How to choose the right diaphragm vessel for your heating system? - step-by-step guide to selecting an expansion tank

Introduction - what is a diaphragm vessel and why it is used?

Diaphragm vessel (a.k.a. diaphragm expansion tank) is a key component for protecting heating, cooling and hot water systems from excessive pressure increases. Its construction is a steel tank divided inside by a flexible diaphragm (diaphragm) into two chambers: water and gas (usually filled with compressed air or nitrogen). This design allows the membrane to take up the increased volume of water that occurs due to thermal expansion - when the water in the system heats up, its volume increases and the membrane flexes, allowing some water into the tank and keeping the pressure within safe limits . As a result, the system works properly, and the safety valve does not drop water every time it gets hotter.

What is the diaphragm vessel used for? To put it simply: a diaphragm vessel protects a closed system from damage caused by an increase in pressure. It collects excess water (or other medium) when it increases in volume under the influence of temperature, and then gives it back to the system when the water cools down. Thus, it compensates for volume fluctuations and stabilizes the pressure in the system . The vessel works automatically - the internal gas "cushion" compresses and expands in rhythm with the changes in the volume of water, it does not require any power or control . In practice, any heating device or heat exchanger in a closed system (boiler c.o., water heater, solar collector, etc.) should have a proper diaphragm vessel installed to ensure safe operation of the system and avoid leaks or failures.

When you need an expansion vessel? If you have a closed heating system (central heating) or domestic hot water preparation system with a boiler, a diaphragm vessel is essential. In modern closed systems, diaphragm expansion tanks are just used. Any installation in which pressure fluctuations may occur (e.g., due to heating and cooling of water) must be equipped with an expansion vessel . For example, if you have a c.w.u. (boiler) with a capacity of 120 liters, it is recommended to match it with a vessel of approx. 10% of the tank capacity - in this case 12 liters . This volume will take over the increased volume of water heated in the boiler, preventing water from dripping from the safety valve each time it is heated. Central heating systems also contain a heating medium that is subject to expansion, so for safety it is necessary to use a diaphragm vessel adequate to the capacity and parameters of the system.

Why choosing the right cookware is important? A vessel that is too small will not be able to accommodate all the expanding water - the pressure will quickly rise to the level of the safety valve tripping and the water will be vented, resulting in losses and the risk of the system being aerated. An oversized vessel, on the other hand, is an unnecessary expense and takes up space (although, of the two evils, it's better to oversize a bit than undersize). A properly selected vessel ensures economical and trouble-free operation of the heating system: it maintains the pressure within the recommended range, reduces the frequency of water refilling in the circuit and protects system components from pressure overloading. In the following tutorial, we will show you step by step, how to choose the right diaphragm tank to your installation - starting with basic selection criteria and ending with an overview of available models (m.in. reflex dishes n, S, DE, DD, DT series) and installation tips.

Detailed technical aspects - step-by-step selection of diaphragm vessel

Key parameters for selecting a diaphragm vessel

The selection of a diaphragm vessel is worth carrying out carefully, taking into account technical parameters of the installation. Specialists have appropriate tables and formulas, but the basic factors that you too can pay attention to are:

• Water volume of the system (volume of the heating medium) - the more water in the system, the greater the amount of water will expand during heating. This is one of the key factors affecting the required liters of the vessel . Indicative for a domestic c.o. with a capacity of ~200 liters of water, the selected vessel can have approx. 18 liters of capacity (assuming typical operating pressures). On the other hand, a small combination boiler with only a few liters of water may require a vessel of only a few liters (often built into the boiler at the factory).

• Maximum increase in water volume (expansion coefficient) - depends on temperature system operation and the type of factor. Water heated from 10°C to 80°C, for example, will increase in volume by about 3-4%. The propylene glycol mixture (used in solar panels) expands slightly differently and has a higher compressibility, so a larger vessel is often recommended for solar panels. It is necessary to estimate the maximum temperature of the medium in the system (for example, 70°C in c.o., 90°C in solar) and, based on this, determine what fraction of the initial volume of water will need to be stored in the vessel.

• Initial pressure (initial pressure in the vessel) - is the pressure to which the gas chamber of the vessel is pre-inflated (given by the manufacturer, usually ~0.5-4 bar, often approx. 1.5 bar factory). This pressure should be adjusted to the static height of the installation (water column). For example, 10 m height corresponds to 1 bar of water column pressure . The rule of thumb is that the pre-pressure of the vessel should be equal to or slightly lower than the static pressure at the connection point of the vessel, so that when filling the system, the diaphragm will allow some water into the vessel. If the pre-pressure is too high, the vessel will remain empty (will not perform its function); if too low - too much water will enter the vessel already cold, reducing its active capacity. Attention: before starting a new installation, it is often necessary to pump or bleed a little gas from the vessel to set the correct pre-pressure (e.g., for a one-story house ~0.9 bar, for a two-story house ~1.5 bar, etc.).).

• Safety pressure (safety valve setting) - standard in c.o. a 2.5 or 3 bar safety valve is installed, in installations c.w.u. often 6-8 bar, and in high-pressure industrial systems even higher. The maximum operating pressure of the vessel must be higher than the setting of the valve, and its capacity must be selected so that when this pressure is reached, the water still does not spill out of the valve. In other words - we select the vessel so that at the permissible highest pressure in the system it still has a reserve capacity. For example, if the valve in c.o. opens at 2.5 bar, and the initial pressure is 0.5 bar (open system during filling), then the vessel must accommodate the volume increase before the pressure rises from 0.5 to 2.5 bar. Conversely, if the system at rest is already 1.5 bar (e.g., tall buildings), and the valve is 3 bar - the difference is smaller, so for the same volume of water you need a larger vessel (smaller pressure buffer).

• Type of installation and medium - A different type of vessel will be selected for the system central heating system, different for hot water, and yet another for solar installation or refrigeration. This is mainly about the membrane material and the design of the vessel. For drinking water, hygienic approvals are required and often a flow-through design (to avoid stagnant water) - about that in the next section. For solar systems, the diaphragm must be able to withstand higher temperatures and the presence of glycol, and for hydrophores (booster systems) the vessels must withstand high pressures of 6-10 bar of continuous operation. Manufacturer Reflex offers a range of models to suit a variety of applications - from classic cookware c.o. to specialized ones for solar and domestic water - which we will discuss below.

Summary, to select a diaphragm vessel: first determine for what type of installation will be applied (c.o., c.w.u., solar, etc.), then collect data on pressures (lift height, safety valve setting) and water volume arranged. On this basis (e.g., using a selection calculator or manufacturer's table), select the capacity of the vessel. If you do not feel confident, it is worth consulting an installer - incorrect selection can result in problems in the operation of the system . In the next section we present an overview of the types of diaphragm vessels available on the market and their applications, which will help you choose a specific model.

Types of diaphragm vessels - overview of Reflex models (N, S, DE, DD, DT)

Company Reflex (part of the Winker Group) is one of the leading manufacturers of diaphragm vessels and offers a full spectrum of solutions for various applications. Below we compare the most popular types of diaphragm vessels reflex/Refix brands and their functions and purposes:

Legend: Reflex - trade name of series to c.o./solar; Refix - the name of the series for domestic water (colloquially, however, they are often all called "Reflex vessels"). Tank colors: blue - standard DHW, green - flow DHW/hydrophore, gray (or white) - c.o./solar.

The table above shows the main features of the different models. In short, if you are looking for a vessel to heat c.o. - choose Reflex N (and for solar installation - Reflex S). If you need a vessel for hot water at the boiler - a typical choice would be Refix DE (or Refix DD, when hygiene requirements are important). For hydrophore systems and large water supply systems, the largest tanks are used Refix DT.

Examples of dish selection in practice: To illustrate, below are two sample scenarios and solutions:

• Example 1: Single-family house with c.o. with a water capacity of ~200 l, 20 kW gas boiler with built-in 8 l vessel, installation height of 5 m (0.5 bar). Here we calculate ~18 liters of total capacity is needed for stabilization . Since the boiler already has 8 liters, just add an external vessel approx. 10 l (it will work well with Reflex N 12 l, because the nearest higher capacity) . As a result, the total capacity of 20 L will protect the system - the pressure will stay below 2.5 bar.

• Example 2: c storage tank.w.u. 300 l in a multi-family building, cold water pressure from the network 4 bar, safety valve for 6 bar. When water is heated from 10°C to 60°C, the volume increase will be approx. 3%. Compensation of ~9 l of water should be provided for. Here it would be recommended 35-50 l vessel, to accommodate this volume at 4→6 bar of pressure increase. Suitable will be, for example. Refix DE 50 l 10 bar (non-flow) installed on the cold water supply to the boiler. However, if it is an installation with high hygienic requirements (such as a hospital), a better solution would be to use two vessels connected in parallel Refix DD of 18 l each(flow), which will ensure the absence of stagnant water.

Installation and operating instructions

Correct installation of the diaphragm vessel is as important as its selection. Here are some practical installation tips:

• Place of connection: In the installation heating c.o. the diaphragm vessel is recommended to be connected at the return (cold return branch), as close to the boiler as possible, but behind the circulation pump (on the suction side, where the pressure is lower and more stable). Often ready-made security groups boiler - these are wall-mounted or boiler-mounted consoles with a safety valve, pressure gauge and spigot for connecting a vessel . In the case of solid fuel boilers operating in a closed system, the vessel should be connected directly to the stub of the boiler - without any shut-off valves between (for security) . In the installation c.w.u. the vessel is installed on the cold water pipeline before the heater (boiler) - usually right next to it on the tee . A check valve must not be installed between the boiler and the vessel (it is to be on the cold water supply to the boiler), because it would cut off the path of water expansion to the vessel.

• Mounting orientation: Standard diaphragm vessels are adapted for installation vertical - usually with the spigot facing down (then the diaphragm is at the top). This position ensures the longest membrane life. Some small models (up to approx. 25 l) can be mounted on the wall with a bracket or mounting tape - make sure they are firmly attached to the wall, as filled with water they weigh a lot. Manufacturer Reflex offers dedicated clamps and straps for mounting 8-25 l vessels on the wall . Horizontal mounting is not recommended unless the manufacturer stipulates such a possibility for a particular model (this applies mainly to some Reflex C flat dishes for special applications).

• A shut-off valve with a dehydrator: To make it easier to service the vessel, it is worth installing a special shut-off valve with a drain (so-called service connector). It allows you to disconnect the vessel from the system and drain it to check the gas pressure or replace the tank, without draining the entire system. It is important that this valve has a lock against accidental closing (in accordance with EN 12828), for example, in the form of a removable handle - so that no one unscrewed would cut off the vessel from the system during normal operation (which would risk a pressure explosion). There are ready-made connection kits with a shut-off valve and drain available on the market, such as the type Reflex SU 3/4" that comply with safety requirements.

• Access and replacement: When installing the vessel, leave some space around it - especially if it has a replaceable diaphragm or you plan to replace the entire vessel in the future. Remember that the life of the membrane is not infinite. Manufacturers recommend checking the condition of the vessel at least once a year. If you notice that the initial pressure drops (despite the pumping up) or the vessel fills up with water all over - this is a sign that the diaphragm may have been damaged (ruptured). In vessels with a non-replaceable membrane (e.g., small Reflex N), it will then be necessary to replace the entire tank with a new one. On larger models with a replacement diaphragm, you may be tempted to purchase a repair kit - a new diaphragm - and replace it according to the service instructions.

• Pre-pressure control: It is a good habit to periodically check and correct the gas pressure in the vessel (at least once a year, such as before the heating season). To do this, you need to: turn off the boiler/pump, disconnect the vessel from the system (by closing the service valve) and completely empty the water from the vessel. Then measure the pressure in the gas chamber (on the car valve at the top of the tank) with a pressure gauge and compare with the required one. If it's too low - inflate with a pump or better with nitrogen due to lower permeability through the diaphragm; if it's too high - drain some air. The preset pre-pressure should correspond to the static height plus a small margin (e.g., 0.2 bar less than the filling pressure of the system). Finally, open the shut-off valve, allow any missing water into the system, and you're done.

The most common mistakes in the selection and use of diaphragm vessels

Although the principle of diaphragm vessels is simple, in practice there are errors in selection and operation. Here's a list of the most common mistakes - it's worth it avoid:

• Undersizing of the vessel - too small diaphragm vessel is one of the most common mistakes. This manifests itself in a constant increase in pressure to the maximum when the boiler is heated up and the safety valve is frequently tripped (water leakage). Solution: always calculate the required capacity with a certain reserve, and in case of expansion of the installation (addition of radiators, a larger boiler, etc.), you will be able to find the required capacity.) check whether the existing dish is still sufficient - if not, add an additional one or replace it with a larger one.

• Oversizing the vessel - Although less threatening, it can be an unnecessary expense and take up space. A vessel that is too large can also cause minimal pressure fluctuations (the system will be very "soft"), which sometimes makes it difficult to vent or diagnose leaks. Try to choose the capacity reasonably for your needs - it does not make sense, for example, an 80-liter vessel for a 10 kW boiler with several radiators (where 18 l would be fully sufficient).

• Unsuitable type of vessel for the medium - vessel confusion c.o. i c.w.u. may result in faster membrane damage or water contamination. Dishes for heating usually have membranes made of rubber not intended for contact with drinking water - they can, for example, emit odor or taste into the water, and do not have an internal anti-corrosion coating on the walls. Conversely, the vessel to c.w.u. used in c.o. will work, but it is more expensive (because it has additional approvals) and often has a higher factory pre-pressure (e.g., 4 bar), which requires its reduction. Therefore, use the utensils as intended: Reflex/Refix to c.o./solar versus Refix (sometimes green or blue) for drinking water.

• No flow in vessel c.w.u. when the required - in larger drinking water systems (especially above 300 l storage tank), regulations may require flow-through diaphragm vessels for hygienic reasons. It is a mistake to use an ordinary non-flowing vessel (Reflex DE type) where there should be Refix DD/DT. This can result in stagnant water in the tank and microbial growth. Always check the requirements of standards (e.g., DIN 1988 in public facilities) and, if necessary, use diaphragm vessels with flow through the vessel.

• Incorrect initial pressure - a common mistake is to leave the vessel with a factory pressure that does not match our installation. As a result, the vessel may not work properly: for example, at a pre-pressure higher than the pressure of cold water in the system - the diaphragm will not yield and "let in" water, which is equivalent to the failure of the vessel to work. Or, conversely, too low a pre-pressure will cause the vessel to fill with water immediately when filling a large portion, so that it loses its active capacity and fills up more quickly when heated. Solution: Always match the gas pre-pressure to the requirements of the installation (see section above on pressure selection). Check them with a pressure gauge before starting the system - often factory vessels have ~1.5-4 bar, which may not match your conditions.

• No periodic service of the vessel - many people forget about the diaphragm vessel for years, meanwhile, it is worth checking from time to time that it is working properly. It is a mistake to wait until the safety valve begins to let go of water - this is a sign that the vessel has most likely lost its air cushion (for example, due to a ruptured diaphragm). Regular inspection and maintenance of proper initial pressure will detect the problem earlier and prevent flooding of the boiler room. In industrial installations, sometimes two vessels are used in parallel - one active, the other as a backup - to allow service without interrupting system operation. At home, simply take a few minutes once a year to check the tank.

• Closing the shut-off valve with the system running - if you have a shut-off valve at the vessel, make sure that it is always open during installation operation. Accidentally cutting off a vessel (for example, mistakenly after an inspection) will result in the system having nowhere to compensate for the pressure build-up - which will very quickly end in the tripping of the safety valve or, in the extreme case, the unsealing of some component. It is a good idea to use valves with a safety feature (removed handle, padlock, etc.).), so that no outsider would screw it up.

Summary

Selecting the right diaphragm vessel for your heating (or water) system is critical to the safe and trouble-free operation whole system. First, you need to determine the parameters of the installation - water capacity, temperature range, operating pressures - and then, on this basis, choose the type and size of the vessel. Remember that different models are designed for central heating than for domestic water (although they operate on a similar principle, they differ in membrane design and materials). On offer Reflex you will find a full range of vessels: from the popular Reflex N to c.o., through Reflex S for solars to the specialized Refix DE/DD/DT for drinking water. This makes it possible to select a product perfectly suited to your needs - whether it's a small domestic boiler or an extensive installation in an industrial building.

We hope our step-by-step guide will help you choose the right expansion tank. Properly selected and installed diaphragm vessel guarantees stable pressure, protects your boiler and pipes from damage and minimizes water loss. If you are looking for proven solutions, check our offer of Reflex diaphragm vessels- you will find models to suit any installation. Don't delay in protecting your installation - choose the right diaphragm vessel and enjoy safe and efficient heating!

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