Connecting the Daline PTE boiler to a storage tank or underfloor heating
Connecting the Daline PTE boiler to a buffer tank or underfloor heating
The electric boiler Buderus Dakon Daline PTE belongs to compact, reliable and technically well-equipped devices that are used in practice not only in simple radiator systems, but increasingly also in combinations with a buffer tank or low-temperature underfloor heating. These connections require a bit more thought in planning, component selection and control settings – and this whole article is about that.
If you have not yet decided what boiler power you need for your house, or you are deciding between models, I recommend reading the articles “What power of the Daline PTE electric boiler do I need for my house" and “How to choose the Buderus Dakon Daline PTE electric boiler – what to pay attention to" – both are available in the same Knowledge Center. Here I assume that you have already selected the boiler and are dealing with hydraulics and control.
Why combine an electric boiler with a buffer tank?
At first glance, it may seem that an electric boiler does not need a buffer tank – after all, electricity is available immediately, you just turn it on and it heats up. But practice says something else, and for several reasons.
1. Use of night tariff (HDO). If you have electricity in the heating tariff (e.g. D7 or D8 in Slovak conditions), you will get cheap electricity only in certain blocks – typically 8 to 10 hours per 24 hours, spread over several windows. The boiler runs at full power during cheap hours, charges the buffer tank and this thermal reserve covers heating during expensive hours. Without a buffer tank, you would have to keep the boiler running outside the tariff, which increases the cost of operation. More on this topic can be found in the article “Daline PTE electric boiler and electricity tariff for electric heating – how to save".
2. Protection of the boiler from short cycles. Underfloor heating has a huge thermal inertia – once heated concrete slab holds heat for hours. If the boiler regulated temperature directly according to the immediate demand, it would turn off and on every few minutes. This burns out the relay, shortens the boiler's lifespan and is energy inefficient. The buffer tank acts as a damper – the boiler heats the tank to the target temperature, turns off, and the system gradually draws heat from the tank.
3. Greater volume of water in the system. Underfloor heating has a larger volume of water than radiators, but if the house is larger or the boiler is smaller, the minimum flow for boiler operation may not be guaranteed. The buffer tank adds volume and stabilizes the hydraulics.
Basic connection scheme: boiler – buffer tank – circuits
The following scheme shows a typical arrangement of a three-circuit system with the Daline PTE boiler, a buffer tank and two heating circuits (radiators + floor):
In this scheme, the boiler heats the primary circuit, which charges the buffer tank. Each heating circuit draws heat from the tank separately. The radiator circuit works at a higher temperature (typically 55–70 °C), the underfloor heating circuit requires a lower temperature (30–45 °C), so a three-way mixing valve (mixing valve) with a pump is used on it.
Size of the buffer tank – specific calculations
This is a question I address in every project as one of the first. The principle is simple: the tank must store enough thermal energy to cover heating during the tariff break without the room temperature dropping significantly.
An approximate formula for the tank volume:
V (litres) = Q (kWh) × 860 / ΔT (°C)
where Q is the energy needed to cover the break in the cheap tariff, ΔT is the temperature difference between the charging temperature and the minimum usable temperature (typically 55 – 30 = 25 °C), and 860 is the conversion factor (kcal/kWh).
Practical example: A family house with a heat loss of 8 kW. The Buderus Dakon Daline PTE 8 kW boiler covers the loss exactly. The night tariff lasts 8 hours per 24 hours, leaving 16 hours outside the tariff. Assume that in mild winter the average consumption is 6 kW. Without a buffer tank, we would need the boiler to run for the entire 16 hours in the expensive tariff (partially). With a buffer tank, we want the boiler to charge the reserve during the 8-hour tariff and cover at least 8–10 hours outside the tariff.
Energy for 10 hours at 6 kW = 60 kWh. V = 60 × 860 / 25 = 2 064 liters. This is a very large tank. In practice, we therefore recommend a compromise – for example, a 500-liter tank, the boiler can briefly overheat during 10 hours outside of the tariff, or we increase ΔT (charge to 80 °C, take down to 35 °C, so ΔT = 45 °C): V = 60 × 860 / 45 = 1 147 liters. A realistic tank for an 8 kW house is therefore 300–500 liters for partial coverage, or 800–1 200 liters for full coverage from the tariff.
For smaller houses: Daline PTE 4 kW for a house with a loss of 3–4 kW, a 200–300 liter tank is sufficient. For larger buildings with Daline PTE 14 kW or Daline PTE 18 kW, we are in the range of 600–1 500 liters.
Floor heating with Daline PTE boiler – what you need to know
Floor heating is a low-temperature system. While radiators typically operate at 60–75 °C, the floor requires a maximum of 45 °C at the supply, ideally 35–40 °C. This difference is key to the entire setup.
The Daline PTE boiler can set the output temperature in the range of approximately 20–85 °C, so technically it could supply water directly to the floor heating circuit. Why is it not recommended to do so without a mixing unit?
- Risk of floor damage: If the thermostat failed or the boiler jumped to a higher temperature (e.g., during a cold start), hot water could damage the wooden floor, expand the concrete, or damage PE-Xa / PE-RT pipes.
- Uneven heating: Without proper regulation for low temperature, the floor could be too hot in some areas (the ČSN EN 1264 standard limits the floor surface temperature to 29 °C in living areas).
- Efficiency: A boiler set to 40 °C operates efficiently, but it cannot simultaneously supply radiators at 70 °C. Without separate circuits, you have to compromise – either overheated radiators or insufficient floor heating.
Mixing unit (three-way valve + pump)
The solution is a mixing (mixing) unit for the floor heating circuit. It is a combination of a three-way thermostatic or motorized valve, a circulation pump, a temperature regulator (sometimes with an external sensor), and other hydraulic components (ball valves, air vent, pressure gauge).
Principle of operation of the three-way mixer: the valve mixes hot water from the supply (e.g., 70 °C from the storage tank) with cold return water (e.g., 30 °C) in such a ratio that the desired temperature (e.g., 38 °C) is achieved at the output to the floor. The mixing ratio is controlled either by a thermostat with a temperature sensor (thermostatic head) or by a motorized drive with a regulator – the latter option is more accurate and suitable for larger houses.
Distributor (distributor/collector) for floor heating circuits
Most floor heating systems have multiple pipe circuits – each circuit for one room or zone. These circuits converge into a distributor and collector (manifold). The distributor has valves to regulate the flow to each circuit separately, or there are thermostatic valves controlled by room thermostats (thermostatic actuators). This way, you can have each room at a different temperature.
Hydraulic separation – primary and secondary circuits
One of the most common problems in practice is hydraulic interference between circuits. If the boiler, radiator circuit and floor heating circuit share one common pipe without separation, the pumps influence each other, the temperature in the system fluctuates and the regulation is inaccurate.
There are two solutions for hydraulic separation:
- Hydraulic equalizer (divider tank, "hydraulic arrow"): A small tank (5–20 liters) installed between the boiler's primary circuit and the secondary circuits. Balances pressures and flows. A cheaper solution, but without thermal storage.
- Storage tank: Performs the function of a hydraulic equalizer and a thermal storage tank at the same time. For systems with an electric boiler and a tariff, this option is clearly more advantageous.
When designing a system, I always recommend preferring a storage tank over a hydraulic arrow, if a technical room with sufficient space is available and the investment in the tank is paid back in energy savings within 2–4 years.
Concrete installation procedure – step by step
Here I describe a typical procedure that has been proven in practice. More detailed installation requirements can be found in the article "Connection and installation of the electric boiler Daline PTE – procedure and installation requirements".
Step 1: Project documentation and calculation
Before any installation, have a hydraulic project or at least a connection design prepared by a specialist. The following must be determined: heat loss of the building, power and flow rates of each circuit, pipe dimensions, size of the expansion tank and safety valve, and the volume of the storage tank. Without this step, you are installing blindly and later problems will be more expensive to fix.
Step 2: Installation of the boiler and storage tank
The Daline PTE boiler is a compact device designed for wall mounting. The storage tank stands on the floor – for a 300-liter tank, you need to plan for a floor space of about 600–700 mm in diameter and a height of 1 400–1 700 mm. Both devices should be as close to each other as possible to keep the primary circuit short and hydraulic losses low. Ball valves must be installed between the boiler and the tank for possible servicing.
Step 3: Primary circuit boiler – tank
The primary circuit connects the boiler with the storage tank. It requires its own circulation pump (the Daline PTE boiler has a built-in pump, but when connected to a tank, it must be checked whether it is sufficient – it depends on the pipe length and pressure loss of the system). The pipe dimensions depend on the power: for 8 kW at ΔT = 10 K, the flow rate is about 0.7 m³/h, which corresponds to Cu 22 mm or steel pipe DN 20.
Step 4: Mixing unit for the floor heating circuit
The mixing unit is installed on the secondary side, between the storage tank and the floor heating distributor. If you also have a radiator circuit, it goes directly from the tank without a mixer (or with a separate mixer, if you want to regulate the temperature according to the outside temperature). Important: the mixing unit for the floor must have its own pump – it must not depend only on the boiler's pump.
Step 5: Electrical installation
The electrical side is very important for Daline PTE boilers. The boiler must be connected to a separate circuit – Daline PTE 18 kW requires three-phase power supply 400 V and a circuit breaker of at least 32 A. The HDO receiver (signal for tariff switching) must be properly connected to the boiler to operate the heating element only during the cheap tariff. More about the electrical installation can be found in the article "Connection and installation of the electric boiler Daline PTE – procedure and installation requirements".
Step 6: Control and setting
This is the step that is most often underestimated and later causes problems. You must set: the charging temperature of the storage tank, the temperature of the mixing unit for the floor, the time programs (when the boiler charges), and possibly an equithermal regulation (when the supply temperature changes according to the outside temperature). Daline PTE has a relatively simple built-in controller – for more advanced control, it is advisable to add an external controller. This topic is discussed in detail in the article "Control and operation of the boiler Buderus Dakon Daline PTE – setting and programming".
Expansion tank and safety valve in a larger system
When you add a storage tank to the system, the total water volume in the system increases significantly. The expansion tank must be dimensioned for the total volume – a standard boiler expansion tank of 8–12 liters, which would be sufficient for the boiler alone, is not sufficient. Approximate calculation: expansion tank = (total system volume in liters × 0.05 to 0.07). For a 500-liter tank + piping of about 80 liters = 580 liters total → expansion tank at least 40 liters (in practice, better 50 liters).
Pressure relief valve must be set to a pressure of 3 bar (standard value for low-energy systems) and must be installed on the boiler, not behind any closed valve. I always recommend installing a pressure relief valve on the buffer tank separately as well.
Typical problems and errors in implementation – examples from practice
Over the years in the industry, I have seen several typical errors that repeat in various projects. I mention them so you can avoid them:
Case 1: Floor heating loop without a mixer. The customer connected the floor heating directly to the boiler set to 65 °C on their own. Result: after two months, cracks in the terrazzo and damage to the parquet in two rooms. The thermal expansion of the pipe at 65 °C instead of 38 °C was too high. Solution: installation of a mixing unit and reduction of the supply temperature, but the parquet had to be replaced.
Case 2: Too small expansion tank. A system with a 400-liter tank and an original 8-liter expansion tank from the boiler. The safety valve dripped every day – water was leaking. The customer thought the boiler had a fault. In reality, the expansion tank was not able to accommodate the volume of the expanded water. After replacing it with a 40-liter expansion tank, the problem disappeared.
Case 3: Boiler runs all day despite the tariff. The HDO receiver was connected, but the customer had the "emergency heating" function activated in the boiler, which ignores the tariff switching at low outdoor temperatures. The monthly electricity bill was 40 % higher than it should have been. Solution: correct setting of the boiler parameters.
Case 4: Hydraulic interference between loops. A house with three loops (radiators, ground floor floor heating, upper floor floor heating) without hydraulic separation. When only the radiator pump was running, the floor heating loops were "drained" – the radiator pump was pulling water from the floor heating branches. The rooms with floor heating were underheated. Solution: installation of a buffer tank as a hydraulic separator.
Adaptive (equithermal) regulation and its benefits
If you are investing in a buffer tank, it is worth considering the addition of adaptive (equithermal) regulation. Equithermal regulation works with an outdoor temperature sensor and automatically adjusts the set temperature of the heating water in the system depending on the outdoor temperature – when it is -15 °C outside, the boiler and mixer deliver maximum temperature; when it is +5 °C outside, a lower temperature is sufficient.
Result: continuous, comfortable room temperature without the need to manually adjust the thermostat. Energy savings compared to a fixed temperature can be 10–20 %. Modern boiler controllers for Daline PTE allow equithermal regulation to be added later, without interfering with the internal electronics of the boiler.
Combination: buffer tank + photovoltaics
More and more customers are asking about combining an electric boiler with a photovoltaic system. The logic is simple: if you have excess electricity from photovoltaics during the day, you can use it to charge the buffer tank – heating for free. The Daline PTE boiler is suitable for this scenario, as a photovoltaic surplus controller (e.g., Fronius, SMA, or a specialized power controller) can smoothly or stepwise control the boiler's power according to the current photovoltaic output.
It is important that the Daline PTE 6 kW or a higher model is capable of stepwise power regulation (not all electric boilers can do this). Daline PTE has 2–4 power steps depending on the model, which is usable for cooperation with photovoltaics. For smooth regulation, PWM or 0–10V power control would be ideal – this needs to be verified directly in the technical documentation of the specific model.
Piping and pump sizing – practical values
For proper system operation, the dimensions of the pipes and the selection of pumps are also important. I provide approximate values for typical single-family homes:
- Primary loop boiler – buffer tank: for power up to 8 kW Cu 22 mm / DN 20, for 14–18 kW Cu 28 mm / DN 25. The length should not exceed 6–8 m (supply + return), otherwise pressure losses must be checked.
- Radiator loop: distribution pipe Cu 22 mm, branches to radiators Cu 15 mm or 18 mm. Pump with parameters of approx. 0.5–1.5 m³/h at a head of 2–4 m.
- Floor heating loop: distribution Cu 22 mm, individual loops from PE-Xa or PE-RT 16×2 mm or 20×2 mm. Maximum length of one loop 80–120 m (depends on pipe diameter and hydraulic calculation). Mixing unit pump typically 0.5–1.2 m³/h at 2–3 m.
Deaeration – the basis of proper operation
Air in the system is the number one enemy. An air pocket in the pump can stop it, and air in the floor heating loop causes some pipes to not heat. After filling the system, you must properly deaerate each branch. In floor heating, air pockets most often settle at the highest points of the manifold – therefore, the manifold must have an automatic deaerator.
For the buffer tank, it is important that the hot water supply enters the upper part of the tank and the return (cold water) leaves from the lower part – this preserves the thermal stratification (layering) in the tank, which increases the efficiency of the entire system.
Most frequently asked questions (FAQ)
Do I need a buffer tank if I want floor heating with the Daline PTE boiler?
Technically, a buffer tank is not necessary for floor heating. You can connect the floor heating system directly to the boiler via a mixing unit without accumulation. However, in practice, a buffer tank is strongly recommended if you want to use the night tariff (HDO) and reduce electricity costs. Without a tank, the boiler runs during expensive hours as well, which increases operating costs. If you don't have a tariff and aren't considering photovoltaics, a hydraulic equalizer (arrow) is a cheaper alternative that solves the hydraulics but does not add thermal storage.
What temperature should be set on the mixer for floor heating?
It depends on the floor composition, the heat loss of the room, and the type of floor covering. General recommendation: supply to the floor heating loop 35–42 °C at an outdoor temperature around 0 °C. On very cold days (-10 to -15 °C), it can be increased to 45 °C. The surface temperature of the floor should not exceed 29 °C in living rooms and 33 °C in bathrooms (standard EN 1264). The correct value is determined by a floor heating calculation, which should be part of the project.
Can I connect domestic hot water (DHW) preparation to the buffer tank?
Yes, there are combined buffer tanks (so-called buffers with an integrated DHW tank or with a heat exchanger). Such a tank can simultaneously accumulate heat for heating and heat domestic hot water. It is an elegant solution for single-family homes where you don't want to have several separate units. Note: the temperature in the tank must be at least 60 °C for DHW preparation (for hygiene reasons – to prevent legionella), which must be taken into account when setting the boiler.
What boiler power is needed if I want to heat and charge the buffer tank at the same time?
The boiler must have sufficient power to cover the heat loss of the house plus the power needed to charge the tank within a specified time. If the house has a heat loss of 8 kW and you want to fill a 500-liter tank from 30 °C to 75 °C in 4 hours (ΔT = 45 °C), the energy for the tank = 500 × 1.163 × 45 / 3 600 × 4 ≈ 7.25 kWh, so the charging power is approx. 1.8 kW/h. Total power = 8 + 1.8 ≈ 10 kW. In this case, you would choose a Daline PTE 14 kW to have sufficient power reserve.
What to do if the floor heating does not heat some rooms evenly?
First: check that all loops on the manifold are open. Second: deaerate the loops where the problem occurs. Third: balance the hydraulics on the manifold – each loop has a different length, and thus a different pressure loss; longer loops require higher flow, which is adjusted with regulating valves on the manifold (so-called balancing). If you have completed these steps and the problem persists, check the mixer and the supply temperature – too low a temperature with a large heat loss in the room is not sufficient. More tips in the article "Common faults of the Daline PTE electric boiler and how to eliminate them".
Is it possible to connect the Daline PTE boiler to an existing floor heating system that originally had a gas boiler?
Yes, the electric boiler Daline PTE can take over the function of a gas boiler in an existing system. The hydraulic scheme remains essentially the same. However, a few things need to be checked: 1) the output of the original boiler and whether the electrical connection allows the same output (electric boilers require significantly more power than a gas boiler of the same output), 2) the condition of the expansion tank and safety valve – in older systems they are often worn out, 3) whether the existing mixing unit for the floor heating is suitable, or whether the valve actuator needs to be replaced, 4) the regulation – gas boilers had a different type of regulation, which needs to be adapted to the electric boiler and possibly to the HDO tariff.
Conclusion
Connecting the Daline PTE boiler to a storage tank or floor heating is not rocket science, but it requires a careful approach during planning, the correct choice of components, and precise regulation settings. If you meet these three conditions, you will get a system that is comfortable, energy efficient, and long-term reliable.
Electric boilers such as Daline PTE 8 kW in combination with a properly dimensioned storage tank and a mixing unit for the floor heating represent a solution that can achieve operating costs comparable to gas heating when using night tariffs or photovoltaics – while eliminating the worries about gas inspections, chimney and combustion.
If you are unsure about the correct model selection, the size of the tank or the hydraulic connection for your specific case, a consultation with a professional designer or installation company is always worth it – the cost of the project is a fraction of what a faulty connection might cost to fix.
Do you have a question on this topic?
Can't decide or are you dealing with a specific situation in your household? Write to us – we are happy to help.