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The first 'smart' oil boiler for off-grid homes


Reece Summerfield

Technical Director

Sapphire Oil Boilers

Most oil boilers are inefficient and all are big carbon emitters, so what is the future for off-grid homes? Reece outlines the changes taking place.


1.6 million homes across the UK have an oil boiler, but despite the size of the market the technology has not really moved on in a decade.

As we transition to greener homes, oil has no place in the future energy mix and from the outside it must look like an industry in decline. Yet pockets of innovation exist and biofuels remain on the political agenda. In this blog, I explore oil boiler options, some pioneering improvements in the technology and the wider shift to greener technologies.


Both gas and oil boilers are ‘condensing boilers’ as standard which means they can boost efficiency from 75% or less (for a non-condensing boiler) up to 90% by recycling some of their waste heat.

However, achieving 90% efficiency is not guaranteed. Like miles-per-gallon for cars, oil and gas boiler efficiency is variable and depends on the success of the technology in the home and to some extent the competency of the installer fitting it. For an optimised performance the boiler needs: 1) lower temperatures to radiators, 2) the right heating control, and 3) a low minimum kW output. Modern oil boilers are missing all three, until now.

1) Low flow temperatures to radiators

If your condensing oil boiler is to stand any change of achieving 90% efficiency it must supply a lower temperature to the radiators of 60degC or less, but low temperatures rarely happen in practice even though most homes are suitable. Condensing boilers tend to be left at their factory setting of 75-80degC, making them little more efficient than old non-condensing models.

This is a part-installer, part-boiler problem. Oil boilers don’t provide any easy way of running a lower temperature for heating without affecting hot water, but even if they did most installers wouldn’t utilise it. This is certainly the case for gas boilers.

2) The right heating control

‘Compensation’ controls are a must-have for condensing boilers, but they simply don’t work with condensing oil boilers.

Compensation controls track the inside and outside temperatures and automatically turn down the boiler's flow temperature at every opportunity whilst still keeping the home warm. For boilers that work with a cylinder, some additional configuration is needed if the flow temperature for heating is to be adjusted without affecting the hot water cylinder.

Gas boilers at least have the potential to work with a compensation heating control, even if they are not fitted as often as they should be.

Condensing oil boilers on the other hand are not compatible with compensation controls, which makes it hard to see how they ever reach their efficiency potential. Instead most oil boilers supply a high, fixed temperature for heating and hot water and households pay 10-15% more for their heating as a result.

3) A low minimum kW output

The size of the boiler (in kW) is in theory based on the size and construction of your home. Larger, older homes, need more kW of power than smaller, well insulated properties.

A quick scan of boiler manufacturer websites shows that the smallest oil boilers have a fixed output of 15-21kW and the largest up to 36kW. Despite the implication that a small home needs 15-21kW, small homes actually need 6-8kW.

Oil boilers, like gas boilers, are massively oversized for our needs and this results in them ‘cycling’ on and off. To return to the car analogy, oversized boilers are like a traffic jam, when what we want is for the boiler to ‘cruise’ along. Gas boilers have been developed to adjust their kW output down when less power is required, enabling them to cruise.

Oil boilers cannot reduce their kW output in this way, which means they waste more energy cycling on and off.

A long awaited oil boiler evolution

A condensing oil boiler that spends little time in ‘condensing mode’ is an technological evolution that has not materialised in practice. A true evolution is getting the boiler to perform in the home as well as it does in laboratory test conditions, and that has been the focus of developing the Sapphire oil boiler.

Sapphire is the first and only boiler to work with a range of compensation controls. More than that, it has been tested to work with different compensation control brands, including EPH, Genius and Honeywell. This is way beyond what most gas boiler manufacturers attempt with third party controls.

With compensation controls enabled, it is for the heating engineer to reconfigure the system to provide a variable temperature for heating without affecting the hot water, which is why Sapphire boilers can only be fitted after installers have completed training.

The Sapphire boiler also behaves more like a gas boiler as it can adjust its kW output up and down according to the demand for heat. It is the first boiler to ‘cruise’ and whilst the smallest boiler is 23kW, it can automatically reduce its output down to as low as 5kW.

Oil - a fuel on its way out

Given the move away from fossil fuel boilers, some might argue it all feels a little late in the day for improving oil boilers.

As it stands, oil boilers do not have a phase out date. An earlier target of 2026 has been scrapped, but a new date is not yet on the table. For the thousands of households that will fit a new oil boiler in the coming years, developments in oil boiler technology remain as important as ever for reducing carbon emissions and energy bills.

Running alongside the need for oil boiler innovation is the development of bioliquids (aka biofuels) as a direct replacement for oil.

Bioliquids are widely viewed as having a key role to play in the transition for those homes not yet ready to move to a heat pump or too large for a stand alone heat pump. However many have expressed concerns over growing crops for fuel instead of food and the use of palm oils.

In response, the industry has developed the PAS5420 2023 standard which specifies that bioliquid fuels for home heating will only use waste-derived sources, such as used fats from cooking. Waste bioliquids reduce carbon emissions by around 88% compared to oil.

No decision has been made as yet on the inclusion of biofuels in the future energy mix, but following the recent Energy Bill Government must consult on the inclusion of biofuels by the end of 2024. It is already possible to buy approved bioliquids, however it is currently around x3 the price of oil.


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