Electric heating element in a storage tank – installation and flange selection
Electric heating element in an accumulator tank – complete guide to installation and flange selection
Installation of an electric heating element (heating rod) into an accumulator tank is a task that at first glance appears simple, but in practice, a whole range of errors are made – from incorrect flange selection through unsuitable power to improper sealing. In this article, I will cover the topic in great detail: from the technical background, through the selection of the correct flange, thread diameter, power, sealing material types, to the step-by-step installation process. If you are interested in a broader context – for example, the difference between a buffer tank and a hot water storage tank or the installation of the entire tank in the boiler room – you will find these topics in other articles in our Knowledge Center.
Why do we even put a heating element into an accumulator tank?
An accumulator tank (buffer tank) serves the role of a thermal energy buffer in the system. The primary source of heat is usually a boiler, heat pump, or solar collector. Despite this, the installation of an electric heating element into the tank is justified for several reasons:
- Backup power source – in the event of a boiler failure or heat pump service, electricity can maintain a minimum temperature in the system, or heat hot water (if the tank is combined with a storage tank).
- Bivalent system – during transitional periods, when the heat pump is not sufficient on its own, the electric element complements the missing power (so-called electric boosting).
- Night electricity – in tariffs with night rates (HDO), it is reasonable to charge the buffer tank with cheap electricity at night and draw stored heat during the day.
- Summer mode – in summer, when solar collectors are not sufficient to cover the need for hot water, electricity serves as a supplement without the need to start the boiler.
Unlike a hot water storage tank (boiler), where the heating element is almost always part of the basic equipment, buffers are delivered in the basic version without an electric heating element. The flange (screwed or threaded) is prepared, but you choose the heating element yourself according to specific requirements. This is an advantage – you can precisely adjust the power, type and material to your application.
Types of flanges on accumulator tanks – what you find in practice
The flange is a connecting element that allows the heating element to be inserted into the tank shell so that the connection is watertight and mechanically strong. On accumulator tanks, you will encounter several types:
1. Threaded flange (screwing)
The most common type on smaller and medium tanks up to about 500 liters. The heating element has an external thread (most often G 1½" or G 2") and is inserted into the tank with a threaded plug or nut with a corresponding internal thread. Sealing is achieved with PTFE (teflon) sealing tape, hemp with glue, or a rubber O-ring according to the design.
2. Flanged connection (flange)
Used on larger tanks (usually from 300 liters and above) and with more powerful heating elements. The flange is welded to the tank shell, the heating element has a corresponding opposite flange, the connection is sealed with a flat gasket (rubber, EPDM, graphite) and tightened with bolts. Advantage: the heating element can be easily replaced at any time without special tools. Disadvantage: higher price and greater space requirements during installation.
3. Welded flange (welded flange)
A standard structural element of larger tanks. It is a steel ring welded to the shell, with drilled holes for bolts. The diameter and number of bolts vary according to the manufacturer – common are flanges DN 50 (5/4"), DN 65, DN 80 and DN 100. The heating element must have the same flange size and hole spacing.
Flange and thread dimensions – what do the abbreviations and numbers mean
When selecting a heating element, you encounter a number of markings that you need to be able to read correctly. It depends on them whether the element physically fits into the tank.
Thread G (BSP – British Standard Pipe)
This is the most common thread on heating elements and flanged connections of buffer tanks in Central Europe. The number after "G" indicates the nominal diameter in inches – not the actual outer diameter of the thread! For example:
- G 1" – outer diameter of the thread approx. 33.25 mm
- G 1¼" – outer diameter of the thread approx. 41.91 mm
- G 1½" – outer diameter of the thread approx. 47.80 mm (very common on medium-sized tanks)
- G 2" – outer diameter of the thread approx. 59.61 mm (common on larger and more powerful units)
Important: the G thread (parallel) must not be confused with the R thread (Rp, tapered). Tapered threads are practically not used on heating elements, but when ordering a nut or reducer, always verify what type it is.
Flanged dimensions DN
With flanged connections, you will encounter the designation DN (Diameter Nominal – nominal diameter). For a heating element in a buffer tank, the following dimensions are common:
- DN 50 – small and medium tanks, power up to approx. 3 kW
- DN 65 – medium tanks, power 3–6 kW
- DN 80 – larger tanks, power 6–12 kW
- DN 100 – high-power applications, sometimes combined units
With flanged connections, you must verify not only the DN, but also the bolt pitch and the number of bolts, which are standardized by EN 1092-1, but not always identical for each manufacturer. Always measure the existing flange on the tank before ordering a heating element.
Heating element power – how to choose it correctly
This is the most discussed question in practice. Customers ask me: "How many kW should I put into the buffer?" The answer depends on the function the electric element is to perform.
Heating element as a backup source
If it is only a backup in case of boiler failure, a lower power is sufficient – usually 1.5 to 3 kW. You do not need to quickly heat the entire buffer, it is enough to maintain a minimum temperature. For a 300-liter tank, a 2 kW element is sufficient to heat water from 20 °C to 60 °C in about 4–5 hours.
Heating element as the main electric source (night tariff)
Here you want to charge the buffer as quickly as possible during the cost-effective night tariff (usually 8 hours HDO). For a 500-liter buffer and a temperature difference of 40 °C (from 20 °C to 60 °C), you need:
Energy = volume × density × specific heat × ΔT = 500 × 1 × 1.163 Wh/(l·K) × 40 K ≈ 23 270 Wh ≈ 23.3 kWh
To cover this amount in 8 hours: P = 23.3 kWh / 8 h ≈ 2.9 kW. In practice, efficiency and tank heat losses are considered, so a 3 kW element is sufficient. For faster charging (4 hours), you would need 6 kW.
Heating element as a boost in a bivalent system
In a system with a heat pump or solar collector, the electric boost is dimensioned as a supplement when the primary source lacks energy. Typically, it is 2–6 kW. For larger homes with an 8–12 kW heat pump, an electric boost of 4–6 kW is a reasonable compromise.
Electrical installation – power requirements
Do not forget that a heating element over 2 kW requires a circuit breaker of corresponding nominal current. A 3 kW heating element at 230 V draws about 13 A → 16 A circuit breaker. A 6 kW heating element at 400 V (three-phase) → 8.7 A per phase → 10–16 A circuit breaker. Three-phase has the advantage of balanced network loading. For households with a single-phase connection, the practical limit is 2.0–3.5 kW with a 16 A circuit breaker.
Heating element material – what resists aggressive water and what doesn't
In buffer tanks for heating (buffer for heat pump systems), treated or demi water with corrosion inhibitors is usually circulated. In DHW storage tanks, it is potable water. The heating element material must be suitable for this.
Brass and galvanized elements
The cheapest option. Suitable for buffers with heat pump water, where pH is maintained in the range of 8–9 and the water does not contain aggressive chlorides. Not suitable for DHW storage tanks in areas with hard water – limescale deposits shorten the lifespan, the element gets clogged and loses performance.
Stainless steel elements (AISI 316)
For DHW storage tanks, stainless steel 316 is a significantly better choice. Resistant to chlorides up to about 200 mg/l, suitable for most municipal waters. The price is 2–3× higher than brass, but the lifespan is significantly extended.
Titanium elements
Premium category for vessels with aggressive water (high chloride content, seawater, industrial applications). You won't encounter them in regular household use.
Elements with a protective glazed layer
Some products have a glazed surface – very good protection against corrosion, but sensitive to mechanical damage. If the glaze is damaged (e.g., by impact during installation), the corrosion process can be faster than with plain steel.
For a standard heat pump buffer such as the buffer tank PUFFER PSS 100 (123 liters, suitable for small boilers and heat pumps), a brass or galvanized heating element is a standard and sufficient choice, provided the water in the system is properly treated.
Sealing during installation of the heating element
Proper sealing is fundamental – a poorly sealed heating element can lead to water leakage into the electrical part, short circuits, or slow flooding of the tank area. This typically happens after years of operation, when the rubber hardens.
PTFE tape (Teflon)
The simplest solution for threaded connections. Wind the tape in the direction of the thread (when looking at the element from the right, winding clockwise). At least 5–6 layers on a clean, degreased thread. Disadvantage: at higher temperatures (above 90 °C), the tape can float and the seal weakens – it is suitable mainly for temperatures up to 70–75 °C.
Flax with sealant
A traditional method, still valid. For buffer tanks with water temperatures above 70 °C, flax with an anaerobic sealant (Loctite 55, Unipak) is more reliable than PTFE tape. Procedure: mix flax fibers into the sealant, wind them onto the thread in the correct direction, tighten, and let it cure for at least 2 hours before filling the system.
Rubber seal (O-ring or flat gasket)
For flanged connections, a flat ring gasket made of EPDM (for hot water up to 140 °C), silicone (up to 200 °C), or graphite (for higher temperatures) is used. The gasket must have the correct inner diameter matching the flange hole and the appropriate thickness. Before installation, check the flange seating surfaces – they must not be cut, scratched, or corroded.
Step-by-step installation of the heating element
Here I describe a practical procedure that everyone should follow when installing a heating element into a buffer tank. It does not matter whether it is a small buffer tank PUFFER PSS 50 (57 liters) or a larger system.
Step 1: Shut down and drain the tank
Before any work on the tank, close all inlet and outlet valves. Open the air vent (at the top of the tank) and drain the water through the drain valve at the bottom of the tank into the sewer or a bucket. Never open a flange under pressure – existing water can be hot (up to 60–80 °C!) and cause serious burns. Check with a pressure gauge that the system pressure is zero.
Step 2: Prepare the flange and thread
Check the flange seating surface and the thread on the tank. Thoroughly remove old sealing material (flax, PTFE) – with a wire brush, scraper, or possibly fine sandpaper. Leftover old sealing material causes leakage in the new joint. Degrease the thread or flange surface.
Step 3: Prepare the heating element
Check that the heating element is new and undamaged. Check that the type (threaded/flanged) and dimensions match the hole on the tank. Check the supply cables – they must be undamaged, without cracked insulation. If you are installing a flanged element, attach a new gasket (never use an old one – always a new one!)
Step 4: Installation
For a threaded element: wind the sealing material (PTFE or flax), screw it in by hand and then tighten with a wrench. Pay attention to the tightening torque – follow the thread size:
- G 1½": 50–60 Nm
- G 2": 70–90 Nm
Never over-tighten – cracking of the thread or the tank neck is a serious (and expensive) fault. For a flanged element: insert the gasket, attach the element, fit the screws and tighten in a cross pattern in two to three rounds evenly. Uneven tightening → the gasket will only seal partially.
Step 5: Electrical connection
The electrical connection must be performed by a qualified person or electrician with a valid qualification. The heating element is a device of class IP55 or higher, but the terminal box must be covered after installation. Ensure that no conductive part touches the wet surface of the tank. For a three-phase element (400 V), connect the phases evenly.
Step 6: Filling and leak check
Slowly open the water supply to the system and let the tank fill. While filling, visually check the area of the heating element flange. After filling and venting the system, increase the pressure to the operating level (usually 1.5–2.5 bar) and monitor the joint area for 15–30 minutes. Only after confirming the tightness, switch on the electricity.
Step 7: First start-up
Set the thermostat of the heating element to the desired temperature (for a heat pump buffer typically 55–75 °C, for a DHW storage tank 60–65 °C, minimum 60 °C to prevent legionella). Check that the thermostat cuts off the heating at the correct temperature – use a contactless thermometer on the pipe at the tank outlet.
Most common mistakes during installation of heating elements
Over the years of practice I keep seeing the same mistakes. Here are the most common ones:
- Wrong flange size – the customer orders a G 2" element, but the tank has a G 1½" opening. Solution: always measure before ordering, never estimate.
- Over-tightened thread – too much force during tightening can crack the cast iron neck or deform the thread in a stainless steel tank. Use a torque wrench.
- Old PTFE tape from the previous installation – they leave the old tape and install a new one on top of it. Result: unstable sealing, it will leak after some time.
- Electrical connection before filling – a heating element turned on "dry" will be destroyed in a few seconds. The heating element must always be completely submerged in water before the first switch-on.
- Missing protective anode – if the tank is equipped with a magnesium (magnesium) anode, sometimes customers skip replacing the anode or checking it during the installation of the heating element. Corrosion then concentrates on the heating element itself. More on this topic can be found in the article "Protective anode in an accumulator tank – what it is, when and how to replace it."
- Inappropriate power for the tariff – the customer installs a 6 kW single-phase element, but the household has a 25 A circuit breaker → when the heating and other appliances are turned on, a power outage occurs. Always consult with an electrician.
Protective anode and its relation to the heating element
The topic of anodes is discussed in detail in a separate article in our Knowledge Center ("Protective anode in an accumulator tank – what it is, when and how to replace it"), but here we will just recap what is directly related to the heating element.
An accumulator tank with a steel body (enamelled or bare steel) is subject to electrochemical corrosion. A magnesium (magnesium) anode acts as a sacrificial metal – it corrodes instead of the tank shell and the heating element. If the anode burns out, corrosion will transfer to the next metal in the system – and that is often the heating element itself, because it is electrically conductive and immersed in an electrolyte (water).
Therefore, the rule applies: every time you replace the heating element, also check the condition of the anode and replace it if in doubt. For example, there is a protective anode 5/4" × 400 mm with a control device suitable for medium-sized tanks, or for larger tanks a protective anode 5/4" × 700 mm. The replacement procedure for the anode is the same as for the heating element – shut off, drain, replace, refill, check for tightness.
Thermostat and safety components of the heating element
Every heating element for an accumulator tank should have an integrated:
- Operating thermostat – adjustable (usually 30–85 °C), regulates the water temperature in the tank. Set the temperature according to the purpose: for a TÚV tank at least 60 °C (protection against legionella), for a buffer tank 55–75 °C.
- Emergency safety thermostat (STB) – disconnects power when the maximum temperature is exceeded (usually 90 °C). After intervention, it must be manually reset. Never take it out of operation!
- Pressure relief valve – not part of the heating element, but must be in the system. For a TÚV tank, the pressure relief valve (6 bar) must be installed on the cold connection and must not be shutoff.
Important: a heating element without a functional emergency thermostat is a fire hazard. It is recommended to check that the STB is working properly before each season – by intentionally increasing the temperature above the set limit.
Approvals, standards and legislation
A heating element for an accumulator tank must meet:
- CE marking – European approval for electrical appliances (Low Voltage Directive 2014/35/EU, EMC 2014/30/EU)
- IP rating – at least IP55 for a humid boiler room environment, IP65 and higher for applications where direct water spray is possible
- Protection class – Class I (protective conductor PE) for all connection terminals
- STN EN 60335-2-73 – safety requirements for stationary immersion heaters
From the installation point of view: electrical connection must be performed only by a person with professional competence according to the Government Regulation No. 508/2009 Coll. (§ 22–24). For the average household user, this means that you can do the hydraulic installation (sealing, tightening) yourself, but the connection to the electrical network must be done by an electrician.
Practical scenarios from practice
Scenario 1: Old wood-burning boiler + 500 l buffer tank, customer wants an electric backup source
The house is heated by a wood-burning boiler, the customer is going on vacation in September (transition period). The boiler is not to be fired. We install a 2 kW electric heating element in the buffer tank, set the thermostat to 45 °C. The element consumes about 2–4 kWh per day to maintain the temperature (depends on the tank insulation and outside temperature). The customer is calm, the system is functional, the lines are not oxygenated.
Scenario 2: Air-to-water heat pump + electric booster
The heat pump 8 kW COP 3 works well down to –5 °C, performance drops at lower temperatures. The customer wants to be sure for winter. We install a three-phase heating element 4.5 kW (3 × 1.5 kW) in the 800 l buffer tank. The thermostat is set to 55 °C. Electricity is turned on only when the heat pump is not sufficient – in practice, this is about 20–30 days a year during extreme cold. The electricity costs are minimal, but the comfort is maximum.
Scenario 3: TÚV tank + boiler failure in winter
The customer has a combined TÚV tank and solar collector. In January, the boiler was under service repair for 5 days. A 2 kW heating element (forgotten since installation, never used) saved the day – without it, there would be no hot water. A note in conclusion: a heating element in a TÚV tank must be manually verified every year (turn on, wait for heating, turn off). A non-functional thermostat or STB will only show itself when you actually need it – and then it is too late.
Choosing a heating element – summary of decision parameters
| Parameter | What to consider | Typical values |
|---|---|---|
| Type of connection | Thread or flange on the tank | G 1½", G 2" / DN 50–DN 100 |
| Power | Backup / booster / main source, tariff | 1.5–6 kW (household) |
| Voltage | Single-phase 230 V or three-phase 400 V | 230 V up to 3.5 kW / 400 V above 3.5 kW |
| Material | Type of water – ÚK / TÚV / aggressive | brass, stainless steel 316, enamel |
| Length of the element | Must fit the depth of the tank | 250–800 mm (measure!) |
| Thermostat | Operating + emergency (STB) | 30–85 °C + STB 90 °C |
| IP rating | Boiler room environment | Min. IP55 |
Most Frequently Asked Questions (FAQ)
Can I install a heating element in a buffer tank myself, without an electrician?
The hydraulic part (screwing in, sealing) can be done by a layperson yourself, if you follow the correct procedure and the pressure system is not compromised. The electrical connection – that is, connecting to the distribution board, circuit breaker, cable – must be carried out by a qualified electrician with valid professional qualifications. According to Slovak legislation (Decree No. 508/2009 Coll.), this is work on an electrical installation that a layperson is not allowed to perform. The fine for violation can be high and the insurance company will refuse to pay out in case of a damage event if the installation was not professionally carried out.
What power rating of a heating element do I need for a 300-liter buffer tank?
It depends on the function. If it is a backup source (not for daily use), 2 kW will suffice. If you want to charge the buffer from 20 °C to 60 °C in 8 hours (night rate), 2.3 kW will be enough – in practice, take 2.5 kW or 3 kW for a reserve. If you want a quick heating in 3–4 hours, go for a 4.5–6 kW three-phase element. Do not forget to consult with an electrician whether your supply and circuit breaker can handle the power.
What is the difference between G 1½" and DN 50 on a flange?
These are two different connection systems. G 1½" is a threaded flange (BSP thread), where the heating element fits into a threaded hole in the tank shell. DN 50 is the nominal diameter of the flanged
Do you have a question about this topic?
Not sure or dealing with a specific situation in your household? Write to us – we are happy to advise.