Are Oil Combi Boilers Good: A Comprehensive Technical Guide

Oil-fired combi boilers have been a subject of debate, with their advantages and disadvantages being extensively discussed. As a technical expert, I will provide a detailed and comprehensive guide on the intricacies of these boilers, covering their technical differences, compromises, advantages, disadvantages, real-life experiences, and technical specifications.

Technical Differences and Compromises

Oil-fired combi boilers have fixed-rate burners, which means they cannot modulate their heat output like gas-fired combi boilers. This leads to several technical compromises:

Integral Heat Store: Oil combi boilers often have an integral heat store, making them larger than conventional heat-only boilers. This heat store typically has a capacity of 12-20 liters and helps cope with additional heating demand, but it can be inefficient due to standby heat losses.
Larger Boiler Input: Oil combi boilers typically have a larger boiler input (typically 25-35 kW) than required for central heating, which can lead to inefficiency. This oversizing is necessary to ensure adequate hot water supply, but it results in the boiler operating at part-load for much of the time, reducing its overall efficiency.
Hot Water Temperature Rise and Flow Rates: The temperature rise and flow rates quoted by manufacturers for oil combi boilers should be treated with caution. In practice, flow rates are often lower, and temperature rise can fall after the integral heat store is exhausted, leading to a reduction in hot water performance.
Hard Water Problems: Oil combi boilers use a small plate heat exchanger to transfer heat from the boiler to the domestic hot water. This heat exchanger is susceptible to scaling from hard water, which can reduce its efficiency and eventually require replacement. A mechanical water softener is essential, but these take up additional space.
Hot Water Back-up: If the oil combi boiler fails, there is no back-up for hot water, unlike conventional systems with an immersion heater. This can leave the household without hot water until the boiler is repaired or replaced.

Advantages and Disadvantages

Advantages:

Versatility: Oil boilers can be flued through chimney breasts and run over long distances, making them suitable for off-grid locations where gas is not available.
High Efficiency: Oil-fired condensing boilers can achieve high efficiencies, with some models reaching up to 89% seasonal efficiency. However, the extra cost of these high-efficiency models may not always be worth the investment.

Disadvantages:

Complexity: Oil combi boilers are more complex and have bespoke components, such as the plate heat exchanger and integral heat store, making them more expensive to maintain and repair.
Limited Hot Water Supply: Oil combi boilers can struggle to keep up with high domestic hot water demand, especially when multiple showers or taps are running simultaneously. This can lead to a reduction in water temperature and flow rate.
Environmental Impact: Oil-fired boilers are generally less environmentally friendly than gas-fired or renewable energy options, with higher carbon emissions and potential for oil spills or leaks.

Real-Life Experiences

System Boiler vs. Combi Boiler: Many users, particularly in rural locations where gas is not available, prefer system boilers for their simplicity and reliability, as they do not have the same compromises as oil combi boilers.
Oil Combi Boiler Performance: Some users have reported issues with oil combi boilers, including struggles with maintaining adequate hot water supply and difficulties with maintenance and repair.

Technical Specifications

Efficiency: Oil-fired condensing boilers can achieve efficiencies in the high 80s, with some models reaching up to 89% seasonal efficiency. However, the extra cost of these high-efficiency models may not always be worth the investment, as the efficiency gains may be relatively small compared to the price difference.
Components: Oil combi boilers have bespoke components, including the plate heat exchanger and integral heat store, which can be expensive to replace. The average lifespan of an oil combi boiler is around 10-15 years, but the integral heat store and heat exchanger may need to be replaced more frequently, adding to the overall maintenance costs.

In conclusion, oil combi boilers can be a viable option for certain situations, such as off-grid locations where gas is not available. However, they often come with technical compromises and challenges that should be carefully considered before making a decision. It is essential to weigh the advantages and disadvantages, as well as the specific requirements of your home and heating needs, to determine if an oil combi boiler is the right choice.