Convert to a Combi Boiler

Written by Tamer Izzet
February 26, 2026

Convert to a combi boiler – is it worth it?

If you’re wondering whether to convert to a combi boiler, you’re not alone.

Many London homeowners with older cylinder systems start looking into converting when their current boiler becomes unreliable or inefficient. In many cases, it absolutely makes sense to convert to a combi boiler – but it depends on your property layout, water pressure and household demand.

This guide explains when it’s worth it, when it isn’t, and how a combi boiler conversion compares with a standard new boiler installation.

What happens when you convert to a combi boiler?

When you convert to a combi boiler, your heating system is simplified.

Existing setup	After conversion	Benefit
Hot water cylinder	Removed	Free cupboard space
Loft water tanks	Removed	Extra loft storage
Stored hot water	Mains-fed instant hot water	No waiting for reheating

Instead of storing hot water, the boiler heats it instantly from the mains supply.

A full combi boiler conversion involves removing tanks, altering pipework and ensuring your incoming water pressure is suitable.

When it makes sense to convert to a combi boiler

Not every home should convert, but in many cases it’s the right move.

Property type	Water usage	Recommendation
1 bathroom home	Moderate usage	Strong candidate
Small 3-bed house	Normal family demand	Usually worth converting
Two bathrooms (rare simultaneous use)	Low overlap	Often suitable

It becomes even more logical if you’re already budgeting for a new boiler installation, as combining the work avoids future disruption and duplicate labour costs.

When you might not want to convert

A combi system relies entirely on mains water pressure.

Scenario	Potential issue	Better option
Two showers used together	Pressure drop	System boiler with cylinder
Poor incoming mains pressure	Weak flow rate	Pressure assessment first
Large family home	High hot water demand	High-output system setup

If your property regularly runs multiple bathrooms at once, a conversion may not deliver the performance you expect.

How much does it cost to convert to a combi boiler?

Combi boiler costs are higher for a full conversion compared to a simple swap.

Work type	Typical London cost	Time required
Convert to a combi boiler (full conversion)	£2,600 – £4,500	2–3 days
Standard combi swap	£1,900 – £3,200	1 day
System boiler replacement	£2,200 – £3,800	1–2 days

If you are comparing options, reviewing a full new boiler installation quote alongside a combi boiler conversion helps clarify the overall investment.

Is it cheaper long term to convert to a combi boiler?

In many cases, yes.

When you convert to a combi boiler, you:

• Remove tank maintenance
• Reduce stored heat loss
• Simplify servicing
• Lower the risk of tank-related leaks

However, upfront combi boiler costs are higher, so the long-term value depends on how long you plan to stay in the property.

Frequently Asked Questions

Is it expensive to convert to a combi boiler?

A full combi boiler conversion is more expensive than a like-for-like replacement because it involves removing tanks and altering pipework. In London, most conversions range between £2,800 and £4,500 depending on complexity.

What If We Run 2 Showers at Once?

A combi may struggle. A system boiler with cylinder could be more suitable.

How long does it take to convert to a combi boiler?

Most homes take two to three days to convert to a combi boiler. Larger or more complex systems may take slightly longer.

Does converting to a combi boiler improve heating performance?

Radiator heating performance is usually similar. The main improvement when you convert to a combi boiler is how hot water is delivered – instantly rather than from stored supply.

Final verdict – should you convert to a combi boiler?

For most small to medium London homes with one main bathroom and good mains pressure, it is usually worth it to convert to a combi boiler.

You gain:

• Space
• Simplicity
• Improved efficiency
• A modern heating setup

However, larger properties with high hot water demand may benefit more from a system boiler.

The key is professional assessment, not assumption.

If you’re deciding whether to convert to a combi boiler, compare the conversion process with a standard new boiler installation and choose the option that best suits your property.