Where oil boiler efficiency actually goes

The quoted figure and the real one

A boiler’s quoted efficiency is measured under laboratory conditions at specified return temperatures and loads. It describes what the appliance can do. What your system achieves depends on how it is sized, commissioned, controlled and maintained, and the difference between the two is frequently in double figures.

The losses, roughly in order of how much they typically cost:

1. Running too hot to condense

This is usually the largest single avoidable loss and almost nobody knows about it.

A condensing boiler gains its efficiency by cooling the flue gases enough that water vapour condenses, releasing latent heat. That only happens when the return water temperature is low enough. If the system runs at a high flow temperature, the return is also high, the gases never condense, and you have paid for a condensing boiler that is not condensing.

Most boilers leave the factory set at or near their maximum flow temperature, and most installers leave them there. Reducing the flow temperature to the lowest setting that still heats the house is free, reversible, and frequently the single most cost-effective change available.

2. Fouling of the heat exchanger

Soot on the combustion side and scale or sludge on the water side both insulate the heat exchanger, which is the one component whose entire purpose is to transfer heat.

A layer of soot a millimetre thick has a measurable effect on efficiency. It builds gradually from slightly imperfect combustion, which is exactly what happens as a nozzle wears through the year. This is the mechanism by which an unserviced oil boiler quietly gets more expensive – not a sudden failure, a steady decline nobody notices.

On the water side, magnetite sludge in the system and, in hard water areas, scale on any surface heating domestic hot water, do the same job from the other direction. Hence system cleaning, inhibitor, a magnetic filter and, in this county, water treatment.

The flue gas temperature reading at each service is the objective evidence of this. A figure climbing year on year means heat is going up the flue instead of into the house.

3. Oversizing and short-cycling

An oversized boiler satisfies demand quickly, shuts down, cools, and fires again. Every cycle includes a pre-purge that blows cold air through a hot heat exchanger and up the flue, and an ignition sequence. The appliance spends a large share of its operating life in the least efficient part of its range and cycles rather than modulating.

Most replacement boilers are oversized, because they are specified from the old appliance’s output rather than from a heat loss calculation, and the old one was very likely oversized too. It compounds with each replacement.

This is not correctable after installation except by better controls, which is why the heat loss calculation at specification stage matters so much – see central heating installation.

4. Poor commissioning

• Pump pressure not set to the data plate, so the boiler fires at the wrong rate.
• Excess air wrong. Too much air and you heat the outdoors; too little and you make soot, which then compounds the problem in item 2.
• Worn nozzle left in place, widening the spray pattern and worsening atomisation.
• Electrode gap wrong, producing unreliable ignition and a poor flame.

All of these are set during a proper service with a combustion analyser, and none of them can be judged by eye. This is what the readings on the service ticket are actually measuring.

5. Distribution and standing losses

• Uninsulated pipework in lofts, voids, garages and underfloor spaces. Heat delivered to a cold loft is heat you paid for and did not use. Pipe insulation is inexpensive and among the fastest-paying improvements available.
• A poorly insulated hot water cylinder, losing heat continuously, twenty-four hours a day, whether you use hot water or not. An old cylinder with a thin jacket is a permanent leak.
• Long runs between buildings, which is a farm and rural estate problem in particular – see agriculture.
• An external boiler’s own standing losses go outside rather than into the house.

6. Controls

Heating a house nobody is in, or heating every room to the same temperature regardless of use, is a straightforward waste that costs nothing to correct.

1. A schedule that matches actual occupancy, rather than one set once and never revisited.
2. Thermostatic radiator valves, so bedrooms are not heated to living room temperature. Cheap, effective, and frequently seized on older systems.
3. Weather compensation, which varies the flow temperature with outside conditions and keeps the boiler in its condensing range far more of the time.
4. Separate hot water scheduling, so the cylinder is not being reheated all day.
5. A balanced system, so heat reaches the rooms that need it rather than the nearest ones – see balancing a heating system.

What to do first

The order matters. Almost everyone starts at the bottom of that table and almost everything worth doing is at the top.