Installation of a thermal storage tank – procedure, connection and placement in the boiler room
Installation of an accumulator tank – procedure, connection and placement in the boiler room
An accumulator tank is the heart of every modern heating system using solid fuel, heat pumps or solar collectors. However, even the best heat storage tank only works properly when it is correctly installed, hydraulically connected and physically placed in the boiler room. In practice, we see many installations where the selection of the tank was well thought out, but the installation itself was underestimated – whether it was incorrect positioning, incorrect hydraulic connection, or poorly dimensioned pipe layouts. The result? The tank does not function as it should: heat does not balance, the boiler short-cycles or heat from the collectors is lost before it is even stored.
This article will guide you through the entire process – from preparing the boiler room and selecting the location through hydraulic connection schemes to commissioning and the first pressure test. If you are looking for an answer to the question of what tank volume you need, read the article What volume of accumulator tank do I need for my boiler or heat pump. Here we will focus purely on the physical installation and correct connection to the system.
Preparation before installation – what you need to know before arriving on site
Every installation of an accumulator tank starts long before the first wrench is opened. It is necessary to have clarity in several key issues:
Dimensions of the tank versus accessibility of the boiler room
Accumulator tanks are cylindrical and usually tall. A tank with a volume of 500 liters has a typical height of about 1,650–1,800 mm and a diameter of 650–750 mm. A 1,000-liter tank can have a height of up to 2,000 mm and a diameter of 850–900 mm. This is critical: the doors of the boiler room, possible stairs or corridors must allow the tank to be transported without dismantling building structures. In practice, this is not so obvious – we have often dealt with situations where the tank was ordered, delivered, and could not pass through the doors in the boiler room. The solution is either to order a tank with a smaller diameter (there are so-called slim versions, e.g. Accumulator tank PUFFER PSS 100 – 123 l), or in extreme cases to solve the entry through a window or a removable wall. Measure twice, order once.
Floor load capacity of the boiler room
Water has a density of 1 kg/liter. A tank with a volume of 800 liters weighs 800 kg when filled plus the weight of the steel tank itself (usually 80–150 kg). In total, we are talking about 900–950 kg on an area of about 0.5 m². This is a load that the floor of the boiler room must be dimensioned for. A standard residential structure in panel buildings usually withstands 150–200 kg/m², which is not enough for a concentrated load. Before installation, check the structural integrity – if you are not sure, consult a building technician. A concrete floor in a family house is usually fine, but never take it for granted.
Project, inspections and permits
When installing new systems in houses with central heating, the installation of an accumulator tank is part of the heating system and is subject to inspection by an authorized person. Systems with pressure vessels (expansion tank, safety valve) are pressure equipment – when operating pressures exceed 0.5 bar and the content exceeds a certain limit, it may be necessary to prepare a pressure equipment and its registration. In practice, most domestic systems do not exceed these limits, but for larger equipment (over 1,000 liters, pressure over 3 bar) address this in advance.
Selection and preparation of the location in the boiler room
The location of the tank in the boiler room is not random. There are several rules whose compliance significantly affects the efficiency of the entire system.
Gravity and height of connection
The accumulator tank should be placed so that the upper outlet (hot water, flow) is hydraulically as close as possible to the boiler and consumers. Most tanks have heating circuit outlets in the upper and lower parts – the upper part is the hottest (stratification), the lower part the coldest. The boiler should supply the lower part of the tank and draw hot water from the upper part – the greater the temperature difference between the layers, the better the overall efficiency. This is especially important for heat pumps, because a low water temperature at the TČ input means a higher COP.
Distance from the boiler and consumers
The shorter the hydraulic connections between the boiler, the tank and the circuits, the lower the heat losses and hydraulic resistances. Ideally, the tank should be as close as possible to the boiler – in the same room. Long lines (more than 5–6 meters) must be insulated with quality insulation (at least 30 mm Armaflex or rigid mineral wool) and pressure losses must be considered when dimensioning the circulation pump.
Access for service and maintenance
There must be free space of at least 500–600 mm on each side of the tank where access is required – for anode replacement, possibly electrical flange or service valves. We recommend at least 700 mm on the side where the inspection opening is located. For more information on anode replacement, see the article Protective anode in the accumulator tank – what it is, when and how to replace it.
Hydraulic connection of the buffer tank – step by step
This is the core of the entire installation. A correct hydraulic connection determines whether the tank will actually function as an energy storage or just as another element in the line that does nothing. We will break the process down into logical steps.
Step 1 – Installing the tank in place
The tank is placed in a vertical position (the manufacturer requires this for almost all types). Place anti-vibration pads of 10–15 mm thickness under the tank – they reduce noise and vibration transmission and also protect the steel bottom from direct contact with concrete, where corrosion could occur. Level the tank – the deviation from vertical should not exceed 1–2°. For some types with an integrated flange for an electric heating element, the correct vertical position is a condition for the operation of thermostats.
For more compact installations, for example in smaller boiler rooms or technical rooms of apartments, the Buffer tank PUFFER PSS 50 – 57 l with smaller dimensions is suitable, which can be placed even in limited space.
Step 2 – Installation of the protective anode
Before any hydraulic connection, check whether the protective anode is installed in the tank. Most tanks have it from the factory, but after installation following a longer storage period, it is worth checking the condition of the anode. If the anode is missing or worn, replace it – for example, Protective anode from magnesium alloy 5/4" × 400 mm with a control device or for larger tanks Protective anode 5/4" × 700 mm. The anode is screwed into the inspection opening in the upper part of the tank with a 5/4" thread. Seal it with Teflon or hemp with sealant – not just Teflon, as it is not sufficient for larger threads. Adhere to the tightening torques specified by the manufacturer, excessive tightening of the thread can damage the neck.
Step 3 – Connecting the boiler circuit (primary circuit)
The boiler circuit is connected so that the output from the boiler (the hottest water) goes into the upper part of the buffer tank, and the return from the lower part of the tank returns to the boiler. This is the basic scheme that must be followed for proper stratification – hot water rises upwards, cold water sinks downwards, and the layers do not mix more than necessary.
Install the following on the supply and return of the boiler circuit:
- Ball shut-off valves – for the possibility of isolating the tank without draining the entire system
- Thermometric probes or contact thermometers – monitoring the temperature at the inlet/outlet is essential for regulation
- Filtration strainers (Y-filter) – on the return side they protect the circulation pumps from impurities
- Check valves – prevent gravitational circulation outside operating hours, which is important especially in heat pump systems
Step 4 – Connecting the consumer circuits (secondary circuits)
Hot water from the buffer tank goes to individual circuits: heating, floor heating, hot water heater, or fan-coils. Each circuit should have its own circulation pump and control valve (thermostatic or motorized mixing valve). Heat is always drawn from the upper part of the tank, and the return flows to the lower part.
For floor heating, where a lower temperature is required (e.g., 35–45 °C instead of 70–80 °C from a wood-fired boiler), a mixing valve with a three-way valve and its own pump is installed. This circuit must be hydraulically separated from the boiler circuit precisely through the buffer tank – this is one of the main reasons for its use.
Step 5 – Expansion tank and safety valve
The expansion tank is a pressure vessel of a closed system. It must be protected by a safety valve (set to the maximum operating pressure of the system, typically 2.5–3 bar for domestic systems). Install the safety valve as close as possible to the tank, on the outlet pipe, with the safety pipe drained into the sewer or a catch basin. Never install closing valves before the safety valve.
The expansion tank must be sized for the total volume of the system including the accumulator tank. If you are adding an accumulator to an existing system, the original expansion tank may be undersized – check and possibly supplement or replace it with a larger one.
Step 6 – Air venting and filling the system
Automatic air vents must be installed at the highest point of each circuit, including the top of the accumulator tank. Air in the system reduces efficiency, causes noise and corrosive damage. When filling the system, proceed slowly – fill from the bottom up, and vent air gradually. We recommend filling the system with soft water or demineralized water with a corrosion inhibitor (e.g. Fernox or Sentinel), especially in systems with steel tanks.
Stratification – the technical basis you must understand
Stratification is a physical phenomenon in which hot water (lower density) remains in the upper part of the tank and cold water (higher density) sinks to the lower part. This is exactly what we want to maximize – the steeper the temperature gradient between the layers, the more efficiently the tank works.
To maintain stratification, it is crucial that the water flow into and out of the tank is as calm as possible. Some tanks have internal diffusers or baffles that prevent turbulent mixing of layers. During installation, ensure that the water velocity in the outlets does not exceed 0.2–0.3 m/s – thus, sufficiently large pipe cross-sections and slow pumps are desirable.
Insulation of the accumulator tank
An uninsulated accumulator tank is a well of heat losses. A steel tank transfers heat to the environment at a rate that depends on the temperature difference between the water inside and the air in the boiler room. At a water temperature of 80 °C and a boiler room temperature of 20 °C, we are talking about a difference of 60 K – this is a very intense heat transfer. Without insulation, the tank can lose 3–5 °C per hour, which means a loss of 30–50 °C overnight – that is almost all the accumulated heat.
Most tanks are delivered with factory insulation (a PU jacket or removable mineral wool wrap). If the tank is not insulated, supplement the insulation with rigid mineral wool of at least 80 mm thickness, covered with aluminum foil or fiberglass. Pay attention to the outlets as well – every pipe passing through the insulation jacket is a thermal bridge. Insulation of the pipes for at least 1 m from the tank is absolutely necessary.
Electric heating element and additional accessories
Many accumulator tanks have a flanged opening (usually DN 89 or DN 65 diameter) for mounting an electric heating element. This serves as a backup heat source – for example, when the wood stove is not in use or when the heat pump is insufficient in strong frost. Details on the selection and installation of the electric heater can be found in the article Electric heating element in an accumulator tank – installation and flange selection.
Another standard accessory is a temperature sensor or a thermostatic control unit. It is advisable to have a separate sensor for each temperature layer (top, middle, bottom), so the control can determine when it is worthwhile to start the boiler or heat pump and when the appliances have enough heat from the tank.
Pressure test and commissioning
After completing all hydraulic connections, before insulating and covering the pipes, perform a pressure test. Fill the system with water and pressurize it to 1.5 times the operating pressure (e.g. at an operating pressure of 2 bar, test at 3 bar). Let the pressure act for at least 30 minutes and monitor whether the pressure drops. Check all connections – threads, flanges, welded joints.
After a successful pressure test, vent the system and fill it to the operating pressure (typically 1.5 bar). Start the boiler or other heat source and monitor how the tank is being charged. Check the functionality of thermostats, regulator and all pumps. We recommend monitoring the first charging and discharging manually and recording the temperatures in the individual layers – it will show you whether the stratification is working and whether the tank is actually accumulating heat.
Most common installation errors with accumulator tanks
From practice, we know that most problems with the operation of accumulator tanks have their roots in installation errors. Here are the most common ones:
- Mixing up the inputs and outputs – the boiler output connected to the lower part of the tank instead of the upper part. Result: cold water goes into the boiler, the tank is not charged properly, stratification does not work.
- Too fast flow – large pumps with high power turbulently mix the layers and destroy the stratification. Proper pump sizing is key.
- Missing or undersized expansion tank – when the water is heated, expansion occurs, pressure rises, the safety valve opens. With every opening of the safety valve, water leaves the system and the system must be refilled.
- Missing protective anode – without an anode, intense corrosion of the steel tank begins, especially in soft or acidic water. After 2–3 years, the tank can be irreparably damaged. For smaller tanks up to 600 liters, a Protective anode made of magnesium alloy 5/4" × 400 mm is suitable.
- Insufficient insulation – heat losses that unnecessarily increase operating costs.
- Forgotten closing valves – without valves before each major component, it is not possible to service the system without completely draining it.
- Inappropriate location for the safety valve – a safety valve behind a closing valve is a fatal error in the safety concept.
Storage tank in combined systems
Modern heating systems rarely operate with a single heat source. A combination of wood-burning boiler, heat pump and solar collectors is not uncommon. Each of these sources connects to the storage tank in a different way and the control logic is also different. For more details on combined systems, see the article Storage tank in a system with a heat pump or solar collector.
In short: a wood-burning boiler or condensing boiler is usually connected directly to the tank via a boiler circuit (without mixing, direct circulation). A heat pump operates at lower temperatures – it can be connected either directly or via an internal heat exchanger inside the tank. Solar collectors operate with a separate circuit using non-freezing fluid and have their own heat exchanger coil at the bottom of the tank.
Service and inspection after installation
After the first heating season, we recommend inspecting the entire installation: check the condition of the anode, verify the function of the safety valve, check the pressure in the expansion tank and the condition of all seals and valves. A detailed guide on regular maintenance can be found in the article How to extend the life of a storage tank – maintenance and regular service.
The anode should be replaced every 2–3 years depending on water hardness and operating intensity. Very soft water (below 8 °dH) is more aggressive and the anode is consumed faster. In such cases, it is worth choosing a longer variant, for example an anode 5/4" × 700 mm, which lasts longer.
Most frequently asked questions (FAQ)
Do I have to install the storage tank vertically, or can I place it on its side?
Almost all standard storage tanks are designed exclusively for vertical installation. A horizontal position prevents stratification, disrupts the function of internal heat exchanger coils and can cause the anode to not be submerged in liquid. An exception are special horizontal buffer tanks designed for low spaces, but these are rare in practice. If you do not have enough height in the boiler room, consider another type of tank (e.g. with a smaller diameter and greater height) or modify the ceiling of the boiler room.
Can I connect two storage tanks in series or in parallel?
Yes, both are technically possible. Parallel connection is more common – two tanks are connected side by side with the upper and lower outlets connected. It is important that the diameters of the connecting pipes are sufficiently large (minimum DN 32–40 for tanks up to 1 000 liters), otherwise one tank will work more than the other. Series connection is used less often – the first tank serves the boiler circuit, the second for consumer circuits. For the correct selection of arrangement, we recommend consulting a designer or read the article How to choose a storage tank – volume, type and connection to the system.
What operating pressure is common for buffer tanks in family homes?
The standard operating pressure for closed heating systems in family homes is 1.0–1.5 bar (static pressure + overpressure). The maximum operating pressure of most common buffer tanks is 3 bar, some have a rating up to 6 bar. The safety valve is set to the maximum allowed system pressure – usually 2.5 or 3 bar. Never operate the system without a safety valve or with a valve set above the tank rating.
Why does my buffer tank cool down quickly during the night, even though it is insulated?
The most common cause is a non-functional check valve on one of the circuits. Without it, gravitational circulation occurs – heat convectively flows from the tank into cold pipes and radiators and returns cold. Another cause may be insufficient insulation thickness of the tank shell or non-insulated connecting pipes. Intentional circulation of the DHW circuit (hot water circulation pipe) also continuously draws heat from the tank. Check the thermometers in the various parts of the system and identify where the heat loss occurs.
Is it necessary to perform a pressure test when replacing an old tank with a new one?
Yes, and it is even more important because during replacement, some connecting pipelines and fittings are typically renewed as well. Every new connection is a potential leak point. A pressure test before covering and insulating the pipes is a standard of proper installation – it detects leaks while they are still easily repairable. Document the test (pressure, time, result) – it may be useful in the case of a warranty claim or insurance incident.
Can a buffer tank operate in a system without a circulation pump, gravitationally?
Theoretically, yes in old gravity heating systems, but in practice, for modern buffer tanks with multiple circuits, heat exchangers, and regulation, gravity circulation is absolutely insufficient. Pipe diameters, heights, and capacities are unsuitable for gravity. At least one circulation circuit (boiler) must have its own pump. Gravity circulation can be a backup function during power outages in simple single-source systems, but it is not a primary solution.
Conclusion
The installation of a buffer tank is not a complex process, but it requires thorough preparation, knowledge of hydraulics, and adherence to priorities: the correct position of the tank, the correct orientation of circuits respecting stratification, system protection with a safety valve and expansion tank, a functional anode, and quality insulation. Each of these requirements has a direct impact on how well the entire system will work and how long the tank will last.
If you are also interested in choosing the right type and volume of tank, see the overview of available solutions in the category buffer tanks and storage tanks or read more articles in our Knowledge Center – for example Buffer vs. hot water storage tank – what is the difference and when to use which or Common faults of buffer tanks and storage tanks – causes and solutions.
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