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A UPS for a combi boiler keeps your boiler running when the power goes out. Even if the gas supply is fine, a combi boiler's pump, fan and control board cannot work without electricity — so a winter outage of a few hours means a cold home and no hot water, which is a real problem for homes with babies, elderly or sick family members.
This guide answers: how to choose a UPS for a combi boiler, how many VA you need, whether a boiler works on a UPS and how long it will run.
Why Does a Combi Boiler Stop Without Electricity?
- Circulation pump: sends hot water to the radiators.
- Fan (room-sealed boilers): supplies combustion air and removes flue gas.
- Control board and ignition: lights the flame and monitors the flame sensor and safety systems.
- Gas valve: closes for safety when power is lost.
The only safe way to keep a boiler running during an outage is an uninterruptible power supply (UPS) that delivers clean, stable power.
How Many VA Does a Combi Boiler Need?
Domestic combi boilers typically draw 80–150 W (see “electrical power” on the boiler label). Because the pump draws more current at start-up, leave a margin.
| Boiler power | Recommended UPS | Note |
|---|---|---|
| Up to 100 W | 650–850 VA | Enough for standard condensing boilers |
| 100–150 W | 1 kVA | Longer runtime and power reserve |
| 150 W+ or boiler + thermostat + modem | 1.2–1.5 kVA | When several devices are connected |
Rule of thumb: choose a UPS rated at 4–5 times the boiler's wattage in VA.
Will a Combi Boiler Work on a UPS? Why Pure Sine Output Matters
Yes — with the right UPS. Some low-cost models produce a square or “modified sine” wave on battery. Boiler pumps and flame-detection circuits are sensitive to this, which can cause error codes or “flame not detected” lockouts, a humming pump and long-term damage to the control board. Always choose a pure sine wave UPS. ELSIPOWER line interactive UPS models offer sine output, automatic voltage regulation (AVR) and a transfer time of about 5 ms.
Check phase–neutral and earthing
The flame sensor needs correct phase–neutral orientation and a solid earth. Use an earthed socket, connect the boiler to the UPS output correctly and test by unplugging the UPS input after installation.
How Long Will the Boiler Run on a UPS?
- 650 VA–1 kVA models with internal batteries: usually 30 minutes to 1.5 hours with an average boiler.
- Long-runtime models with external batteries: from several hours to all night, depending on the battery bank.
Where outages are long and frequent, an online UPS with external batteries is the better investment.
Checklist for Choosing a Boiler UPS
- Pure sine wave output
- Correct size: 4–5× the boiler wattage in VA
- AVR for areas with low or high voltage
- Fast transfer time so the boiler does not reset
- Quiet, compact design
- A dry, ventilated location away from water and direct sunlight
- Battery check before every winter — batteries are usually replaced every 3–5 years
Frequently Asked Questions
What is the minimum UPS size for a combi boiler?
A 650 VA pure sine UPS is usually enough for a standard domestic boiler; 1 kVA is recommended for longer runtime or extra devices.
My boiler shows an error on the UPS — why?
The most common causes are a non-sine output, reversed phase–neutral or poor earthing. A pure sine UPS and correct wiring solve most cases.
Can the boiler stay connected to the UPS permanently?
Yes. The UPS runs from the mains and keeps its batteries charged, switching over automatically during an outage.
Let us choose the right UPS for your boiler
Send us your boiler model and the runtime you need, and we will recommend the right ELSIPOWER UPS. Call +90 212 210 75 51 or message us on WhatsApp.
