Should I replace my conventional boiler with a combi?

If you have just bought a new home, your existing boiler has broken down or the boiler that you have is over 12 years old, then you should maybe consider a replacement. Combi boilers are fantastic, as they provide space heating as well as water on demand – you can do away with the hot water tank.

If you have a regular heat-only boiler is it worth making the change?

Your existing heating system

First of all, it is important to establish the existing heating system you have and the type of arrangement you are looking for. This will impact on whether or not a combi boiler is appropriate to install in your home.

cold tank

When talking about ‘conventional systems’, we are normally referring to a heat-only boiler, which has two cold-water tanks in the loft space and a hot water tank, normally found in the airing cupboard.

A system boiler is very much like a heat-only boiler, except you don’t require the feed and expansion cold-water tanks in the loft.

A combi boiler, as discussed previously, works by providing hot water without the need for a hot or cold water tank. Basically, if you turn the hot water taps on, the boiler will fire up and heat the water coming out of the pipes. Since you don’t need to store the hot water before you use it (i.e. there is no hot water tank), these tend to be considered more efficient.

What are the reasons for installing a combi boiler?

One of the main reasons for installing a combi boiler is the fact you can do away with the water tanks and this obviously frees up a lot of space. This is particularly important in flats and small houses where space is at a premium. Imagine suddenly being able to use your existing airing cupboard for additional storage space. You still need a space for the boiler of course, but it is typically similar in size to a standard heat-only boiler and could go in the same space.

Related to the previous point is the fact that the combi boiler doesn’t produce and store hot water. With a regular boiler, hot water tends to be produced twice a day, regardless of if it gets used or not – in the morning and in the evening. Now obviously this requires the boiler to fire up to produce the hot water, and if for whatever reason it doesn’t get used the water simply cools down, which is a waste of energy. Combi boilers only produce the hot water when you need it (i.e. you turn a tap or the shower on), so obviously for many people this produces substantial energy savings.

>>> How much should a new boiler cost? >>>

Combi boilers are great when you have relatively small hot water demand at any one time. If you require hot water feeds in multiple outlets at the same time– say that you want to run 2 showers at the same time in the morning, then a combi boiler won’t be suitable. In this case a regular or system boiler might be more suitable.


The final point is that with a regular boiler or system boiler once you have used all the hot water, that is it; you’ll need to wait until the boiler fires up and produces new hot water as per your programmer settings. With a combi boiler you could in theory produce limitless hot water although the mains water pressure defines the output.

Obviously it is worth mentioning efficiency – a new combi boiler should be 90% efficient (as would a new system or regular boiler), so if your boiler is particularly old you might want to install a new boiler at this point.  You can run some calculations to see if this might be relevant for you – you can learn more about this by clicking here.

When should I install a combi boiler?

You can in theory change your heating system any time, but the best time to do this is probably during a general refurbishment (provided the boiler is working okay!).  Carrying out this work when you refurbish will help minimise upheaval since things like running new pipes under floors are complicated if there is a nice carpet in place! Adding any new radiators to the system will obviously make this a bigger job, as will a system conversion when you move from a system or heat-only boiler (with tanks) to a combi.

If you are carrying out refurbishment work, you should also try and install as much insulation as you can at the same time as installing a new heating system – this will lower your gas demand since heat will leave the home more slowly and therefore you should see even bigger energy savings.

>>> How much should a new boiler cost? >>>

We also recommend looking to install a boiler before the winter, since at this time there tends to be a big rush on plumbers, which pushes the price up a little – changing the boiler during the summer months should therefore be a little cheaper.

The final point is related to positioning the new boiler – with regards to where the existing boiler and flue currently sit. If the boiler is being replaced then the flue will have to sit on an outside facing wall, which may require physically relocating it. This will add to your costs. A plume diverter may also need to be installed depending how close the current flue is in proximity to your local neighbours.

Condensing combi boilers

Condensing boilers are pretty much standard these days for all domestic applications – any boiler you install now, be it combi, heat only or system will be condensing.

A condense pipe connected to a downpipe.

Condensing boilers are highly efficient and most are now manufactured with an efficiency rating of 90% or better. They use heat in the waste flue gas to pre-heat the cold water going into the boiler, hence this high level of efficiency.

A condensing combi boiler (like all condensing boilers) will have an additional condensing pipe that allows the condensed vapours to be drained away as the boiler is working. This shouldn’t be a problem as long as the installer can attach the pipe outlet to a drain, either internal or external. If this isn’t possible then the boiler may have to be relocated, which will add to the overall installation cost.

Combi boilers and flue gas heat recovery systems

A Flue Gas Heat Recovery System (FGHRS) works by using a further heat-exchanging unit to take advantage of the heat within the waste flue gases, which any boiler will produce. This recovered heat is used to preheat the cold water entering the boiler, thereby lowering the amount of energy needed to warm the water up to the required level.

Even the most efficient boilers available on the market today are only 90% efficient, as a result of heat lost in the waste flue gas; however the installation of a FGHRS on even a brand new boiler can help further drive up energy efficiency, helping you save money on your bills.

However, depending on the installation a separate FGHRS will add to the overall installation cost, and with the GDHIF being scrapped in 2014, there are few incentives to draw on to ensure the installation process is cost-effective.

>>> What is the best brand of boiler to install? <<<

How much do combi boilers cost?

There is a blog discussing the cost of boilers here.

  • Straight swap with existing combi – starting £1,600 + VAT (labour & parts)
  • Combi conversion (going from heat only or system) – starting at £2,500 + VAT (labour & parts)

Is it worth swapping to a combi boiler?

If you have a small house and you are already tight for space, then combi boilers are a fantastic solution. Also if you are just moving into a property and will be carrying out extensive refurbishment, then having a new heating system installed at this time is ideal.

On the other hand, if you have a large house with extensive hot water demand at any one time, you may want to go for a system/regular boiler.

Also if you are on a tight budget, the system conversion will add to the overall costs.

Installing a new boiler

Are you thinking about getting a new boiler? We have scoured the country for the best tradespeople, so that we can make sure we only recommend those we really trust.

If you would like us to find you a local installer to install a new boiler in your home, just fill in the form below and we will be in touch shortly!

Need a new boiler?

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