Solar Immersion with a Combi Boiler: Can It Work?

The problem

Solar diverters like the myenergi Eddi and Solar iBoost+ work by sending surplus solar electricity to an immersion heater element inside a hot water cylinder. The cylinder stores the hot water for later use.

A combi boiler heats water on demand — it has no hot water storage cylinder and no immersion element. There's nowhere to send the diverted electricity. This means roughly half of UK homes (combi boilers are the most popular type) can't simply plug in a diverter.

The options

Option 1: Add a hot water cylinder

The most straightforward solution is to install a dedicated hot water cylinder alongside your combi boiler. The combi continues to provide central heating and backup hot water, while the cylinder stores solar-heated water.

How it works:

• A 100–150 litre unvented hot water cylinder is installed (typically in an airing cupboard or utility room)
• An immersion heater element is fitted in the cylinder
• A solar diverter connects to the immersion
• The cylinder feeds hot water to your taps, prioritised over the combi boiler
• When the cylinder has hot water, the combi doesn't fire for hot water — saving gas

Cost:

The honest assessment: This is a significant investment. The annual savings from diverting surplus solar to hot water are typically £100–£250. At the high end of installation costs, payback could be 8–15 years — much longer than a simple diverter-only installation on a home that already has a cylinder.

Option 2: Thermal store / buffer tank

A thermal store sits between the combi boiler and the hot taps. Solar-heated water warms the store, and the combi only fires when the store temperature drops below the required level.

This is more complex than a simple cylinder and typically more expensive (£1,500–£3,000 installed). It's rarely cost-effective solely for solar diversion.

Option 3: Inline electric water heater

A small inline electric water heater (2–3kW) can be connected to a solar diverter. When surplus solar is available, the inline heater pre-heats water before it reaches the combi boiler. The combi then needs less gas (or none) to bring the water to the set temperature.

Pros: Lower installation cost than a full cylinder (£300–£600 for the heater + diverter) Cons: Less efficient than a cylinder (no storage — only works while surplus solar is available). Limited flow rates.

This is a niche solution with limited products on the UK market.

Option 4: Don't bother with diversion

For many combi boiler homes, the pragmatic answer is: skip the diverter and invest in a battery instead.

Why a battery is usually better:

• A 5kWh battery costs £1,800–£3,000 installed
• It stores surplus solar as electricity (more versatile than hot water)
• The stored electricity can power any appliance, not just hot water
• Self-consumption savings are typically £250–£450/year
• Payback: 5–8 years

Compare this to adding a cylinder + diverter at £1,100–£2,300 for savings of £100–£250/year. The battery delivers better value in most cases.

What about Eddi's non-immersion uses?

The myenergi Eddi can divert surplus to any resistive heating load — not just an immersion heater. Combi boiler homes could potentially use an Eddi for:

• Electric towel rails — uses 100–400W, warm towels from surplus solar
• Electric panel heaters — useful in shoulder seasons for background heating
• Electric underfloor heating — if you have wet-room or bathroom electric UFH

These loads don't require a cylinder. However, the energy diverted to towel rails and panel heaters is much less valuable (you mainly need heating in winter when surplus solar is minimal) than hot water (needed year-round).

If you're choosing between a battery and a diverter for your combi boiler home:

GivEnergy All-in-One 9.5kWh Battery

£5,500

capacity kwh

9.5

usable capacity kwh

8.6

chemistry

LFP

cycles

6000

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myenergi Eddi Solar Diverter

£185

max power w

3000

modes

power_divert,timed_boost

outputs

2

priority

configurable

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Affiliate link — we may earn a small commission at no extra cost to you

The decision framework

Install a cylinder + diverter if:

• You have space for a 100–150L cylinder
• You're planning a boiler replacement anyway (system boiler + cylinder)
• You have a large solar system (6kW+) generating significant surplus beyond what a battery can absorb
• You already have a battery and want to maximise surplus use

Skip the diverter and get a battery if:

• You have a combi boiler and limited space
• Budget is a primary concern
• You want the most versatile energy storage option
• You don't already have a battery

Do nothing if:

• Your solar surplus is small (small system or high daytime consumption)
• You're happy with SEG export income from surplus
• Adding complexity to your system doesn't appeal

For most UK combi boiler homes, a battery is the better first investment after solar panels. A diverter + cylinder can come later if you want to squeeze every last bit of value from your solar surplus.

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