Every October the same thing happens across Doncaster. The first cold snap arrives, thousands of people turn the heating back on after five months off, and a fair few of them discover it does not work. The boiler will not fire, the radiators stay cold, or everything comes on but the house never gets warm.
Most switch-on problems are minor and some you can sort yourself in five minutes. This guide walks through the common ones in the order we would check them, what you can safely try, and the point at which you need a Gas Safe registered engineer.
One rule before anything else: if you can smell gas, do not touch any switches. Open the windows, turn the gas off at the meter if you can, leave the property and call the National Gas Emergency line on 0800 111 999. Never take the boiler casing off yourself — working on a gas appliance is only legal for a Gas Safe registered engineer.
Why Heating Fails After the Summer
A boiler that has done nothing but heat water since April has had its heating side sitting idle for months. Three things tend to go wrong in that time.
Pumps and diverter valves stick. The circulating pump and the diverter valve (the part inside a combi that switches between hot water and heating) can seize after a long period of not moving, particularly on hard-water systems like most of Doncaster's.
Pressure drops. Sealed systems lose a little pressure over time. Over a summer it can drift low enough that the boiler locks out the moment it is asked to heat.
Air and sludge settle. Air collects at the top of radiators and sludge settles at the bottom. Neither shows up until the heating is asked to run.
Seven Checks to Try First
1. Is the boiler getting power and gas? Check the display is lit, the fused spur next to the boiler is on, and other gas appliances such as the hob are working. If you are on a prepayment meter, check there is credit.
2. Check the pressure gauge. On most combi boilers the gauge should read between 1 and 1.5 bar when the system is cold. Below about 0.8 bar many boilers will refuse to fire. Your boiler manual will show how to top up using the filling loop. If the pressure keeps dropping after you top it up, there is a leak somewhere and it needs looking at.
3. Check the thermostat and timer. It sounds obvious, but this is the single most common "fault" we get called out to in October. Clocks change on the last Sunday of the month, programmers reset after power cuts, and smart thermostats can drop off the Wi-Fi or run flat batteries over the summer. Turn the room thermostat a few degrees above the room temperature and set the heating to constant on.
4. Reset the boiler once. If the display shows a fault code, look it up in the manual and press reset once. If it locks out again, stop — repeated resetting can mask a genuine fault, and the code is useful information for the engineer.
5. Check the condensate pipe. This is more of a winter problem than an October one, but if the boiler shows a blockage or ignition fault on a frosty morning, the plastic pipe running outside may have frozen. Pouring warm (not boiling) water along the outside section usually clears it.
6. Bleed the radiators. If radiators are cold at the top and warm at the bottom, they have air in them. With the heating off, open the bleed valve with a radiator key until water appears, then close it. Recheck the boiler pressure afterwards — bleeding lets pressure out.
7. Check the radiator valves. A thermostatic radiator valve that has sat on one setting all summer can stick shut. Turn it fully open and back a few times. If the pin underneath the head is stuck, a gentle tap often frees it.
Signs You Need an Engineer
Stop and call a Gas Safe engineer if any of these apply:
The boiler keeps locking out on the same fault code after one reset.
You can hear the boiler firing but the radiators stay cold. That usually points to a seized pump or a stuck diverter valve.
Hot water works but the heating does not, or the other way round. On a combi this is very often the diverter valve.
The pressure keeps dropping after you top it up, which means a leak in the system or a failing pressure relief valve.
Banging, kettling or whistling from the boiler once the heating runs, which usually means sludge or scale on the heat exchanger.
Radiators are cold at the bottom across several rooms. That is sludge, not air, and bleeding will not fix it. Our powerflush guide explains the options and what they cost.
Water leaking from the boiler, from any pipe under it, or from the overflow pipe outside.
What a Switch-On Repair Typically Costs in Doncaster
Every repair we do is quoted as a fixed price after diagnosis, before any work starts. Our diagnostic visit is £80–£120 inc VAT, and it is credited against the repair if you go ahead. As a guide, the common October faults sit in these ranges, fitted and inc VAT:
Diverter valve: roughly £180–£280, depending on the boiler.
Pump replacement: roughly £220–£360.
Pressure sensor or pressure relief valve: roughly £120–£180.
Expansion vessel (a common cause of pressure that keeps dropping): roughly £150–£240.
Full ranges for every common boiler repair, and the point at which replacing the boiler makes more sense than repairing it, are on our boiler repair page and in our repair-or-replace guide.
The Cheapest Fix: Have It Serviced Before You Need It
Almost every fault in this guide is easier and cheaper to catch at an annual service than in the middle of a cold snap. A service checks the pressure, the expansion vessel, the condensate route, the pump and the diverter, and it is when we check your system's inhibitor levels too. Our annual boiler service is £114 inc VAT, and booking it for September or early October means the heating is ready before you need it, not after.
If your boiler is out of warranty and over ten years old, the service is also the right moment to ask whether it is worth keeping. We will give you a straight answer, not a sales pitch.
Why Use a Local Gas Safe Engineer
Yorkshire Green Heating is a family-run, Gas Safe registered business based in Doncaster (Gas Safe number 638592). We are rated 9.84/10 across 641 verified Checkatrade reviews and 5.0 on Google. Every repair is quoted as a written fixed price before work starts, and our engineers carry common parts for the boilers we see most often in Doncaster homes, so many switch-on faults are fixed on the first visit.
If the checks above have not sorted it, call us on 01302 963007 or book online.