Installation and connection of a backup power source to the boiler
Why proper installation of a backup power supply is just as important as its selection
When a customer is dealing with power outages during the heating season, most of the attention is focused on selecting the right power and capacity of the backup power supply. This is understandable - the question "what power do I need for the boiler" is crucial. Less is said about what happens after the power supply arrives home in a box. And yet, it is precisely the way of installation and connection that determines whether the device will actually fulfill its purpose when a real power outage occurs - in the cold, at night, during the weekend, when the service technician is not immediately available.
Over the years of working with customers, we have seen dozens of cases where the customer bought a technically completely correct backup power supply, but connected it in such a way that the boiler still failed during the first power outage - either because the power supply was connected to the wrong circuit breaker, or because the cable led through an extension cord with a switch that someone accidentally turned off. This article is therefore focused exclusively on the practical aspects of installation and connection - not on product selection, which we cover in other articles in the Knowledge Center.
How a backup power supply actually works - the principle you need to understand before installation
Backup power supplies of the Avansa type, commonly used for boilers and circulation pumps, are so-called off-line (or line-interactive) UPS backups. This means that under normal operation, electricity from the grid passes through the power supply directly to the appliance (boiler), while the power supply continuously charges an internal or external battery. When a power grid voltage outage occurs, the electronics in the power supply detects this change and switches the power to the battery via an inverter (inverter), which generates alternating current 230 V from the direct current of the battery within a few milliseconds (typically 2-10 ms).
For the boiler, this transition is practically imperceptible - the boiler electronics does not register it as an outage, the circulation pump does not stop, the burner does not go out. This is a crucial difference compared to relying, for example, on a motor-driven power plant, where a real voltage outage occurs for several seconds during startup.
Energy flow diagram during normal operation and during an outage
From this diagram, you can see the principle that must be followed in every correct installation: the power supply must be physically connected between the socket and the appliances (boiler, possibly circulation pump, if it is not part of the boiler). It is not possible to connect it "beside" or connect it to only one of the appliances if you want to back up both - more on this below.
Preparation before installation - what you need to check
Before you proceed to the actual connection, we recommend going through the following checklist. This phase is often underestimated, yet most complaints of the type "the power supply does not work" stem precisely from incorrect preparation, not from a product defect.
1. Determine the actual power consumption of the boiler and the pump
Most common condensing gas boilers have a power consumption of 80 - 150 W during operation, but during the burner start-up (ignition, fan start-up), there may be a short-term increase to 200 - 300 W. A separate circulation pump (e.g., in floor heating, where there are multiple pumps or a mixing valve) can add another 40 - 90 W per unit. The exact value is always found on the boiler or pump's production label - look for the value "P max" or "power consumption" in the technical data table. A detailed procedure on how to determine this consumption and calculate it is covered in a separate article How to Determine the Consumption of a Boiler and Pump for Backup Selection.
2. Check the type of socket from which the boiler will be powered
A standard boiler is powered from a standard 230 V/16 A socket. Check that this socket is functional, grounded, and not connected to the same circuit breaker that often trips (e.g., together with kitchen appliances with high consumption). We recommend that the boiler has its own separate circuit - if this is not your case, it is advisable to have it resolved by an electrician before installing the backup.
3. Check the physical space around the boiler
A backup power supply (especially a model with an external battery, for example, Avansa 500 W + battery 46Ah) needs a dry, well-ventilated space, protected from splashing water and with a temperature within the range recommended by the manufacturer (usually 0 to 40 °C, ideally 10-25 °C). Batteries are sensitive to heat - placing them directly above the boiler, in a space with a constant temperature above 35 °C, significantly shortens their lifespan.
Step by step: installation and connection of the backup power supply
The following procedure applies to a standard installation of one backup power supply to one boiler, or a boiler with a circulation pump. It is a standard procedure that even a layperson with basic electrical skills can handle, but in any case of uncertainty, we recommend calling an electrician.
Step 1: Safe Disconnection of the Boiler from the Power Supply
Before any handling, switch off the circuit breaker that powers the boiler in the distribution box (not just the switch on the boiler itself). Verify with a voltage tester or multimeter that there is indeed no voltage in the socket. This is an elementary safety step that, unfortunately, is occasionally skipped during "quick" installations in practice.
Step 2: Placement and Connection of the Battery
For models with an external battery (typically for higher power outputs such as Avansa 700 W or Avansa 1050 W, or for the variant Avansa 500 W + battery 46Ah), place the battery on a stable, flat surface next to or under the power source itself. The battery is connected via supplied cables with a fuse (usually 15-30 A depending on the type) directly to the terminals - make sure to observe the correct polarity (red cable is positive, black is negative). Reversing the polarity can damage the control electronics or blow the fuse. For models with an internal battery (e.g., Avansa 300 W), this step is not necessary, as the battery is part of the device body.
Step 3: Connecting the Power Source to the Electrical Network
The power source is plugged into a standard 230 V socket using a power cable included in the package. We recommend not using extension cords with a switch or low-quality extension leads with overvoltage fuses - the switch on the extension cord is one of the most common places where unintentional disconnection occurs (e.g., during cleaning, when someone accidentally switches the switch to "0"). If you need to extend the distance between the socket and the power source, use a high-quality extension cable without a switch, with a conductor cross-section of at least 1.5 mm².
Step 4: Connecting the Boiler (and Possibly the Pump) to the Power Source Output
The boiler is powered from the output socket (or sockets) on the body of the backup power source. With sources having multiple outputs (e.g., Avansa 1050 W usually has 2-3 output sockets), you can connect the boiler and a separate circulation pump at the same time - provided the total power consumption is within the safe reserve relative to the rated power of the source. This is exactly where we recommend calculating the consumption in advance, ideally with at least a 30 % reserve relative to the continuous power, due to inrush currents.
Step 5: First Power On and Battery Charging
After connecting, switch on the power source using the main switch. Before you start using it and relying on it, let the battery charge fully - for new batteries, this can take 6-10 hours depending on the capacity and the charging current of the power source. Most models indicate the charging status using LED indicators or a display.
Step 6: Functionality Test During Simulated Power Failure
We recommend a controlled test: switch off the circuit breaker supplying the backup power source (not the boiler!) and observe whether the boiler remains in operation, the boiler display does not turn off, and the pump continues to run. This test will reveal any connection errors before a real power outage occurs. At the same time, you can verify how long the power source will actually last under real load - approximate values and calculation methods can be found in the article How Long Does a Backup Power Source Last During a Power Outage.
Step 7: Reconnection and Final Check
After a successful test, switch the circuit breaker back on and check whether the power source automatically switches back to grid power and starts charging the battery. Most Avansa models do this automatically without user intervention.
Connecting Multiple Devices - Boiler and Separate Circulation Pump
Many heating systems, especially in underfloor heating or larger homes with multiple heating circuits, use a separate circulation pump in addition to the built-in pump in the boiler (e.g., for a water heater circuit or a mixing station). In such cases, it is necessary to consider whether one backup power source will power both devices or whether a solution with two separate power sources is more appropriate.
As shown in the diagram, when using one source with multiple outputs, it is crucial that the total actual power consumption (including inrush peaks) does not exceed the rated power of the device. For a standard boiler with a built-in pump (up to 150-200 W total), a smaller model such as Avansa 300 W or Avansa 500 W is sufficient. For a boiler with an additional separate circulation pump, or for a mixing station with its own pump, it is more appropriate to calculate with a reserve and choose a model such as Avansa 700 W or Avansa 1050 W, which usually has more output sockets and a more robust inverter capable of handling pump inrush currents.
A detailed comparison of the various power ranges can be found in a separate article Comparing Avansa Backup Power Sources by Power, where we also discuss differences in battery capacity and backup duration.
Placement of the Backup Power Source in the Boiler Room - Practical Recommendations
The physical placement of the power source affects not only its lifespan but also the safety of operation. We recommend the following guidelines, based on common installation practices:
- Place the power source in an upright position according to the manufacturer's recommendation, usually not directly on the floor (risk of moisture), but on a shelf or bracket at least 20-30 cm above the floor.
- Leave at least 10 cm of free space around the ventilation openings of the device for cooling the inverter.
- Do not place the power source or the battery in close proximity to the boiler's chimney or flue, where the surface temperature can exceed 40 °C for long periods.
- Protect the battery (especially in external variants) from direct sunlight and freezing below 0 °C - the capacity of lead-acid batteries significantly decreases at low temperatures.
- Ensure that the input and output cables are not mechanically stressed (bending, loading with furniture), and that they are out of reach of children and pets.
Most common installation errors we see in practice
During service interventions and complaints, we repeatedly encounter several typical errors. Knowing them will help you avoid unnecessary problems right from the start.
Connecting via an extension lead with a switch or surge protector with automatic disconnection
Certain cheaper extension cords and "surge protectors" have built-in fuses or circuit breakers that disconnect the entire circuit including the backup power supply in case of a minor overvoltage (e.g., during a thunderstorm). If this happens during a power outage, the power supply will switch to the battery normally, but after the power is restored, it will not start charging until someone manually resets the extension lead. Solution: connect the power supply directly to the wall socket, or use a high-quality extension lead without automatic disconnection.
Underestimating the inrush current of the pump
Circulation pumps, especially older types with asynchronous motors, have an inrush current at startup that can be up to 3-5 times higher than the rated current. If the power supply is undersized, this can cause an overload and the inverter to shut off during pump startup. This will manifest as "the power supply is not working," while in reality it is a matter of incorrect power selection, not a fault. Details on the calculation can be found in the article "What backup power supply capacity do I need for my boiler?"
Forgotten or insufficient ventilation in the battery area
Lead-acid batteries (especially AGM or gel types) emit a small amount of gas during charging. Although this gas production is minimal in high-quality sealed batteries, an enclosed, unventilated space (e.g., a built-in cabinet with no air vents) is not ideal. We recommend leaving at least minimal air circulation.
Lack of regular testing
A power supply that is installed and "forgotten" may have significantly reduced runtime after one or two years due to natural battery aging. We recommend performing a controlled test of disconnection from the grid at least once every 3-6 months and monitoring how long the power supply actually lasts. The topic of battery lifespan and maintenance is discussed in detail in the article "Battery maintenance and lifespan of a backup power supply," where you will also find signs of an approaching end of battery life.
Mixing up input and output
In some models with separate terminals for grid input and appliance output, confusion occurs, especially if they are not color- or shape-coded. Always verify in the attached manual which socket/terminal is input and which is output before connecting – mixing them up can lead to damage to the power supply.
Specifications according to boiler type
Not every boiler is connected to a backup power supply in exactly the same way – there are small but important differences.
Gas condensing boiler (typical combi boiler in an apartment or house)
Most modern condensing boilers (e.g., common brands such as Vaillant, Viessmann, Baxi, Protherm) are powered via a standard 230 V plug. Connection is therefore the simplest – it is enough to move the boiler's power cable from the wall socket to the output of the backup power supply. There is no need to interfere with the boiler's electronics or change any settings.
Electric boiler
Electric boilers have a significantly higher power consumption (units to tens of kW), so a standard backup power supply such as Avansa is not designed to back up the heating power of the electric boiler itself – this would require a power supply with much higher power and capacity, which already exceeds typical home solutions. In this case, the backup power supply usually backs up only the control electronics and circulation pump of the electric boiler, not the heating element itself.
Wood-burning boiler with built-in fan and pump
In wood-burning boilers, backup is especially important – without electricity, the primary and secondary air fan and the circulation pump do not work, which can lead to boiler overheating during ongoing combustion. Here we recommend a higher power supply (Avansa 700 W or 1050 W) due to the simultaneous operation of the fan and pump, which together can consume 150-250 W.
System with multiple heating circuits and mixers
In larger homes with floor and radiator heating, controlled by an equithermal regulator and servomotors of mixing valves, it is advisable to back up not only the boiler but also the entire control unit and servomotors. In this case, it is worth consulting the specific configuration, as the total power consumption can increase faster than it might seem at first glance.
Connecting to an external battery – what is different
Models with the option to connect an external battery, such as Avansa 500 W + battery 46Ah, offer the advantage of significantly longer backup time at the same power supply capacity. During installation, you should keep in mind several specifics:
- The battery is connected with cables of a defined cross-section (usually at least 4-6 mm² for higher currents) – do not use thinner cables than recommended by the manufacturer, as this could lead to overheating and voltage drop.
- The fuse between the battery and the power supply must be dimensioned exactly according to the instructions – a lower rating will cause unnecessary shutdowns, while a higher rating loses its protective function.
- An AGM or gel battery should not be stored or operated for a long time in a discharged state – this significantly shortens its lifespan compared to classic starter batteries.
- With a capacity of 46 Ah and a voltage of 12 V, this is about 550 Wh of energy, which at a boiler load of around 100 W theoretically lasts several hours – the exact calculation with inverter efficiency (usually 85-90 %) can be found in the article "What battery capacity is suitable for a circulation pump?"
Connecting to a system with a lightning rod and surge protection
If the house is equipped with a lightning rod and surge protection (SPD – Surge Protective Device) in the main distribution board, the backup power supply complements this system fully, it does not replace it. The power supply solves power outages, not surges caused by lightning strikes. We therefore recommend combining both systems – keep the surge protection in the distribution board active and connect the backup power supply after it.
Inspection after installation – what to focus on during the first heating season
After completing the installation, ideally before the start of the heating season, we recommend performing a full inspection:
- Visual inspection of all connections and cables – no loose terminals, damaged insulation.
- Check the battery charge level on the display or via LED indicators.
- Simulated power outage test with measurement of the time during which the power supply actually kept the boiler running.
- Check the audible signalization (beeping) of the power supply – verify that you can distinguish the "operation on battery" signal from the "battery almost empty" signal, so you can react correctly in case of a longer outage.
- Record the installation date and the recommended date for the next inspection in a calendar or directly on the device label.
When to entrust the installation to a professional
Although connecting a backup power supply is relatively simple and can be done by a skilled layperson for a standard boiler with a simple plug, there are situations when we recommend calling an electrician or service technician:
- If the electrical installation in the boiler room is older, without a separate circuit breaker for the boiler, or shows signs of wear.
- If you plan to back up multiple appliances at once (boiler, pump, mixing station, control unit) and are unsure about the total power consumption.
- If it is a boiler with a more complex connection, for example, with direct connection to the terminal block instead of a standard socket.
- If you need to connect the power supply in such a way that it also backs up part of the home network (e.g., a Wi-Fi router for remote boiler control) – in this case, it is necessary to correctly divide the outputs according to priorities.
Frequently asked questions about the installation and connection of a backup power supply
Do I need to turn off the main circuit breaker in the apartment or just the boiler circuit breaker when connecting the backup power supply?
It is sufficient to turn off the circuit breaker of the branch that powers the specific socket for the boiler. There is no need to turn off the main circuit breaker of the entire apartment or house, unless you are directly working on the distribution board.
Can I connect the backup power supply to a multi-socket outlet together with other appliances?
We do not recommend this. The power source should have its own, preferably dedicated, socket to prevent overloading the circuit or unintentional disconnection when handling other devices on the same circuit.
Is it necessary to adjust anything in the boiler's electronics after installation?
No, for standard condensing gas boilers. The boiler treats power from the backup source the same as power from the grid and does not require any changes to the settings.
What to do if the power source does not charge the battery after a power outage?
Check whether the power source is actually connected to a functioning socket under voltage (verify with a multimeter), whether it is not plugged into an extension cord with a switched-off socket, and whether an error indication is not lit on the display. If the problem persists, it is likely a technical fault - more typical faults and their solutions can be found in the article Common Faults of Backup Power Sources for Boilers.
Can one backup power source be used simultaneously for the boiler and the router to keep the internet running?
Yes, provided the power source has spare capacity in terms of power and battery capacity in addition to the boiler's requirements. Routers usually have low power consumption (5-15 W), so with models such as Avansa 500 W or higher, this is usually not a problem, but it is always advisable to calculate this in advance.
How often should the connection be checked once it is working?
We recommend a visual and functional check at least twice a year - in the autumn before the start of the heating season and in the spring after its end. This frequency also coincides with the recommended battery condition check interval.
Summary
Proper installation and connection of the backup power source determines whether the device will actually perform its function when you need it most - during a real power outage in winter. The key principles are simple: connect the power source directly to a wall socket without switches along the path, ensure proper dimensioning in relation to the boiler's and possibly the circulation pump's total power consumption, place the battery in a well-ventilated space with an appropriate temperature, and always perform a controlled test simulating a power outage after installation. If you are unsure about the correct choice of power or capacity, we also recommend reading the articles How to Choose a Backup Power Source for a Boiler and Circulation Pump and What Power of a Backup Power Source Do I Need for My Boiler, which cover the topic of selection in more detail. By following the described principles, the installation is simple, safe, and the device will reliably serve you for many heating seasons.
Do you have a question on this topic?
Having trouble deciding or dealing with a specific situation in your home? Write to us - we are happy to help.