Are you sick and tired of your old combi boiler compromising the flow rate to your large property?

If you’re having problems with running two showers at once, the bath is taking a while to fill, or your house starts to go cold because the combi boiler is using up all its energy to heat your hot water, it could be time for a change.

One of the major benefits of installing a system boiler with unvented cylinder is that you’ll have 24-hour access to a high pressure system, and all the heating and hot water your large property will ever need.

Below, I’m going to talk you through everything you need to know when switching from a combi boiler to a system boiler with unvented cylinder.

How easy is it to switch?

The good news is, it’s actually pretty straightforward to switch to a system boiler with unvented cylinder, as it should be able to go in the same space as your old combi.

The only thing you really need to make sure of is that you have a cupboard or attic space for your unvented cylinder, as it will require additional space. You must also be able to supply hot and cold water pipes to the cylinder.

What is involved?

Firstly, the combi boiler has to be removed, and the system boiler fitted in the same location – this switch can take up to one day.

Then, the unvented cylinder has to be fitted in a cupboard or attic space. Hot and cold supplies will have to be run to the cylinder and the cylinder has to be plumbed in, before a control pack is fitted – this essentially means valves so the hot water can be switched on when needed, along with time controls and thermostats for your heating and hot water.

Fitting the unvented cylinder can take an additional day of work, but it will be worth it for the fantastic water pressure your whole family can benefit from.

What complications could there be?

Occasionally there can be complications when switching from a combi boiler to a system boiler with unvented cylinder.

The two main complications are:

1. You have stone floors which cannot be disturbed

If you have flooring which cannot be disturbed, such as polished stone or wooden flooring, it can be particularly difficult to feed the safety discharge pipe through to the outside of the house – a safety requirement for unvented cylinders.

Because of this, engineers may take longer to come up with a solution to get around the flooring and make sure the pipe reaches the outside of the property.

2. You have an old 15mm pipe

Manufacturers of unvented cylinders recommend the incoming cold water main supply pipe is 22mm in diameter. This is a problem in a lot of older properties in Edinburgh where the old mains pipe used to be half inch lead – or 15mm.

This means the flow rate from the unvented cylinder may suffer as a result, or alternatively the cold water mains pipe may need to be replaced so that a larger pipe can be fitted. This may involve excavating your driveway or garden to install the new pipe.

How much does it cost?

This is the part that not everyone likes to hear.

Switching from a combi boiler to a system boiler with unvented cylinder can cost you roughly between £3,500 and £4,000.

This is mainly because an unvented cylinder, supplied and fitted, will cost an average £1,100 on top of the price of your new system boiler and motorised valves and control pack


If your old combi boiler simply isn’t up to the job of supplying your large property with hot water, and your flow rates are suffering as a result, it could be time to make a change. A system boiler with unvented cylinder guarantees fantastic flow rates and all the heating and hot water your family will ever need – and it’s fairly simple to switch.

Your number one concern is to ensure you have the additional cupboard space for your new unvented cylinder, and that you don’t mind engineers taking a couple of days to get everything fitted correctly – it’s a much bigger job than simply installing a new boiler.

If you have stone floors which cannot be disturbed, or an old-fashioned 15mm mains pipe to your property, be prepared for additional delays/complications and extra labour costs this might cause.

Finally, enjoy your new heating system and fantastic water pressure!