External combi boilers – Natural gas or LPG-fired

External combi boilers provide both central heating and instant hot water without the need for a hot water cylinder. Combination boilers are the most popular type of boiler in the UK and Ireland due to their high efficiency for both heating and hot water production and easy installation. Our external combi boiler versions are available in either natural gas or LPG (propane)-powered options and are a great alternative to oil boilers.

As standard, our gas-fired outside or outdoor combi boiler products are manufactured to ErP Class “A” ratings for heating & domestic hot water.

Our external combination gas boilers have an IP5XD weatherproof rating (for protection against rain and weather) and an antifreeze protection system that automatically starts the boiler when the internal water temperature in the stainless steel heat exchanger falls below 8°C, firing up the boiler until the water temperature reaches 30°C within the heat exchanger and preventing it from freezing, as long as you have the gas supply and the power supply available.

Advantages of outside combi boilers

An external combination boiler is a type of combination boiler that is designed to be installed outside the property, usually in a weatherproof casing. This provides one unit for both central heating and domestic hot water without the need for a separate hot water cylinder or cold water storage tank.

One of the big advantages is saving space indoors. The boiler is externally located to allow space within the home, utility room, garage or plant area. This is especially useful in smaller homes, workshops, offices or properties with limited internal space.

This also means less noise in your home. Because the boiler is external, it keeps normal operational noises out of the living or working space. This makes it a particularly suitable choice for homes where the boiler would be near to bedrooms, kitchens or occupied rooms otherwise.

Outside gas-fired combi-boilers can also make servicing and maintenance easier, as the engineer can often access the boiler without needing to enter the property. It is useful for landlords, commercial properties, holiday lets and rural properties.

They are also useful when replacing older heating systems, especially where there is no suitable indoor wall space, flue route or plant room available. The outer casing protects the boiler from the weather and enables it to operate safely outdoors.

Off-gas areas often have external LPG combi boilers which provide both heating and hot water in one small outdoor appliance.

Simply put, the main benefits of an external combi boiler are space saving, quieter operation in the home, easier access for servicing, installation flexibility and no need for a separate hot water cylinder. They are a practical choice if internal space, noise or flue positioning is problematic.

Greatly reduce the risk of carbon monoxide poisoning

Unfortunately, it is commonly reported that around 7 people a year die in the UK from carbon monoxide poisoning caused by poorly fitted, maintained or vented gas appliances and flues. This is the figure provided by the Health and Safety Executive (HSE)

Because the boiler is located externally, it significantly reduces the risk of carbon monoxide (and thus carbon monoxide poisoning) entering the building compared with an internally installed gas boiler. However, correct flue positioning, installation, servicing and annual safety checks are still essential, as no gas appliance can be described as completely risk-free.

Additional benefits of mounting the combi boiler higher up on the wall 

The primary benefit is flood protection. By placing the boiler above the expected flood level, the risk of flood water reaching the burner, fan, PCB, gas valve, pump, heat exchanger and electrical components is reduced.

This can lessen the likelihood of a total boiler breakdown following floods and can allow the home to recover more rapidly. The UK flood resilience guideline often stresses limiting damage and speeding up recovery during floods. Government flood-risk advice also often suggests raising sensitive parts of buildings above the expected flood levels when possible. You can do this with our 

Please bear in mind that you must also provide safe service access, especially if you mount the boiler high up. The gas, electrical and condensate pipework also needs protection from frost, weather and potential flood damage.

Damage protection / avoidance

A high-level mounted outdoor combi boiler can also assist with security and with unintentional damage. When you mount it above easy reach, it is less likely that cars, storage items or garden equipment will knock, tamper with, vandalise or destroy it.

External combination boiler sizing 

The main considerations when sizing an external combination boiler are the property’s heating demand, and domestic hot water demand

Heating system demand

The first thing to sort is the space heating load: the boiler must be powerful enough to heat the building on the coldest days of the year. Don’t believe the hype about just counting the radiators; a proper heat loss calculation is always the way forward.

The major benefit of our external combi-boiler range is their ability to modulate output, meaning they can adjust the boiler’s output performance to match the heating demand, which is essential for reducing your heating bills and household comfort.

For example, 

24kW combi boiler version

Our 24 kW external combi boiler will ramp all the way down to a minimum of 2.75 kW of heating output if run on an 80/60°C heating circuit or 3.02 kW if run on a 50/30°C heating circuit (it works in full condensing mode, recovering a lot of latent heat from the flue gases at these temperatures – hence the larger kW output figure). 

28kW combi boiler version

Our 28 kW external combi boiler will ramp all the way down to a minimum of 3.52 kW of heating output if run on an 80/60°C heating circuit or 3.83 kW if run on a 50/30°C heating circuit, i.e., in full condensing heating mode, as mentioned above.

34kW combi boiler version

Our 34 kW external combination boiler will ramp all the way down to a minimum of 3.94 kW of heating output if run on an 80/60°C heating circuit or 4.34 kW if run on a 50/30°C heating circuit, i.e., in full condensing heating mode.

So in short, all of our outside combi boilers can modulate down to very low levels of heating output, which means you can’t really oversize them for the heating side of things. It’s the domestic hot water requirements that really decide what output combination boiler you will require.

Hot water heating demand

Next up is domestic hot water demand, which tends to be the main factor these days for sizing externally mounted combi boilers. Because a combi boiler produces hot water instantaneously, its output has a direct impact on the hot water flow rate. A small one-bathroom place might get by with a small combi, but a big house with loads of bathrooms or high hot water usage needs something more powerful.

In some cases, a combi just isn’t the right fit; a system boiler with an unvented cylinder might be a better bet (we also supply these gas-fired external system boilers – please see this webpage)  External gas boilers – and we also supply the indirect unvented hot water cylinders – please see this webpage: Unvented indirect hot water cylinders 

We always recommend sizing on the coldest mains water temperature, which tends to be 10°C for most of the UK; however, for Scotland and Northern Ireland, the lowest cold water incoming temperature can be as low as 5°C in the winter months.

So use the temperature rise (shown in the below DHW performance chart as “ΔT”) of 30°C for the majority of the UK , and please use the 35°C for Scotland and Northern Ireland 

Hot water outlet flow rates 

A typical shower head requires seven or eight litres of hot water per minute, while “green band” shower heads require six litres.

Please be advised that drench showers can have extremely high flow rates, ranging from 15 litres to almost anything; 30 litres per minute is not unusual.

 

If you have a “drench shower”, it’s best to use our external system boilers and an indirect hot water cylinder installation.

Other hot water outlets’ flow rates

Taps in kitchen sinks: 4 to 6 litres per minute

Standard washbasin taps: 4 litres per minute.

Water-saving “spray taps” for basins and cloakrooms, 2 litres per minute,

Bath mixer taps: The demand rate for modern bath taps is around 12 litres per minute.

You may thus determine your hot water demand by combining all your possible hot water outlets and then using a diversity factor, which accounts for the fact that not all outlets will be used simultaneously.

Combi boiler product options  

We have three models in our outdoor combi-boiler range: a 24 kW, 28 kW, or 34 kW model, all of which can be supplied for either natural gas or propane (LPG) gas, i.e., an external LPG combi boiler.

The appropriate manuals are here for direct download:

24kW R2K Rain external gas combination boiler manual (H version) from Flexiheat UK

28kW and 34kW Rain external gas combination boilers manual from Flexiheat UK

All units have the same external dimensions; just the boiler inside is different. These dimensions are 

Oil boilers?

Why use an external gas combi boiler over an oil boiler version? 

Gas boilers also avoid the need for a large oil storage tank. With an external oil boiler, the property still needs an oil tank, base, pipework, fire separation clearances and periodic checks. A gas boiler, especially on mains gas, removes the need to monitor fuel levels, arrange oil deliveries, or worry about running out during winter.

In terms of maintenance, gas boilers are often simpler and cleaner to service. Oil boilers require oil filters, nozzles, pump pressure checks and combustion adjustment. Gas boilers still need annual servicing, but they generally involve less messy fuel handling.

Running costs can also favour gas, particularly where mains natural gas is available. Oil prices can fluctuate significantly and depend on delivery costs, order quantity and seasonal demand. Mains gas is usually more convenient and often cheaper per unit of heat than heating oil.

Outdoor combination boilers for Sale in the UK & Ireland

If you require any further assistance or more information on our outside / outdoor / external combi boilers, please click here to contact us via email or call us directly on 01202 822221.