Liquid thermal media - water, glycol? What to use in a heating system?

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Posted on: 2024-04-12
 

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 Europe. 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.

Specific heat capacity of water and its thermal conductivity coefficient

☑️ For those curious, an interesting fact is that water changes its specific heat capacity depending on the temperature it reaches. The specific heat capacity of water 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. Similarly, the thermal conductivity coefficient varies depending on the temperature, ranging from 0.558 W/mK to 0.681 W/mK (for temperatures typical in a household installation, ranging from small positive temperatures to +80°C).

However, let's be honest - how many plumbers (unless they are engineering graduates from a Technical University) are aware of this and use this data as practical knowledge necessary for designing installations? None, because in practice, knowledge of basic principles in which exceptions to the use of water are made is sufficient for designing efficient and effective heating systems. The specific heat capacity of water and the thermal conductivity coefficient belong to a higher level of knowledge, which practitioners do not typically reference. This is our own assessment.

Disadvantages of water in a central heating system

✅ Inconveniences in using water in some installations, more commonly industrial than domestic, also arise from the fact that it changes its state of matter around 0 degrees, and begins to boil and intensely evaporate around 90-100 degrees, significantly raising the pressure in the installation. Therefore - installations must certainly be protected against significant cooling (although in practice, to freeze water in heating, the house would have to be left off for many days) - as well as against high pressure in the case of water evaporation. In summary, we cannot allow water in the system to boil, which could damage it - this is less likely with modern heat sources with temperature controls, but it is possible with fireplace installations or old boilers.

The potential of water to create deposits that weaken our heating installations - such as boiler scale, etc.

✅ Among the properties of water that affect hydraulic devices is also the content of dissolved salts in it (which can provoke the formation of boiler scale) and its pH level (related to the potential occurrence of ☑️ corrosion on steel equipment, such as tanks, distributors, or plate heat exchangers). Due to the harmful effect accelerating equipment corrosion - both chemists and users should be interested in the content of oxygen and carbon dioxide in water. The content of these gases - changes with changes in water temperature, which is a phenomenon evident in every installation. This, in turn, causes a change in pressure in the installations and the precipitation of the above gases, which manifests as air bubbles in the installation.

✅ Air bubbles will always accumulate in the heating system when water is used - and they must be removed from the installation, which determines how we design the water installation and how we anticipate the possibility of their removal.

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

In water, salts are dissolved, and their concentration depends on the source of the water. Magnesium and calcium are considered the worst. They have relatively poor solubility but have the ability to decompose (dangerous for our installation). ✅ The decomposition of salts in water becomes stronger at high temperatures, starting to occur to a lesser extent already at 30-40 degrees Celsius, so practically every installation will provide conditions for the decomposition of magnesium and calcium (i.e., for the formation of scale deposits). In heating systems, as the temperature increases, water evaporation intensifies in boilers - and as a result of salt decomposition, sediment is precipitated, which settles on heat exchange surfaces (i.e., on boiler walls, pipes, underfloor heating loops, hydraulic equipment, etc.). This sediment is the infamous antagonist of water installations - boiler scale.

Water hardness

✅ The content of salts in water is referred to as water hardness. It can vary in degree. Hardness is distinguished as temporary (carbonate) and permanent, or non-carbonate hardness. Temporary hardness occurs in water when we deal with acidic calcium and magnesium carbonates, which decompose into soluble salts at a certain temperature. Therefore, they are not permanent. However, if the hardness is caused by neutral calcium and magnesium salts, then they will not decompose as the temperature of the medium increases - and we have permanent, non-temporary hardness.

Because we use water - installations become scaled over time, as the overwhelming majority of areas in Europe have hard or very hard water. This is an invisible enemy, as hard water will not be recognized by us without analysis, i.e., physicochemical analysis. However, we can know that over time, the presence of water (because it will likely be hard anyway, unless properly treated) will significantly shorten the lifespan of devices such as a heating boiler, heat pump, home UV water lamp, plate heat exchanger, storage tank, underfloor heating manifold, and even radiators.

✅☑️ Over time, pipes and channels in our installation overgrown with scale cause our heating system to have such poor thermal conductivity that our heating bills or fuel consumption can increase by up to 30-40%. Fortunately, there are methods to neutralize hard water as well as devices like water softeners.

Water softener filter - possible solutions

Until recently, the main solution against hard water was conventional water softeners, which cost starts at 600-800 €. Their downside, besides the price, was that they took up a lot of space in the boiler room. Additionally, it was a device that required occasional fiber replacement, so it was not completely maintenance-free. Nevertheless, it must be added that such water softeners were quite effective. These devices are not hard water filters; it is a different method of operation. However, in recent years, hard water filters mounted at the entry point of water into the house have also appeared on the market.

✅ The current trend is the use of flow-through galvanic softeners - hard water filters using a technology called IPSE. This device takes the form of a tube that is mounted on the incoming water pipe. The IPSE hard water filter is best used for prevention. However, a heavily scaled installation will still pose a serious problem. The advantage of hard water filters is their maintenance-free operation and long service life, typically up to 10 years. They operate on the principle of a small galvanic cell, with higher quality devices reinforced with a silver layer. By generating a small voltage of around 5-6 V, the cell breaks down mineral salts responsible for scale and water hardness.

Alternative to water in heating systems. Is there a better option? Dowtherm, glycol, mineral oil? Are they a better solution?

✅ First and foremost, their use eliminates the need for water treatment in the installation and minimizes the risk of scale formation in heating pipes and equipment, which leads to more frequent replacement and decreased heating efficiency. However, they also have several drawbacks.

In practice, weighing the pros and cons of these fluids leads us to the conclusion that they are more specialized heat carriers, which are predestined for specific applications, such as industrial settings. Feel free to read more if you're curious.

Diphenyl ether

✅ Exotic-sounding but impractical as a heating medium in household 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 capacity ranging between 1.55 kJ/kgK and 2.76 kJ/kgK would be a very interesting parameter. Similarly, its freezing point even as low as -55 degrees Celsius would allow for its use in intermittently heated spaces. Unfortunately diphenyl ether is hazardous to human health and flammable. This excludes its use in household installations, but even in industrial buildings where activities generating sparks, such as welding, are conducted. We provide information about it more as a curiosity. Despite being invented in the early 20th century, it is still used today.

Mineral oil

✅ The main advantage of mineral oil is that it will not freeze in our climate. With a freezing point even as low as -50 degrees Celsius, it can be used even in Scandinavia or the northern regions of Russia. It remains liquid over a very wide temperature range, as its boiling point ranges between 280 and 350 degrees Celsius, and its ignition temperature is practically unattainable in the discussed application. Naturally, like other oils, it does not cause scale or corrosion. However, it has its drawbacks; it is significantly more expensive than water, especially when dealing with an extensive installation - and over time, the deposition of oily residues on heat exchange surfaces (radiators, coils, plate heat exchangers) can be observed. It relatively easily oxidizes, meaning that such installations need to be vented more frequently. Widely used - as known - also in refrigeration.

✅ Ethylene glycol in heating installations

Ethylene glycol is a potential real alternative to water in installations (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 form boiler scale. The freezing point ranges from about -17 degrees to even -49 degrees Celsius, which is sufficient to protect the installation in our climate. The freezing point depends, of course, on how diluted it is because...

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

☑️ For the curious:

  • the boiling point of ethylene glycol is 197°C.

Glycol in installations has its drawbacks and disadvantages

Glycol has its advantages - and its disadvantages. Firstly, it is definitely more expensive than water, which is natural. Before deciding to use it, we must know that glycol 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 about 8% lower when using a 30% glycol solution, and even up to 15% lower with a 50% glycol solution. This results in greater inertia of the installation and a longer wait time before it heats up, as well as slightly higher fuel consumption (i.e., higher heating operating costs).

This explains why it is diluted - diluted glycol achieves the anti-freeze properties we need while reducing its undesirable characteristics (and... it becomes cheaper). Additionally, the loss in heat transfer, those mentioned 8-15%, can of course be largely offset by increasing the size of heat transfer devices - such as the surface area of radiators and heat exchangers, but this means real additional investment costs.

✅ Glycol for underfloor heating - glycol for radiators - analysis

For the reasons described above, we can assume that glycol is certainly not a good solution for underfloor heating, but rather potentially for standard radiator heating systems. Opting to use glycol in underfloor heating can make heating up a cold system a painstaking process. Glycol is not suitable for underfloor heating and should only be used with it as a last resort. For example, when it comes to a summer cottage - but we must be aware of its drawbacks.

☑️The myth that raising the pressure in the system is enough to neutralize the thermal inertia of glycol in underfloor heating should be dismissed. Fighting the laws of physics, unfortunately, ends without victory.

✅ However, if anything makes sense, it is - if at all - using glycol for radiators. Glycol for radiators is a better solution than for underfloor heating because radiators are just a thin layer of steel - not a concrete floor. The limitation of heat transfer caused by glycol will be less harmful here.

✅ Glycol for heat pump - analysis

Perhaps you came across this article because you are considering using glycol in the circulation of a monoblock heat pump, as suggested by an installer or salesperson. A monoblock heat pump, part of which is located outside, is indeed susceptible to freezing, which can lead to rather unpleasant financial consequences. So, is it worth using glycol for a heat pump?

There are several aspects to consider - assessing the risk of the heat pump freezing. The main causes (besides severe frosts) are power failures, preventing defrosting of the heat pump. In areas where power outages occur - for example, in some mountainous regions - it is worth being prepared for this. Additionally, we must remember that a heat pump is a device intended to serve us for a dozen or even twenty years. We do not know what kind of winter will occur in 10-15 or 20 years. Furthermore, the Polish power grid is overloaded, we have years of neglect behind us, and according to widely available information, problems with its efficiency may increase in the coming years.

Until recently, we also did not consider the risk of deliberate destruction of energy infrastructure or power plants - i.e., the risk of war (see the war in Ukraine). Although it still seems unlikely, after Russia's aggression against Ukraine, it may be worth securing this aspect - of course if we live in area where there is a proper winter.

The third aspect - very important - is the heat pump manufacturers who require the use of glycol in the external circuit of heat pumps. In the past, practices like this were used, for example, by the Rotenso company. It is worth considering whether it is worth using glycol for a heat pump - since it is not only a recommendation but even a requirement from some manufacturers.

✅ Glycol in heat pump - separation from the water-based heating system

When using glycol for a heat pump, it should be remembered that filling the entire house system with glycol is not recommended due to its inertia. In this case, it is worth separating the system with a heat exchanger to create independent glycol and water-based systems that exchange heat with each other. In this way, glycol used for the heat pump will not significantly hinder or weaken the quality of our heating system.

Separating the house system into water-based and glycol-based systems from the heat pump is possible using a plate heat exchanger. This, however - increases the installation cost - but we pay for the full safety of our monoblock. We should take this into account when calculating the risk of the heat pump freezing.

The downside is that heat exchangers for glycol-based monoblock heat pumps are relatively large, so they are not the smallest devices priced at 50 or 100 €. A heat exchanger for the glycol circuit of a heat pump should have a minimum of 1.5m2 of plate surface per 10 kW of device for heat pumps operating at around 50°C. Such heat exchangers start at around 180-200 €, but in the case of low-temperature heat pump systems (30-35°C), the heat exchangers should be significantly oversized, and thus more expensive.

✅ Glycol - when there is a risk of the system freezing

✅ For the reasons described above - if we are planning to connect an additional facility to the installation, where 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 advisable to separate the circulation of the freezing-prone area (likely a garage, workshop, or warehouse) from the main boiler circuit using a plate heat exchanger. It should be sized so that roughly 0.35m2 of heat exchange surface of the plate heat exchanger is allocated for every 100 square meters of such a facility. This coefficient allows for heating the traditional installation with glycol as well, but if it is an outbuilding where we do not need temperatures as high as in residential areas, a smaller heat exchanger may also suffice. The cost of the heat exchanger alone to separate the glycol circuits is approximately 60-80 € per 100m2 of the facility, based on the prices of plate heat exchangers from the Nordic Tec manufacturer.

☑️ Unfortunately, there is another downside - perhaps we expected that, unlike mineral oil, glycol would not contribute to equipment corrosion. Unfortunately, although glycol solutions are not electrolytes, they degrade during the operation of the installation, which causes certain corrosive properties. However, this should not be overstated. For full information - the corrosive potential of glycol is slightly higher than that of water (!), but it still does not have much significance in modern systems in practice.

Selecting pumps for Glycol

✅ When opting for glycol, it's also worth noting whether the manufacturer of the circulation pump recommends it to operate with glycol. Some manufacturers specify that the pump is not intended for use with glycol, which may be worth considering. Most pumps will cope with weakly concentrated glycol, but the warranty issue remains. There are also special pumps available on the market declared as suitable for glycol - however, they do not differ significantly in their construction.

Conclusion - What is the best heating medium for a single-family home heating system?

✅ Considering the above, we must conclude that the fact that water is used as the heating 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 up the house would take a long time, and attempting to neutralize the weakening of heat transfer would require a larger investment. Especially with the trendy underfloor heating today, which already exhibits greater inertia, glycol is not suitable. Mineral oil, on the other hand, is too expensive and not very functional.

✅ Water, despite its many drawbacks, is cheap and readily available. It easily releases its heat, whether on radiators or underfloor heating. Its main disadvantage, high hardness level, can be mitigated (albeit at a cost). The investment cost in a water softener is around 2500-4000 PLN (for a single-family home), or water can be treated with an electric beam or using hard water filters within the range of 600-800 PLN. This is the best solution to protect our heating boilers, heat pumps, storage tanks, heating distribution manifolds, heat exchangers, and circulation pumps from damaging scale buildup while ensuring efficient heating that consumes minimal fuel or generates minimal heating costs.

✅ Opt for glycol only when there is a risk of the installation freezing, such as in empty houses, e.g., garden sheds, or when connecting additional rooms that are not permanently occupied.

If you are looking for heat exchangers with specific parameters or applications, including those used for separating glycol and water circuits, check out products available in the following categories:

____________

Glossary of Terms

Aragonite - a form assumed by mineral salts from hard water when subjected to the action of hard water filters. Aragonite is considered incapable of forming scale, hence harmless.

Hard water filter - a flow-through device installed on water pipes to eliminate salts responsible for water hardness (or more precisely, to neutralize them and convert them into a form called aragonite, which does not have the ability to form scale in the installation).

Boiler scale - mineral deposits, mainly calcium and magnesium carbonates, which accumulate on the walls of boilers and pipes used in heating installations. It is caused by hard water, and it is assumed that scale precipitates during its heating, even at relatively low temperatures around 30-40°C.

HP - acronym used for: heat pump

Monoblock heat pump - a type of heat pump that has part of the circuit outdoors, making it sometimes exposed to adverse weather conditions, including freezing in extreme cases.

Water hardness - a parameter indicating the content of dissolved minerals, mainly calcium and magnesium salts, in water. Hard water can affect various aspects of water use (e.g., washing, cooking, as well as the operation of household appliances such as kettles or washing machines). High water hardness shortens the lifespan of devices in contact with it, including central heating boilers or heat pumps. If the water has high hardness, it will have a greater potential to generate boiler scale.

Plate heat exchanger - a flow-through device used for heat exchange between two mediums, resembling a block with four connections, composed of steel plates. In the context of the article, it is most often used to separate glycol circuits from water circuits (to exchange temperature but not mix). An example of such a device is a heat exchanger used for solar panels.

 
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