In this guide, I’ll walk through what boiler efficiency actually means, how to determine how efficient your boiler currently is, what can cause drops in efficiency, and how to fix each issue properly and safely. I’ll also cover long-term prevention, when you must call a Gas Safe engineer, and practical FAQs.
How Efficient Is My Boiler?
Boiler efficiency refers to how efficiently your boiler converts fuel (gas, oil, LPG) into usable heat for your home and hot water. In simple terms, a boiler is efficient if it wastes very little energy through flue gases, poor combustion, or heat loss.
Boiler efficiency is measured using:
- ERP rating (A–G) – modern standard for energy labelling
- SEDBUK (Seasonal Efficiency of Domestic Boilers in the UK) – older but still referenced
- % efficiency (e.g., 92%)
A modern A-rated condensing boiler typically operates at 90–94% efficiency, while older non-condensing boilers may operate anywhere between 55–80%.
Why your boiler might be less efficient than expected?
Below are the most common real-world causes of falling boiler efficiency, along with clear explanations and fixes.
Is my boiler inefficient because it’s an older non-condensing model?
The answer is often yes. Non-condensing boilers (mostly pre-2005) lose a significant amount of heat through the flue because they cannot capture and reuse latent heat produced during combustion.
What causes inefficiency in old non-condensing boilers?
- No heat recovery during combustion
- Outdated burner and thermostat controls
- Natural build-up of soot and scale
- Poor insulation and heat loss
How to fix this cause
1. No heat recovery
Fix:
There is no real fix for this issue—this design limitation means the boiler can never reach modern efficiency levels. Upgrading to a condensing boiler is the only way to achieve efficiency improvements.
2. Outdated controls
Fix:
Install modern heating controls such as:
- Smart thermostats
- Load compensation
- Weather compensation
These can improve seasonal efficiency by 5–15%.
3. Soot and scale build-up
Fix:
A Gas Safe engineer can:
- Clean the burner
- Replace seals
- Check combustion ratios
This can improve efficiency noticeably in older units.
4. Poor insulation
Fix:
Improve lagging on pipes, cylinders, and surrounding areas. This reduces heat loss and improves overall efficiency.
Is your boiler inefficient because the boiler flow temperature is set too high?
When homeowners wonder how efficient is my boiler, flow temperature is one of the most overlooked factors. A boiler with a high flow temperature cannot condense properly, meaning it loses efficiency.
What causes high flow temperature inefficiency?
- Boiler set to 75–80°C by default
- Radiators not balanced
- Installer never optimised it for condensing mode
How to fix this cause
1. High flow temperature settings
Fix:
Reduce flow temperature to:
- 50–55°C for radiators (ideal for condensing mode)
- 60°C for hot water cylinders
This alone can significantly reduce energy use.
2. Unbalanced radiators
Fix:
Balancing ensures even heat distribution so the boiler doesn’t need to run hotter than necessary. A heating engineer can adjust lockshield valves to optimise flow.
3. Poor installer setup
Fix:
A simple commissioning visit can:
- Activate load compensation
- Tune flow temperatures
- Improve pump modulation
Is your boiler inefficient because of system sludge?
Sludge is one of the top hidden culprits. Sludge forms from corrosion inside radiators and restricts water flow.
What causes sludge-related inefficiency?
- Corrosion inside radiators
- Blocked pipework and heat exchangers
- Pump strain and poor circulation
How to fix this cause
1. Corrosion inside radiators
Fix:
A power flush or chemical flush removes sludge and improves heat transfer.
2. Blocked heat exchangers
Fix:
A Gas Safe engineer can:
- Clean or replace the plate heat exchanger
- Add chemical inhibitors to prevent future build-up
3. Pump circulation problems
Fix:
Replace or service the pump to restore proper flow.
Is your boiler inefficient because of poor insulation in the property?
Many people overlook the building itself. Even a highly efficient boiler will struggle in a poorly insulated home.
What causes heat loss inefficiency?
- Uninsulated lofts and walls
- Single glazing
- Draughts around doors and floors
- Unlagged pipes
How to fix this cause
1. Poor loft and wall insulation
Fix:
Upgrade insulation, which can reduce heat demand by up to 30%.
2. Inefficient windows
Fix:
Install double glazing or secondary glazing.
3. Draughts
Fix:
Seal gaps and install draught proofing.
4. Unlagged pipes
Fix:
Add pipe lagging to reduce heat loss in airing cupboards and loft spaces.
Is your boiler inefficient because it’s not serviced regularly?
Boilers lose efficiency gradually when they are not inspected and serviced annually.
What causes efficiency loss without servicing?
- Dirty burner or combustion chamber
- Incorrect gas-to-air ratio
- Failing seals
- Limescale build-up
How to fix this cause
1. Dirty burner
Fix:
A service includes cleaning the burner and restoring clean combustion.
2. Incorrect gas-to-air ratio
Fix:
Engineers use a flue gas analyser to tune the boiler for optimal efficiency.
3. Failing seals
Fix:
Replace seals immediately—faulty seals reduce efficiency and can be dangerous.
4. Limescale
Fix:
A scale reducer or heat exchanger clean may be needed, especially in hard water areas.
How to Prevent Boiler Efficiency Loss in the Future
To maintain peak efficiency:
- Get annual boiler servicing
- Keep flow temperatures optimised
- Balance radiators every 1–2 years
- Install smart controls
- Use chemical inhibitors
- Insulate pipework, lofts, and cavity walls
- Bleed radiators when needed
- Consider upgrading old non-condensing boilers
These steps protect your investment and reduce long-term energy costs.
FAQs
Does weather compensation really improve boiler efficiency?
Yes. Weather compensation adjusts the boiler’s flow temperature based on outdoor temperature, helping it stay in condensing mode for longer and reducing gas usage.
Can adding thermostatic radiator valves improve efficiency?
Yes. TRVs help you control room temperatures individually, reducing unnecessary heating and improving overall efficiency.
Does turning my boiler off at night save energy?
Usually no. Modern boilers operate more efficiently when maintaining stable temperatures rather than reheating a cold system from scratch.
Can poor water pressure affect boiler efficiency?
Yes, very low pressure can reduce water circulation through the heat exchanger, leading to reduced performance and wasted fuel.
Do combi boilers lose efficiency faster than system boilers?
Combi boilers can show efficiency losses sooner because they work harder and contain plate heat exchangers that can scale up, but proper maintenance prevents this.
Stay warm, The Boilers2Go Team
I’m Penny North, a home energy heating expert. My mission is to demystify new boilers and complex heating systems to help you achieve a warm, cosy home with lower energy bills and a smaller environmental footprint.