Why September is the Most Expensive Month for Your Boiler
August 27, 2025
As households prepare to switch their boilers back on after summer, heating engineers are warning that September is often the most expensive month of the year for boiler breakdowns, repairs, and inefficiency.
Rob Jones, heating engineer at UK Radiators, explains:
“After months of being idle, boilers are suddenly put under pressure when temperatures drop in September. It’s the equivalent of asking someone to run a marathon after sitting on the sofa all summer – parts fail, pressure drops, and efficiency suffers. That’s why we see a huge spike in callouts every September.”
The Hidden Costs of the September Switch-On
- Pressure Loss – Idle boilers often lose pressure slowly over summer. When switched back on, this can cause heating not to fire properly.
- Sludge Build-Up – With water sitting still in the system, rust and debris settle, leading to cold spots and higher bills.
- Stuck Valves – Radiator valves and pumps can seize up when unused, forcing costly repairs once the heating is needed again.
- Air Locks – Radiators switched on after months off often need bleeding; without it, boilers work harder and waste energy.
- Undetected Leaks – Small leaks go unnoticed over summer but become expensive emergencies as the system is pressurised again.
Why September Bills Spike
According to UK Radiators, homeowners often use more energy in September than they realise:
- Days are warm, but evenings are cool, leading to boilers being switched on and off repeatedly (which uses more energy than consistent running).
- Families are back from summer holidays and schools reopen, meaning more showers, laundry, and hot water demand.
- Old habits — like turning unused radiators fully off — increase the risk of condensation, damp, and higher repair costs later.
Engineer’s Advice to Avoid the “September Surge”
Rob Jones advises:
- Check boiler pressure now – top it up if it’s dropped below 1 bar.
- Bleed radiators before switching on to avoid airlocks.
- Run your heating for 15 minutes once a month in summer to prevent valves and pumps seizing up.
- Book a service before the cold snap – engineers are busiest in September, so waiting can mean longer waits and higher emergency fees.
Rob Jones adds:
“The cost of a simple service in August or early September is far less than the repair bill we see when boilers give up later in the month. A little prep saves a lot of money.”
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