Combi, System, or Heat Only: Which Electric Boiler Is Right for Your Home?

If you're shopping for an electric boiler for the first time — or switching from gas — you'll quickly run into three terms: combi, system, and heat only. Most people know vaguely what a combi boiler does, but the differences between all three types, and why one might suit your home dramatically better than another, are less commonly understood.

This guide breaks down each type clearly, using the ECA Arceus range as the reference point throughout.

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The Three Types at a Glance

The ECA Arceus electric boiler comes in three distinct configurations:

Let's look at each in detail.

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The Combi Electric Boiler (MT)

How it works

A combi boiler handles both central heating and domestic hot water from a single wall-mounted unit. When you open a hot tap, the boiler detects the demand and switches to heating water for that tap instantly — no storage cylinder needed.

Who it suits

The combi is the right choice if:

• You have a relatively small to medium-sized home (one or two bathrooms)
• You want to save space — no hot water cylinder or cold water tank required
• Your hot water demand is moderate rather than simultaneous across multiple outlets
• You want a straightforward installation with fewer components

Hot water performance

On the Arceus, combi functionality begins at the 12kW model and scales up to 27kW. Hot water output depends on which model you choose:

For a single shower or bath at a time, even the 12kW combi handles the task well. If you regularly have two showers running simultaneously — as in a busy family home in the morning — a higher output model or a system boiler with stored water may suit you better.

Comfort Mode and Eco Mode

One of the more practical features on the Arceus combi is the ability to switch between Comfort Mode and Eco Mode for domestic hot water.

In Comfort Mode, the water inside the boiler's heat exchanger is kept close to your set hot water temperature at all times. The moment you turn a tap, hot water arrives more quickly. It's ideal for households where hot water is used frequently throughout the day.

In Eco Mode, the heat exchanger isn't kept pre-heated, so the boiler is more energy-efficient during periods when hot water isn't regularly needed — during a holiday, for example, or in a home where demand is predictable and concentrated.

You can toggle between the two modes with a simple button combination, giving you flexibility based on your routine.

Summer Mode

The Arceus combi also has a dedicated Summer Mode. In this setting, central heating is disabled entirely, but the boiler still responds to hot water demand. This means you don't have to turn the boiler off completely in warmer months — you keep instant hot water without accidentally triggering your radiators.

Things to consider

• 6kW and 9kW models are not available as combis. If your home only requires a small output, you'll need a system or heat-only configuration with a separate hot water solution.
• Combi boilers work best when hot water flow stays above 2.5 litres per minute. Low-flow taps or very old pipework can cause temperature instability.
• In a hard water area, a scale reducer is worth considering to protect the internal heat exchanger.

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The Heat Only Electric Boiler (CH)

How it works

A heat-only boiler — sometimes called an open vent boiler or regular boiler — does exactly what it says: it heats your central heating circuit and nothing else. Hot water for taps and showers is handled separately, typically by an immersion heater in a hot water cylinder.

Who it suits

The CH configuration is the right choice if:

• Your property already has a hot water cylinder and you're replacing an older boiler without wanting to change the whole system
• You're installing in a property where the hot water demand is handled by another source (solar thermal, immersion heater, etc.)
• You want simplicity in the boiler itself, with the hot water system managed separately

The CH model is available across all seven output ratings — 6kW right through to 27kW — making it the most flexible in terms of sizing.

How hot water works alongside it

Because the CH model has no domestic hot water function built in, you need to pair it with a cylinder and a separate heat source for that cylinder. The most common approach is:

• A standard vented hot water cylinder heated by an immersion heater
• Or a heat-only boiler feeding a coil within an unvented cylinder, with the boiler providing indirect heating

If your existing setup already works this way (which is common in older UK properties), simply replacing the old heat-only boiler with a CH-model Arceus is the most straightforward upgrade path.

Things to consider

• Requires an existing or new hot water cylinder — factor this into your total installation budget
• Hot water capacity depends on cylinder size, not the boiler's output
• Generally a good choice for properties where the hot water demand outstrips what a combi could realistically deliver instantaneously

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The System Electric Boiler (ST)

How it works

A system boiler occupies a middle ground between the combi and the heat-only. Like the combi, it handles central heating directly. But for hot water, rather than heating it on demand as a combi does, it works with an external unvented hot water cylinder that stores a ready supply of hot water.

The boiler heats the cylinder, keeping it at your set temperature. When you open a tap, water draws from that stored volume — so multiple outlets can be used simultaneously without a drop in temperature or pressure.

Who it suits

The system boiler is the right choice if:

• You have a larger home with two or more bathrooms
• Multiple people regularly need hot water at the same time
• You want strong, mains-pressure hot water at every tap without relying on instantaneous heating
• You're installing a new build or a whole-system replacement where an unvented cylinder is practical

Anti-legionella protection

A notable feature on the Arceus ST model is built-in anti-legionella protection. This automatically raises the stored water temperature periodically to a level that prevents bacterial growth in the cylinder — an important safety consideration for stored hot water systems that is handled automatically rather than requiring manual intervention.

The cylinder question

Because the ST model requires an external unvented cylinder, your installation involves two components: the boiler on the wall and the cylinder in an airing cupboard or utility room. This takes up more physical space than a combi, but the payoff is effectively unlimited hot water capacity relative to cylinder size.

Unvented cylinders must be installed by a qualified engineer under Building Regulations (Approved Document G). If you're considering the system boiler route, budget for both the Arceus ST unit and an appropriately sized cylinder — your installer can advise on the right cylinder volume for your household.

Things to consider

• More components than a combi — higher initial installation cost
• Requires space for the cylinder
• Once the stored volume is exhausted, recovery time is needed before full hot water capacity is restored
• Best suited to larger households where the storage capacity justifies the setup

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Side-by-Side Comparison

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Real-World Scenarios

Scenario 1: One-bedroom flat, single occupant, single bathroom A 12kW or 15kW combi (MT) is ideal. No cylinder required, compact installation, and the hot water output is more than enough for one person.

Scenario 2: Three-bedroom semi, couple with young children, one bathroom A 15kW combi covers this comfortably in most cases, assuming the home has reasonable insulation and hot water demand isn't simultaneous across multiple outlets.

Scenario 3: Four-bedroom detached, family of five, two bathrooms and en-suite A system boiler (ST) at 18kW or 24kW is likely the better choice. The stored hot water cylinder means morning rush-hour demand — three people showering before school and work — is met without temperature loss or pressure drops.

Scenario 4: Older property with existing hot water cylinder and immersion heater A heat-only boiler (CH) slotting into the existing system is the simplest and most cost-effective approach, preserving the existing hot water setup.

Scenario 5: New build with underfloor heating throughout A system boiler (ST) pairs well with underfloor heating because the stored cylinder provides reliable hot water while the UFH circuit operates at low temperatures (30–45°C) — the Arceus supports this range natively.

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What About the Controls?

All three Arceus variants — MT, CH, and ST — support the same range of external controls, including:

• Voltage-free (low voltage) room thermostats
• High-voltage room thermostats
• Programmable timers
• Outdoor air temperature sensors (for weather-compensated heating curves)
• Zone valve configurations for multi-zone heating

The outdoor sensor allows the boiler to modulate its output based on external temperature — a practical energy-saving feature that prevents the system from overheating your home on mild days.

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Making the Decision

Use these three questions to narrow down your choice:

1. How many bathrooms do you have? One bathroom → combi is likely fine. Two or more → consider a system boiler.

2. Do you already have a cylinder? Yes → heat-only is the least disruptive option. No → combi or system depending on home size.

3. What output do you need? If sizing tells you 6kW or 9kW → system or heat-only only (combi starts at 12kW). If 18kW or above → three-phase supply required.

If you're still not certain, ECA's technical team is available on 0800 640 9988 and can help you assess the right configuration for your specific property and usage patterns.

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Explore the full ECA Arceus electric boiler range at ecaonline.co.uk.