Thermostatic head: liquid vs. gas - which one is better
Thermostatic head: liquid vs. gas - which one is better?
When you are renovating your heating system or replacing old valves, you will sooner or later come across a question that is often underestimated by most people: what type of thermostatic head should you actually buy? At first glance, it seems like a detail - after all, all heads look almost the same, have a numbered scale, and screw onto the valve body. In reality, however, there are two fundamentally different physical principles on which these devices operate, and your choice between them directly affects how accurately, quickly, and reliably your heating system regulates the room temperature.
In this article, we will look very closely at both types - liquid and gas thermostatic heads. We will explain what happens inside each of them, how they differ in practice, where each type has an advantage and where it fails. We will add specific numbers, scenarios from practical experience, and help you make an informed decision - not just based on price, but based on what you actually need.
What is a thermostatic head and how does it work in general
A thermostatic head is a control element that is mounted on a thermostatic valve (for example, on a thermostatic radiator valve direct EK x 1/2" or on a corner version EK x 1/2") and automatically opens or closes the flow of hot water according to the current room temperature. It is not a programmable device with electronics - it is a purely mechanical device that uses the physical properties of a certain substance to respond to changes in ambient temperature.
The basic principle is simple: inside the head, there is a sealed sensing element filled with a substance that expands when heated and contracts when cooled. This expansion/contraction moves a pin (needle), which directly presses on the valve seat. When the room is too warm, the substance expands, the seat closes, and the water flow through the radiator decreases. When the room cools down, the substance contracts, the seat opens, and the radiator starts heating more again. The whole thing works without electricity, without batteries, and without a control unit.
The difference between liquid and gas heads is precisely in what the sensing element is filled with - and this seemingly small detail causes significantly different behavior in practice.
Liquid thermostatic head: how it works and what makes it different
A liquid head has a sensing element filled with a special liquid - most often a mixture based on wax (paraffin) with additives, or directly a liquid heat transfer medium with a high coefficient of thermal expansion. Some manufacturers use capillary liquids based on alcohol. The key point is that liquids generally have very good thermal conductivity and react to temperature changes faster than gases.
When the surrounding heat warms up the sensing element of a liquid head, the liquid expands - the volume increases, the pressure in the element rises, and this pressure mechanically moves the pin downward. The pin presses on the valve seat and restricts the flow. When the room cools down, the liquid contracts, the pressure drops, the spring in the valve pushes the seat back, and the flow is restored.
Advantages of a liquid head
- Faster response to temperature change: Liquid conducts heat better than gas. The sensing element heats up faster and also cools down faster. In practice, this means that the head reacts to opening a window or sunlight in a shorter time interval - typically 20–40 % faster than a gas head of the same class.
- Higher regulation accuracy: Thanks to the fast and linear reaction of the liquid, a liquid head can maintain the room temperature with a deviation of ±0.5 °C to ±1 °C from the set value. This is a significantly better result than with standard gas types.
- Smaller hysteresis: Hysteresis is the difference between the temperature at which the valve starts to close and the temperature at which it opens again. In liquid heads, this difference is smaller (typically 0.5–1 K), which means less "overheating" and "underheating" of the room.
- Stable behavior over a wide temperature range: Liquids are less sensitive to changes in ambient barometric pressure, which is an advantage, for example, in mountainous areas or in buildings with large height differences.
Disadvantages of a liquid head
- Higher price: Liquid heads are more expensive in most cases - the difference can be 30–80 % compared to a comparable gas head of the same brand.
- Greater sensitivity to damage: If the sensing element is mechanically damaged, the liquid can leak out and the head will stop working completely. A gas head may continue to function partially for a longer time with a slow gas loss.
- Longer stabilization time after installation: After the first installation or replacement, a liquid head may need more time to "stabilize" - for the liquid to adapt to the typical conditions of the room.
Gas thermostatic head: how it works and what makes it unique
The gas head contains a sensing element filled with gas or a mixture of gases. Most often, it is nitrogen, a mixture of hydrocarbon gases, or special refrigerants. The gas reacts to temperature changes by changing pressure (according to Gay-Lussac's law), and this pressure increase moves the valve needle in the same way.
Gases have a significantly higher temperature pressure coefficient than liquids, which would theoretically mean a faster reaction – but in practice, the situation is the opposite. Gases are excellent thermal insulators. The sensing element of a gas head heats up and cools down more slowly, because the gas itself conducts heat poorly. The result is a slower, yet smoother and "calmer" regulation – without sudden jumps.
Advantages of the gas head
- Lower price: Gas heads are a traditional standard and are produced in large volumes. For common use in a family home or apartment, their price is more favorable.
- Simple construction and reliability: Fewer moving parts, a simpler element. Under normal conditions – without extreme temperature fluctuations – they work for decades without problems.
- Smother regulation in stable conditions: If the room has no large heat gains (sunlight, fireplace, many people) and the temperature changes slowly, the gas head regulates completely adequately.
- Easier service and replacement: Due to their widespread use, gas heads are widely available and most plumbers know them well. Replacement without draining the system is commonly feasible – the topic is also covered in a separate article Replacing a thermostatic head without draining the system – is it possible?.
Disadvantages of the gas head
- Slower reaction: The main technical drawback. In case of rapid temperature changes (opening a window in winter, direct sunlight), the gas head reacts with a delay of several minutes – this means unnecessary heat loss or overheating of the room.
- Greater hysteresis: The difference between the closing and opening temperature is higher in gas heads, typically 1.5–2.5 K. The room thus heats up a bit more than the set value before the valve reacts.
- Sensitivity to atmospheric pressure: In mountainous areas (elevation above 800–1000 m a.s.l.), the lower atmospheric pressure can affect the setting of the gas head – the actual regulation temperature may slightly differ from the set value.
- Less accuracy at extremes: At very low (below 5 °C) or very high (above 28 °C) ambient temperatures, the accuracy of the gas head decreases.
Tabular comparison: liquid vs. gas head
| Parameter | Liquid head | Gas head |
|---|---|---|
| Reaction speed | Fast (3–6 min.) | Slower (8–15 min.) |
| Regulation accuracy | ±0,5 – 1 °C | ±1,5 – 2,5 °C |
| Hysteresis | 0,5 – 1 K | 1,5 – 2,5 K |
| Price (approx.) | Higher (+30–80 %) | Lower |
| Sensitivity to barometric pressure | Minimal | Mild (elevation >800 m) |
| Durability in case of damage | More sensitive | More durable |
| Suitability for drafty rooms | Excellent | Sufficient |
| Typical use | More demanding applications, energy optimization | Standard for most homes and apartments |
Practical scenarios: when to choose which
Scenario 1: A family house with significant solar gain
Imagine a house with large glazed areas facing south or southwest. When the sun shines in January, the temperature in the living room can rise by 4–6 °C within an hour – without any contribution from heating. In such a house, a gas head cannot react quickly enough – the valve closes too late, and the room overheats. The pleasant solar heat arrives, but the radiator still delivers energy you don’t need – and you pay for unnecessary heat.
In this case, a liquid head is a significantly better choice. Its faster response means the valve starts closing earlier, thus smoothly utilizing passive solar gains and saving energy. From experience, I know that in such houses, the right choice of heads can reduce natural gas or pellet consumption by 8–15 % per heating season – this is not a marginal saving.
Scenario 2: An apartment building with stable conditions
An apartment in a panel or brick building, where neighbors are heating from all sides, windows are small and facing north. The temperature in the rooms changes slowly and evenly. Solar gains are minimal, and wind does not penetrate through gaps. Here, a gas head is absolutely sufficient. The regulation is slow, but in an environment where the temperature also changes slowly, this is not a problem. The radiator opens and closes at a comfortable pace, and the room is warm without the precision of a liquid head.
In such an apartment, investing in more expensive liquid heads is economically unjustified – the return on investment would be very long, and you would hardly notice the comfort difference.
Scenario 3: Bathroom, toilet, basement
The temperature in the bathroom changes significantly – a shower, a bath, a hair dryer. The temperature here can rise by 8–10 °C in 10 minutes. A liquid head can really prevent unnecessary heating during a shower here. Moreover, bathroom radiators often have a drying function as well, and the speed of response plays a role in comfort and consumption.
For a bathroom – if you don’t want a programmable electric solution – I recommend a liquid head. For valves in the bathroom, for example, the VK corner valve EK x 1/2" is suitable, which is installed precisely in walls with bottom connections, typical for bathroom ladder radiators.
Scenario 4: Renovation of an older house with a limited budget
An older house, complete replacement of all valves on 10–15 radiators. The budget is limited. Here it is realistic to consider a compromise: place liquid heads only on radiators in rooms with variable thermal gains (living room, a child’s room with solar exposure), and use quality gas heads in other rooms (bedroom, hallway, toilet). This selective approach gives a good investment-to-result ratio.
Scenario 5: Mountain cabins and recreational buildings
In a building at an altitude of 900–1200 m a.s.l., the lower atmospheric pressure can affect the accuracy of a gas head. The typical difference is small (0.5–1 °C), but in buildings where heating runs unattended and energy efficiency is key, it makes sense to prefer a liquid head. Its setting remains accurate regardless of altitude and pressure fluctuations.
How a thermostatic head reacts to the placement of the radiator
One aspect that is rarely mentioned when choosing between a liquid and a gas head is the position of the head on the valve. Standard installation is horizontal – next to the valve needle. If the radiator is deeply embedded in a recess, behind a curtain, or the head is turned upwards (vertically), the air around it is not free and the measuring element does not measure the room temperature, but the temperature influenced by the radiator itself.
In such a case, a liquid head is less prone to error, as it reacts faster and "does not remember" the heat from the radiator as long as a gas head does. If the radiator or curtain creates a problematic installation situation, a better solution than changing the head type is the installation of a remote sensing element – a capillary head, where the sensing element is connected to the mechanism using a thin capillary and can be placed directly in the room, not on the radiator. This is already a separate chapter, also covered in the article How to set a thermostatic head to the correct temperature.
Choosing according to the valve body: what to know before buying
A thermostatic head – regardless of whether it is liquid or gas – must be compatible with the valve body installed on the radiator. Most heads on the market use the standard connection M30x1.5, but there are exceptions. Before buying a head, always check:
- Thread diameter on the valve (most commonly M30x1.5 or M28x1.5)
- Manufacturer of the valve body – some manufacturers (e.g. Danfoss, Heimeier) have proprietary connections and require an adapter or their own head
- Connection direction (straight vs. corner valve body) – this does not affect the choice of head type (liquid/gas), but the shape of the valve body you choose for your installation
If you are renovating the entire system and want to be sure of compatibility, a practical solution is to use complete sets, for example, SWING2 PN10 set in straight version or SWING2 PN10 set in corner version, where the valve, regulating element, and accessories form a verified combination without the risk of incompatibility. Complete sets, what they include, and when they are worth buying are described in the article SWING2 set: what it includes and when it is worth buying a complete set.
For direct radiator connection (pipe comes out of the wall or floor directly under the radiator), use the thermostatic radiator valve straight EK x 1/2". For corner connection (pipe comes out of the wall to the side), the thermostatic radiator valve corner EK x 1/2" is suitable. More on choosing between straight and corner versions can be found in the article How to choose a radiator valve: straight vs. corner – what decides.
Energy savings: Can a type of valve head really save energy?
This is a question that customers often ask, and the answer is: yes, but it depends on the context. The thermostat head itself – whether liquid-filled or gas-filled – does not consume energy. Energy is consumed by the boiler and the pump. The head only regulates how much of that energy reaches a specific room.
A liquid-filled head saves energy compared to a gas-filled one by reacting faster to external heat gains and preventing a room from overheating. When a room is overheated by just 1 °C above the set temperature, heat consumption increases by approximately 6 % (a rough rule of thumb). If a liquid-filled head can shorten the overheating period by a few hours a day, the savings will accumulate into a noticeable amount over the heating season.
In practice, switching from a gas-filled to a liquid-filled head can save 5–15 % on heating costs, with higher savings in rooms with significant solar gains or with rapid internal temperature changes. For a standard apartment without large windows and with stable temperatures, the savings will be lower.
If you are looking for more significant savings and are willing to invest more, consider programmable electronic heads (with an electric motor and digital thermostat). These are a further category, and their comparison with mechanical types is a topic for a separate article.
Installation and replacement: Is there a difference?
From an installation point of view, there is almost no difference between a liquid-filled and a gas-filled head. Both types are screwed on to the valve body in the same way, using an M30x1.5 nut (or another standard connection). The replacement procedure is identical, and a regular plumber does not need special tools or knowledge.
Important note: Replacing just the head (without interfering with the valve body) does not require draining the heating system. Simply unscrew the old head, fit the new one, and tighten it. This applies equally to both liquid- and gas-filled types. A detailed procedure is described in the article Replacing a thermostatic head without draining the system – is it possible?. If you are also replacing the valve body, then yes – the system must be isolated and the relevant section drained. A step-by-step guide can be found in the article Installing a radiator valve yourself: step-by-step guide.
Most common mistakes when choosing a thermostatic head
From experience, I know that people make several typical mistakes when choosing a head:
- They buy a head based only on price, without considering the type of room. The cheapest gas-filled head in a living room with large south-facing windows will cost more in unnecessarily burned energy than if they had invested in a liquid-filled type.
- They ignore compatibility with the valve body. Not every head fits every valve. Always check the thread and possible need for an adapter.
- They set the head to maximum and are surprised that the room does not overheat. The numbered scale on the head does not correspond directly to degrees Celsius – it is just a position (1 = approx. 12 °C, 3 = approx. 20 °C, 5 = approx. 28 °C). More about calibration and correct setting can be found in the article How to set a thermostatic head to the correct temperature.
- They leave the head covered by curtains or behind a cabinet. The sensing element measures the air temperature around it – if it is covered, it measures the temperature behind the curtain (higher) and the valve closes prematurely. A solution is a remote sensor or repositioning the curtain.
- They forget to bleed the radiator after installing the valve. An air pocket in the radiator reduces performance even with a correctly set head. This topic is covered in the article Maintenance and bleeding of radiator valves – how and how often.
Most frequently asked questions (FAQ)
Can I replace a gas-filled head with a liquid-filled one without replacing the valve?
Yes, in the vast majority of cases – if both types of heads have the same connection (usually M30x1.5). The valve body remains in place; you simply unscrew the old head and fit the new one. Some valves require an adapter for specific head brands, so before purchasing, check the type of valve or take a photo of it and compare it with the technical specifications of the head.
Is a liquid-filled head suitable for an old cast iron radiator?
Yes, the type of radiator (cast iron, steel panel, aluminum) does not affect the choice between a liquid-filled and a gas-filled head. The head regulates the water flow through the valve and it does not matter what is on the other end. Naturally, cast iron radiators have a higher thermal inertia – they heat up and cool down more slowly – which reduces the effectiveness of regulation regardless of the head type. Here, a liquid-filled head is slightly more advantageous, as its faster response compensates for the slow thermal response of the radiator.
How can I tell if I have a liquid-filled or gas-filled head at home?
Visually, they are hard to distinguish – most manufacturers indicate this on the product itself (on the box, in the catalog, or directly on the head body with an abbreviation, e.g., "L" for liquid/liquid-filled or "G" for gas). A more reliable method is to look at the technical specifications of the model, where the type of sensing element is always stated. If you do not know what type of head you have on the radiator, simply assume it is gas-filled – liquid-filled types were less common and their installation was usually known.
Can a thermostatic head (of any type) completely replace the boiler thermostat?
No. A thermostatic head regulates heat distribution in a specific room, but it does not communicate with the boiler. The boiler runs according to its own thermostat or control unit. An ideal system works so that the boiler thermostat (or an equithermal regulation) controls heat production, and the thermostatic heads on the radiators control its distribution. If you have only thermostatic heads without a boiler thermostat, the boiler could run unnecessarily even when all the heads are closed – which is energy inefficient and could damage the equipment (high pressure situation).
What happens if a liquid-filled head freezes or is mechanically damaged?
If the sensing element of a liquid-filled head is damaged (broken capillary, mechanical impact), the liquid leaks out and the head loses its ability to regulate – it usually remains in the position it was in when it failed, or the spring opens it fully. In any case, replacement is necessary. A gas-filled head may still function partially for some time in case of a slow gas leak, but it also needs to be replaced. Frost resistance is similar for both types – the sensing element itself does not freeze, as it is not filled with water. A problem could occur only in extreme cold (-20 °C and lower) in unheated spaces, where the liquid in the element could freeze – which is unlikely in normal use in a heated building.
Does it make sense to buy an expensive liquid-filled head for a cheap valve body?
Yes, the combination is functional and not problematic. The valve body and the head perform different functions – the valve body is a passive element (opening and closing the flow), while the head is an active regulating element. The quality of the head affects the accuracy and speed of regulation, while the quality of the valve body affects tightness, durability, and hydraulic characteristics. If your budget is limited, it is more sensible to invest in a good head rather than a premium valve body, because the head has a direct impact on comfort and consumption.
Conclusion: Liquid-filled or gas-filled – so what then?
To summarize after years of working with heating systems of various sizes and types: there is no one "better" type. There is a type that is more suitable for your specific case.
If you live in a house with large windows, a southern exposure, a fireplace, or another source of variable heat – go for a liquid-filled head. Its faster response and higher accuracy will pay off in energy savings and better comfort. If you live in a typical apartment in a panel building or a brick house, where temperatures change slowly and conditions are stable – a quality gas-filled head is a perfectly sufficient and economically reasonable solution.
Do not forget that a thermostatic head is just one link in the chain. Proper function also depends on the hydraulic balancing of the system, a functional boiler thermostat, clean and bled radiators, and correctly dimensioned valves. All these aspects together form a system that heats efficiently, reliably, and economically – regardless of whether you have a liquid-filled or gas-filled head on the valve.
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