in the application under discussion. Naturally, like other oils, it does not cause scaling or corrosion. However, it has its drawbacks; it is significantly more expensive than water, especially when dealing with an extensive installation - and over time, there is observed the deposition of oil residues on the surfaces of heat exchange devices (radiators, coils, plate heat exchangers). It relatively easily oxidizes - which means that such an installation must be vented more frequently. Massively used - as we know - also in refrigeration.

Glycol in heating installations

This is a potential real alternative to water in the installation (or rather in its part - the one exposed to freezing - such as an additional utility building connected to the system additionally, not permanently heated). Naturally, like mineral oil, it does not create boiler scale. The freezing temperature ranges from about -17 degrees to even -49 degrees Celsius, which in our climate is enough to completely protect the installation. The freezing temperature depends, of course, on how diluted it is, because...

It is rarely used in undiluted form. Most often, it is mixed with water, in concentrations depending on the need - 30% glycol and the rest water, 35% glycol and the rest water, or if the situation requires it, 50/50% with water.

Glycol in the installation, however, has its drawbacks. First of all, it is definitely more expensive than water, which is natural. Before deciding to use it, we need to know that it limits heat transfer. It does not freeze, which means it also does not release heat energy as easily as water. It is estimated that heat transfer is 8% lower in a 30% glycol solution, down to 15% in a 50% glycol solution. This means greater inertia of the installation and a longer wait for it to heat up, as well as slightly higher fuel consumption (i.e., higher heating costs). This explains why it is diluted - diluted glycol achieves non-freezing parameters that we need and reduces its defective characteristics. Additionally, this loss in heat transfer, the aforementioned 8-15%, can be largely neutralized by increasing the heat transfer devices - such as the surface area of radiators and heat exchangers, which means real higher investment costs. For the reasons described above, we can assume that it is certainly not a good solution for underfloor heating, but rather possibly for standard radiator heating systems.

For the above reasons - if we plan to connect some additional facility to the installation, in which, due to the risk of freezing, we decide to use glycol - we do not recommend the decision to use glycol in the entire system. In practice, it is necessary to separate the circuit of this freezing-exposed room (probably a garage, workshop, warehouse hall) from the main boiler circuit using a plate heat exchanger. It should be selected so that for every 100 square meters of such a room, there is roughly 0.35 square meters of heat exchange surface of the plate heat exchanger. Such a coefficient allows for heating of a traditional installation with glycol as well, but if it is a utility building where we do not need temperatures like in residential rooms, it may also be a smaller exchanger. The cost of the heat exchanger alone for separating the glycol circuits is about 50-80 euros per 100 square meters of space, based on the prices of heat exchanger manufacturers Nordic Tec.

Unfortunately, there is another downside - we probably expected that unlike mineral oil, glycol would not contribute to the corrosion of devices. Unfortunately, although glycol solutions are not electrolytes, they do break down during installation operation, which causes some corrosive properties, but their strength should not be overestimated.

When deciding to use glycol, it is also worth paying attention to whether the manufacturer of the circulation pump recommends it to work with glycol. Some manufacturers note that the pump is not intended for glycol, which may be worth considering. Most pumps will cope with weakly concentrated glycol, but there remains a warranty issue. There are also special pumps available on the market declared as intended for glycol - but they do not differ much in their construction.

Conclusion - what heat transfer medium is the best solution for a single-family home central heating system?

In view of the above, we must conclude that the fact that water is used as a heat transfer medium in 99% of households has its clear justification in the advantages of water. Glycol is too inert to be used throughout the installation; heating the house would take a long time, and attempting to neutralize the thermal conductivity weakening would require a greater investment from us. Especially with today's trendy underfloor heating, which already has greater inertia, glycol is not suitable. Mineral oil is too expensive and not functional enough.

Water, on the other hand - despite its many drawbacks - is cheap and easily available. It easily releases its heat, whether on radiators or underfloor heating. Its main drawback - namely high hardness levels - can be (albeit at a cost) neutralized. The investment cost in a water softener is about 600-1000 euros (for a single-family home), or water can be treated with an electric beam. And this is the best solution to protect our boilers, heat pumps, accumulators, heating manifolds, heat exchangers, circulation pumps from deadly scale - and to have efficient heating that consumes minimal fuel or generates minimal heating costs. Glycol should only be used when there is a risk of the installation freezing, i.e., in houses standing empty, e.g., in garden sheds - or when connecting additional rooms that are not used permanently.

Developed based on: Marian B. Nantka, Heating and District Heating, Volume I, Publishing House of the Silesian University of Technology, Gliwice 2010