What Type of Boiler Do I Have?

May 4, 2021

This is a question we’re asked - a lot!

From family members to friends, to customers and random emails waiting in the office for us in the morning.

It seems everyone wants to know what type of boiler they have.

It’s no wonder, even engineers we know get confused between heat only boilers and system boilers and if there’s an unvented cylinder does it mean I have a system boiler.

Below we’ll try to simplify the process of determining what boiler you have and list the pros and cons of each.

There are three main types of boiler commonly installed in the UK,

Heat Only, System and Combi

Heat Only

The heat only boiler is the oldest type of boiler installed in the UK and is a comparatively simple appliance.

They’re also referred to as regular boilers and older versions would have been floor standing open flued appliances, like an Ideal Mexico CF.

Modern versions can be wall hung room sealed appliances, like the Vaillant 400 series.

They will typically be installed on an open vent heating system and will be accompanied by a hot water cylinder and two water tanks in the loft.

In the cylinder cupboard, you’ll also have an external pump and zone valves.

It is not uncommon for the heating system to be converted from an open vent type to a pressurised system, in this case, you’ll also have a red expansion vessel external from the boiler and have one large tank in the loft to refill the cylinder as you use hot water.

A heat only boiler will have 2 copper pipes coming from the top and one feeding gas into the bottom, if it’s newer it will also have a plastic condensing pipe coming from the bottom.

They are best suited to larger family homes with 2 or more bathrooms with large hot water demands.

The main benefits of a heat only boiler are the large hot water capacity and the backup immersion if the boiler breaks down, also, you’re less likely to experience problems if there is a small leak on the heating system.

The main drawbacks are they take up lots of space, the external components are not covered by the manufacturer guarantee and they are typically the least efficient boilers available.

System Boiler

A system boiler is like a heat only boiler in as much as it will be installed with a hot water cylinder.

The system boiler will be installed on a pressurised heating system meaning you only need one tank in the loft to refill the hot water cylinder, unless you have an unvented cylinder then you’ll have no tanks in the loft.

A system boiler will have three copper pipes going into the bottom and a newer condensing boiler will also have a plastic pipe leaving the bottom of the boiler.

They will nearly always be wall hung, room sealed heating appliances that are safer and save space.

Again, they are best suited to larger family homes with high hot water demands.

Because the heating system is sealed from the atmosphere there is less chance of oxygen being pulled into the heating system meaning they are generally more efficient, and the heating system will deteriorate slower.

The main benefits of a system boiler are the pump and expansion vessel are incorporated inside the boiler meaning they will be covered by the manufacturer guarantee and save you some space and time on the installation. You’ll also have a backup immersion on the cylinder so if the boiler breaks down, you’ll be able to have hot water while you wait for the boiler to be repaired.

The main drawback is if you have a tiny leak, or the heating system loses pressure the boiler will fail to operate.

Combi Boiler

A combi boiler will be a wall hung; room sealed heating appliance.

It will be installed on a pressurised heating system and incorporates all the external components inside the boiler including a diverter valve, pump, and expansion vessel.

With the addition of a second heat exchanger known as a plate-to-plate heat exchanger it can give instantaneous hot water on demand and there is no need for a hot water cylinder or tanks in the loft.

The combi boiler is more efficient than both a heat only boiler and system boiler as there is no large external cylinder to heat, so it only provides hot water when it’s required.

It will have 5 copper pipes going into the bottom and a newer condensing model will also have a plastic pipe leaving the bottom.

It is best suited to flats or single bathroom houses with lower hot water demands.

The main benefits of a combi boiler are it saves lots of space and as all the external components are inside the boiler, they will all be covered by the manufacturer’s guarantee.

The main drawbacks are it is limited to how many taps it can supply hot water to at one time and if the boiler breaks there is no backup immersion to provide hot water.

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Thank you for reading.