System boiler guide 2021 | We compare prices on 64 boiler models

What is a system boiler? Who offers the best system boiler prices in 2021?

System boilers commonly work with unvented hot water cylinders (but can work with vented copper cylinders) and often found in modern homes with multiple bathrooms. Prices range from £580 for an entry-level boiler to £1,600 for a premium model. They can also be a good replacement for heat only boilers and work with standard cylinders where you need better hot water performance. The best system boiler for you will be based on your property size and your budget. In this guide we compare prices, sizes, efficiencies and warranties on 64 models of system boiler from nine leading manufacturers, including Worcester Bosch, Viessmann and Vaillant.

What is a system boiler?

System boilers work with a hot water cylinder, which can be either a steel unvented hot water tank or a standard vented cylinder. They are often used in properties with multiple bathrooms as they can supply good hot water flow rates simultaneously to two or more outlets (e.g. two showers) providing the incoming flow rates are sufficient.

Unlike heat only boilers, all of the components that would usually be found external to the boiler (pumps, valves etc) are inside a system boiler. This means more of the heating system is covered by the boiler manufacturer's warranty. System boilers require good incoming water pressure and good internal flow rates to operate at their stated capacity. If you currently have a heat only boiler on a gravity system you will need to check your water flow rates to make sure a system boiler will work.

When is a system boiler right for my home?

System boilers suit properties with multiple bathrooms, particularly where they are in use simultaneously, for example people showering in the morning. System boilers provide good hot water flow rates from a high performance cylinder. Many new build properties have system boilers with unvented hot water systems, replacing your boiler will be a straight swap. For those considering installing a system boiler in place of their combi boiler, this is worth while when you are extending and adding bathrooms. You will need to find a spot for the hot water tank and preferably close to the bathrooms. For those with a conventional heat only boiler, you will need to change the boiler and the copper cylinder.

Find your best system boiler

Use the tables below to find your best system boiler. The tables are arranged so that you can easily find your property size and then find who offers the best boiler package for your budget. The tables show boiler performance alongside warranty period and price so you can find the best value boiler. Most boiler manufacturers produce one or two models of system boiler with outputs between 12kW and 40kW. At the smaller end, the price difference between the budget and premium models is quite small. However the disparity grows significantly for the larger models.

Guide to using the boiler tables

In the example table below, it is easy to see that the Vaillant ecoTEC plus at £772 is great value with a 7-10 year guarantee, compared the next cheapest and next most expensive, which only offer a 2 year warranty.

Boiler tables

1-3 bed properties, small hot water cylinders

The average UK home needs 6-8kW of heat at peak times. It is not currently possible to buy a 6-8kW boiler, the smallest heat only boiler available is 12kW, but you will likely need around 12kW of capacity to heat a small hot water cylinder in a reasonable amount of time. Your hot water requirements will be determined by the attending engineer.

2-5 bed houses, larger hot water cylinder

If you have a larger hot water cylinder then you will need a larger output boiler. As boiler sizes increase however, so does the lowest minimum output. The best boilers are those that can go down really low (see right hand column) whilst still providing enough capacity for a bigger hot water cylinder. The most expensive boiler is the most efficient - amazing work by Worcester Bosch. However the best buys go to Vaillant and Ideal for a great price and guarantee period on the boiler

3-5 bed houses, multiple bathrooms

Here we see Potterton and Ferroli enter the group offering two incredible cheap boiler models and the Ferroli comes with a 5 year warranty. Viessmann still offers the best efficiencies in the pack. NB ignore that some boilers have much larger maximum outputs than others - the important figure is how low the boiler will go (right-hand column).

3-5 bed houses, multiple bathrooms + very large hot water cylinder

You will only need a 25kW boiler for high hot water demand households. Prices at the top end of this group are almost three times that of the entry level boilers. The Ideal Logic + System is a great all-round buy for larger hot water requirements whilst the Viessmann 200-W provides the ultimate efficiencies. Both Ravenheat boilers are amazing value, although warranty periods are slightly shorter.

Even 'the best' combi boiler is not A-rated out of the box

Combi boilers are not A-rated out of the box, they require a competent installer to set them up correctly and 99%

of installers have not been trained to do this. Whilst we guide you on best buys and best performance, they will only fulfil their potential if fitted by an installer trained in correct heating system design principles. Find out more about our work to deliver A-rated heating systems at our Heating Heroes page.

Sizing your system boiler

Most boilers are oversized for the homes they serve. The average UK home requires just 6-8kW of heat on a very cold day. The most popular system boiler is 15-18kW, more than twice the capacity most homes will ever need. So what is wrong with oversizing, just in case?

• Firstly the boiler will cycle on and off more than it was ever designed to, accelerating wear and tear.
• Secondly it will rarely operate at its most efficient, i.e. in 'condensing mode', because the return temperatures to the boiler will be too hot. Condensing boilers need the temperature of the water as it flows back to the boiler to be 50 degrees C or less. This requires a correctly sized boiler AND a perfectly balanced system with TRVs that signal the boiler to reduce its output further (modulate) as each room gets warm enough.

The good news is that most modern boilers can be 'ranged rated' down, i.e. the output reduced to more closely match your property's heat requirement, which is calculated by the attending engineer. Make sure you check that the engineer has done this. For more see our Guide to Boiler Sizing.

NOTE In many cases you will require a larger boiler however based on the capacity of your hot water cylinder coil. Coils (like an element in a kettle) may need 12kW or 25kW, depending on their size. The attending engineer will be able to assist with sizing for your hot water requirements.