When a Grant oil boiler won’t fire, the lamp in the burner’s reset button is usually lit, which means the burner tried to light and gave up for safety. The cause is most often low oil, air in the line or a blocked filter or nozzle. Grant’s overheat thermostat and the fire valve can also stop the boiler, and those two need more care.
Grant’s Vortex boilers are about as common as oil boilers get in the UK, so the faults are well known. Below are the questions Grant owners ask us most, with the short answer first.
Which Grant boiler do I have?
Check the label inside the front panel. The current range is split into Vortex Pro and Vortex Eco models, plus the Vortex Boiler House for garages and plant rooms. Older homes may still have a VortexBlue or an earlier Vortex, which Grant no longer lists. The model matters because the burner, the reset and some of the safety devices differ.
| Grant range | Where you find it | Burner (per Grant) |
|---|---|---|
| Vortex Pro (internal, external, combi) | Kitchens, utility rooms, outside walls | Riello RDB low NOx |
| Vortex Eco floor-standing | Utility rooms and external housings | Riello/Ecoflam RDB2.2 BX |
| Vortex Eco wall-hung | Kitchens, smaller utility rooms | Max 1 LN |
| Vortex Boiler House | Garages, cellars, outbuildings | Riello RDB |
| VortexBlue and older Vortex | Installed before the current range | Varies by age |
Why is my Grant oil boiler not firing up?
The likeliest reasons, most common first:
- Low or empty tank. Always rule this out first. A tank that has run very low can also pull air and sediment into the line.
- No power or no call for heat. A tripped isolator, a blown fuse, flat thermostat batteries or a programmer upset by a power cut all look like a dead boiler.
- Burner lockout. The lamp in the reset button is lit because the burner didn’t get a flame, or lost it.
- Air in the oil line. After a run-out, a delivery or a loose joint, the pump can’t prime until the line is bled.
- Blocked oil filter. Its job is catching dirt, so eventually it clogs.
- Worn or partly blocked nozzle. The spray goes ragged and the flame struggles to light or stay lit.
- Dirty photocell. The burner lights, can’t “see” the flame and shuts down within seconds.
- Worn or sooty electrodes. A weak spark gives late ignition, sometimes with a thud.
- Water or sludge in the oil. Lockouts keep coming back however many times the filter is changed.
- Closed supply valve or blocked tank vent. The tank is full but oil isn’t flowing.
Less common faults include a seized motor coupling or tired capacitor (the motor hums but won’t turn), a worn oil pump, a sticking solenoid valve, a failed ignition transformer or a failed control box.
How do I reset a Grant oil boiler?
Press the reset button on the burner once, firmly, then wait. The burner should run its pre-purge, spark and light within a minute or so. If the lamp comes back on, don’t keep pressing. Leave it a few minutes and try one more time at most. Repeated resets can flood the combustion chamber with unburnt oil, which then lights all at once. Our general guide on boiler lockouts explains why manufacturers limit resets.
What is the overheat thermostat on a Grant boiler?
It is a safety cut-out that switches the burner off if the boiler water gets far too hot, at around 110°C according to Grant. It has its own manual reset under a small cap, separate from the burner reset. Resetting it is easy. Finding out why it tripped is the real job, because the cause is usually a circulation problem such as a failed pump, a closed valve or heavy sludge in the system. If it trips again soon after, call an engineer rather than resetting it.
Could the fire valve have shut off my oil?
Yes, and it is easy to mistake for an empty tank. The fire valve sits on the oil line and closes if it senses excess heat around the boiler, starving the burner even when the tank is full. It should be checked by an engineer before reopening, because it may have closed for a real reason.
Why does my Grant combi heat the radiators but not the hot water?
On a Vortex Pro Combi, heating without hot water usually points to the diverter valve or a scaled plate heat exchanger, especially in hard water areas. It isn’t a burner fault, so the lockout lamp won’t be lit. An engineer can free or replace the valve and descale or swap the heat exchanger.
Why does my Grant lock out on frosty mornings?
On condensing models the condensate pipe can freeze, and the boiler shuts down when the water can’t drain. Pour warm, not boiling, water over the outside section of the pipe, then reset. External Vortex models and tanks on exposed sites can also suffer from oil thickening in a hard frost. Lagging the pipework helps, and our frozen condensate pipe guide shows the steps.
Is my Grant still under warranty?
It depends on the model and who fitted it. Grant offers up to 10 years on Vortex Pro boilers installed from 1 February 2023, provided a Grant G1 installer fitted and registered it within 30 days and a Grant Mag One filter was fitted. Vortex Eco and Boiler House models carry up to five years on similar terms. Outside those schemes the standard cover is shorter. Keep your service records, because a missing service history is one of the easiest ways to lose a claim. Check Grant’s current terms for your exact model before booking a repair.
Should I repair or replace an old Grant?
Repair it if the faults are the usual service parts and the boiler is otherwise sound. Grant parts are widely available, which helps older boilers. Replacement starts to make sense when the heat exchanger is corroded, the lockouts are frequent despite repairs, or the boiler is an old non-condensing model burning noticeably more oil. A new Vortex Pro is a like-for-like swap in most homes, and we fit them as part of our oil boiler installation service. If you’re comparing brands, see our guide to the best oil boilers.
Not a Grant? Our guides to a Worcester Bosch oil boiler not firing up and to oil boilers not firing in general cover the same ground for other makes. You can also look up specific Grant boiler error codes.
Grant oil boiler FAQs
Can I bleed my Grant boiler myself?
Some owners do, but it means removing the casing and working on the burner pump. If you are not confident, or the boiler locked out for any reason other than running out of oil, leave it to an OFTEC registered engineer.
Does a Grant Vortex run on HVO?
Grant says all its floor-standing Vortex boilers are HVO compatible, but they need converting first. That usually means a new nozzle, oil line and filter, and burner adjustments. Some older models can’t be converted.
How long does a Grant repair take?
Filters, nozzles, electrodes and photocells can usually be changed in one visit. Motors, pumps and control boxes sometimes need ordering in.
Why does my Grant work in summer but fail in winter?
Winter brings frozen condensate pipes, thicker oil in exposed pipework and more demand on a burner that may already be borderline. A service before the cold sets in catches most of it.
With years of experience in boilers and home improvements in general, Myles Robinson is a passionate advocate for making home heating simpler, safer, and more affordable for households across the UK. With years of experience in the heating industry, Myles combines hands-on knowledge with a focus on customer-first service, helping homeowners choose the right boiler, understand their systems, and keep their heating running efficiently all year round. His articles break down complex topics into clear, practical advice you can trust.