Changing a Conventional Boiler to Combi: Complete Guide

Should I Replace My Conventional Boiler with a Combi?

Deciding whether to undertake a conversion boiler project requires understanding the fundamental differences between the two systems.

A conventional boiler system is likely what you grew up with. It includes three key parts: the boiler, a hot water cylinder often kept in the airing cupboard, and a cold water storage tank, which is usually placed in the loft. This system stores hot water in a tank, so once the supply runs out, you will need to wait for the cylinder to heat up again.

In contrast, a combi boiler is a single compact unit that delivers both central heating and hot water, making it a practical choice for smaller homes.

If you are in doubt about replacing your boiler, call a professional first. They can assess the situation and guide you toward the right choice.

Why Change to a Combi Boiler?

There are several compelling reasons why changing from a conventional boiler to a combi is the standard for modern home upgrades:

1. Energy Efficiency

Modern combi boilers are highly efficient condensing boilers. Because they heat water only when you need it, you are not paying to keep a water cylinder hot all day. This helps cut your carbon footprint and brings down your monthly energy bills in a steady, noticeable way.

2. Space Saving

By removing the hot water cylinder and the loft tanks, you reclaim significant space in your home. This can be transformed into extra storage, a larger bathroom, or even a loft conversion.

3. Unlimited Hot Water

Since the water is heated instantly from the mains, you will never run out of hot water. There is no need to program a timer or boost the immersion heater.

4. Mains Pressure Showers

Combi boilers deliver water at mains pressure, which often eliminates the need for a separate shower pump to get a powerful spray.

Which Is Better: A Combi or a Conventional Boiler?

The answer depends entirely on your household's usage habits and the size of your property.

For the vast majority of average-sized UK homes, replacing a boiler with a combi is the superior choice for efficiency and convenience.

Check out our Boiler Reviews section to learn more about different boiler types and the features they offer.

How to Change a Conventional Boiler to a Combi?

The process of changing from a conventional to a combi boiler is more involved than a like-for-like swap. It requires a qualified Gas Safe engineer to alter your heating infrastructure significantly.

Here is what you can expect during the installation:

1. Removing the Old System

The engineer will drain your existing central heating system. They will then take out the old boiler along with the hot water cylinder in the airing cupboard, and remove the cold water feed and expansion tanks from the loft.

2. Upgrading Pipework

This is a crucial step. Conventional systems often use 15mm gas supply pipes, whereas combi boilers usually require 22mm pipes to ensure sufficient gas flow for the higher output required to heat water instantly. The water pipes may also need reconfiguration to bypass the old tank locations and connect directly to the mains.

3. Installing the New Unit

The new combi boiler is mounted (often in a kitchen cupboard, utility room, or the space where the cylinder used to be). The flue is installed to vent exhaust gases safely outside.

4. System Flushing

A chemical flush or power flush is performed to get rid of sludge and debris from your radiators and pipes. This ensures the new high-efficiency boiler isn't contaminated by old system dirt.

Do I Need New Pipes for a Combi Boiler?

This is a common concern. As mentioned above, you may need to upgrade your gas supply pipe from 15mm to 22mm. Regarding your heating pipes (radiators), the existing copper piping is usually sufficient.

However, the key consideration is pressure.
Conventional systems rely on gravity and operate at low pressure. Combi systems operate at high mains pressure. Before installation, an engineer should check that your existing radiators and valves are in good condition.

Occasionally, the increased pressure can find weak spots in old pipework, causing leaks. A professional assessment by Serviceteam can identify if any pipework upgrades are necessary before the installation begins.

Can You Replace a Back Boiler with a Combi Boiler?

Yes, replacing an old back boiler with a combi boiler is not only possible but highly recommended. Back boilers, units fitted behind a gas fireplace, are obsolete, inefficient, and difficult to repair due to a lack of parts.

Converting a back boiler is a more complex job than a standard conversion. It involves removing the fire front and the boiler unit from the chimney breast, making good the fireplace, and finding a new location for the combi boiler (since combis are not installed behind fireplaces). While the upfront effort is higher, the efficiency gains from ditching a back boiler are substantial.

Which Conventional Boiler is Best?

If you decide that a combi is not right for you, perhaps you have three bathrooms in regular use, you might opt to stick with a modern conventional or system boiler.

If you are retaining a tank system, look for modern "system" boilers. Unlike older regular boilers, system boilers have many major components (like the pump and expansion vessel) built into the unit, making them quicker to install and easier to service than traditional heat-only setups.

Leading brands like Worcester Bosch, Viessmann, and Ideal offer high-efficiency system boilers that work perfectly with unvented hot water cylinders to provide high-pressure hot water to multiple outlets.

For more guidance, you can read our article titled ‘Which Is the Best Combi Boiler for Your Home’.

What Is the Disadvantage of a Combi Boiler?

To provide a balanced view, it is important to look for the potential downsides before committing to changing from a conventional to a combi boiler:

• Water Flow Limitations: Because the water is heated on demand, flow rates can drop if multiple outlets are used at once. If someone flushes the toilet while you are in the shower, you might notice a temperature or pressure fluctuation (though modern high-end combis handle this better).
• No Backup: With a conventional system, if the boiler breaks, you may still have a tank of hot water or an electric immersion heater as a backup. With a combi, if the boiler breaks, you lose both heating and hot water immediately.
• Mains Pressure Dependency: If the water pressure in your area is poor, a combi boiler will not function effectively. An engineer must test the mains flow rate before installation.

For more guidance, you can read our dedicated blog titled Combi Boiler vs Conventional Boiler: Pros and Cons.

Choosing the Right Combi Boiler Size

If you proceed with the conversion, selecting the correct kW output is vital. A boiler that is too weak will struggle to provide hot water; one that is too powerful is an unnecessary expense.

Note that for combi boilers, the kW rating refers primarily to the hot water output. If you have a high demand for hot water (e.g., a deep bath that you want to fill quickly), opting for a higher kW model is advisable even in a medium-sized home.

Check out our blog What Is the Best Combi Boiler for a 3 Bedroom House in London to learn which models offer the best performance, efficiency and value for your home.

How Much to Replace a Conventional Boiler with a Combi?

The financial aspect is often the deciding factor. Replacing a conventional boiler with a combi model usually costs more than a like-for-like swap because of the additional labour and materials required.

Typically, the cost to convert from a conventional system to a combi boiler ranges between £2,500 and £4,500.

This price variation depends on:

1. Relocation: Are you putting the new boiler in the same place, or moving it to a new room? Moving piping takes time and money.
2. The Boiler Model: Premium brands (like Worcester Bosch or Viessmann) cost more than budget options but often come with longer warranties.
3. Complexity: Removing water tanks from a difficult-to-access loft or removing a back boiler will increase labour costs.
4. Upgrades: The cost usually includes a new flue, magnetic filter, system flush, and smart controls.

Back Boiler Conversion Costs

If you are removing a back boiler, expect the cost to sit at the higher end of the spectrum, typically £3,000 to £5,000, due to the structural work involved in the chimney breast and the complete relocation of the heating hub.

While the upfront cost is significant, the return on investment comes through lower energy bills and the elimination of maintenance costs for ageing cylinders and tanks.

However, it is not a "one size fits all" solution. Assessing your mains water pressure and household demand is critical to ensuring a combi boiler meets your needs. Because of the technical complexities regarding gas pipe sizing and system flushing, this is a job that requires experienced hands.

Serviceteam Heating Experts

At Serviceteam, we specialise in complex heating conversions. Our qualified engineers can assess your current system, recommend the ideal boiler for your usage, and carry out a seamless installation that keeps your home warm for years to come.

Do not forget to read our blog ‘Things to Consider Before Boiler Installation’ for helpful guidance before you choose and install your new system.

Frequently Asked Questions

Will removing my water tank affect my shower?

If you currently have a gravity-fed shower (which often feels weak without a pump), switching to a combi boiler will likely improve your shower pressure significantly, as it will utilise mains pressure. However, if you have a power shower (which has a built-in pump), this will need to be removed or replaced, as power showers are not compatible with combi boilers.

Can I install a combi boiler in the airing cupboard?

Yes. Many homeowners choose to install the new combi boiler in the airing cupboard where the hot water cylinder used to be. This simplifies pipework connections. Alternatively, you can reclaim that cupboard for storage and have the boiler installed in the kitchen or utility room.

How long does the conversion take?

A standard boiler swap takes 1 day. However, a combi conversion typically takes 2 days. Day one involves removing the old tanks and updating pipework; day two involves hanging the boiler, flushing the system, and commissioning the new setup.

Is a smart thermostat included?

Most reputable installers, including Serviceteam, will recommend or include a smart thermostat (like Nest, Hive, or Tado) or a load-compensating thermostat to ensure your new system operates at peak efficiency.