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Boiler types explained - combi, heat only and system Boilers.

Deciding on the Right Boiler for Your Home

Figure technical drawing of an Unvented cylinder
Unvented cylinder

There are primarily three types of boilers available: the combi boiler, heat-only boiler, and system boiler. Heat-only boilers, often known as conventional or regular boilers, work in cooperation with a cylinder situated in your airing cupboard. System boilers, favoured in modern residences, are distinguished by the use of an 'unvented' hot water cylinder. In contrast, combi boilers provide hot water on demand, instantly. This guide will assist you in understanding what each boiler type offers, including their advantages and disadvantages. This information can be crucial when considering whether to retain your current boiler or explore a new type.

Boiler types explained Below: Understanding the Combi Boiler

A combi boiler types is an all-inclusive unit catering to the heating and hot water demands of a home. This system operates without necessitating hot water tanks. At DDWilson, we provide a like for like combi swap service starting from £1,999.

The Heat-Only (Regular) Boiler: An Overview

Heat-only boilers are solely responsible for heating. They operate with a hot water cylinder, and typically, you may find water tanks housed in the loft. Our services for a similar replacement utilizing a 15kw heat-only boiler commence from approximately £1,299.

Getting to Know the System Boiler

A system boiler contains all components of a combi boiler apart from hot water production facilities. It uses either a steel hot water cylinder or an unvented g3 cylinder. With DDWilson, you can get this installation starting from £1,399 with a 15kw system boiler.

Our Customers' Favourite: The Combi Boiler

How does a combi boiler function? This combined furnace and water heater warms water directly from the cold mains via an integral heat exchanger, delivering hot water instantly. This technology deviates from traditional systems, which store hot water in tanks for future use. Combi boilers do not require a hot water cylinder or additional loft tanks, integrating all operations through a single, typically wall-hung unit.

Advantages of a Combi Boiler:

  • Frees up space by eliminating old hot water tanks

  • Lowers maintenance costs as the integral heating and hot water mechanism fall under the boiler manufacturer's warranty/guarantee

  • Provides hot water without significant wait times

Disadvantages of a Combi Boiler:

Hot water flow rates may decrease when supplying hot water to multiple outlets at once, although this issue primarily affects households with two or more showers

For guidance through the selection process, contact us at 01517398945, option 1.

An Overview of the Heat-Only Boiler

Regularly called a 'regular' or 'conventional' boiler, a heat-only boiler supplies heat directly to the radiators while working with a cylinder to deliver hot water. They usually function on an open-vented heating system, meaning there is a Feed and Expansion tank in the loft, though they can also work with a 'sealed' system.

The Back Boiler: A Consideration

A back boiler, located within a chimney breast with a fire on the front, is a heat-only boiler. It's currently unfeasible to replace a back boiler at the exact location. If you need to replace an existing back boiler, opt for a wall-hung heat-only boiler or consider a combi boiler at a different location in the house.

Advantages of a Heat-Only Boiler:

  • Compatible with older radiator systems susceptible to leaking due to high water pressure from system or combi boilers

  • Low replacement cost due to its relative affordability

Disadvantages of a Heat-Only Boiler:

  • Only the boiler is under the manufacturer's warranty/guarantee, while other system components may require replacement or break down

  • A combi boiler generally offers the best advantages if your home benefits from good water pressure.

The Functioning of a System Boiler

Although system boilers work with a hot water cylinder like regular boilers, the setup doesn't use an open-vented system, meaning there are no tanks in the loft. Additionally, all components outside a heat-only boiler (pumps, valves, etc.) are integrated within the boiler, representing a modernized version of the regular boiler.

Advantages of a System Boiler:

  • Ideal for properties with high hot water demand, as the unvented tank can deliver substantial hot water flow rates to multiple outlets at the same time

  • The manufacturer warranty/guarantee thoroughly covers the majority of the system's components

Disadvantages of a System Boiler:

  • Substantial hot water flow rates are reliant on solid incoming mains water pressure

  • Requires more space than a combi boiler, thus needing a suitable location for the hot water cylinder if replacing a combi boiler

Factors to consider:

Families taking concurrent showers, tight budgets favouring like-for-like boiler swaps (starting from £1,299), low maintenance costs preferring combi boilers with a 10-year manufacturer's warranty/guarantee or property extensions adding bathrooms (should consider a system boiler with unvented hot water tank). This is why it's essential to explain boiler types. Boiler types explained (we hope). If not, we will cover different boilers types in future blogs.

With thousands of five-star reviews, DD Wilson Gas Engineers is ready to meet your boiler and heating system needs to the highest standard. Whether you're updating or considering a new installation, we're here to advise. Please get to know us more at


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