Locking out? It’s the fuel.
Most oil boiler problems aren’t the boiler — they’re what’s inside the tank. Water, sludge and aged fuel cause lockouts, smoky burn and nozzle failures. Professional tank polishing clears the lot, with sample evidence before and after.
Sound familiar? Probably your tank.
Most of the calls we get for “broken boilers” turn out to be fuel problems. If your boiler is doing any of the below, a polish is almost always cheaper than the repair you’re heading for.
Boiler keeps locking out
You press the reset button, it fires for a few hours, then locks out again. Classic fuel contamination signal — water or particulate disrupting the burn, safety circuit cutting the boiler off before damage.
Sooty burn
Black deposits inside the combustion chamber, on the flue, or around the boiler. Bad combustion caused by contaminated fuel — water in the mix, sludge being drawn up the supply, wrong air/fuel ratio.
Won’t ignite at all
Boiler tries, fails, locks out. New nozzle didn’t fix it. Often the supply itself is so contaminated the boiler can’t get a clean flame — no amount of service work fixes the cause until the fuel is cleaned.
Water visible in tank
You’ve checked the tank with a dipstick or pump, and there’s a clear layer of water at the bottom. Sometimes inches of it in old steel tanks. Water sits below the fuel, gets drawn up first when levels are low, kills the boiler.
Fuel looks or smells off
Cloudy fuel, dark sediment when you dip it, strange smell from the tank vent. Could be microbial growth, oxidation, or sludge being disturbed by recent deliveries. Visual signal that polish is overdue.
Frequent nozzle blockages
You’ve had two or three nozzles replaced in the last 18 months, each one not lasting the year. Sign that whatever’s in your supply is faster than the filter at killing nozzles. Polish solves the root cause.
It’s not the fuel that’s delivered.
The fuel that comes off the tanker is fine. It’s what happens to it once it sits in your tank — and especially what’s in the tank before it arrives — that causes the trouble.
Condensation
Tank cools overnight, warm air inside meets cold steel walls, water condenses out. Drips down, settles at the bottom, builds over years. Old steel tanks are worst — plastic less so, but still affected.
Sludge buildup
Heavy fractions of kerosene settle out over time. So do contaminants from delivery hoses, the inside of the tank, old gaskets. Builds into a black sludge layer at the base. Stirred up by every refill.
Microbial growth
Where fuel meets water, microbes thrive — particularly in summer. Creates a slimy biomass at the fuel/water boundary. Clogs filters, blocks nozzles, produces acidic byproducts that corrode the tank.
Fuel age
Kerosene degrades over 12–18 months. Tanks that don’t get fully cycled — typical for low-usage holiday homes or properties with multiple heat sources — accumulate aged, oxidised fuel that burns poorly.
From contaminated to clean.
Fuel polishing is a misunderstood term. It’s not a fuel additive. It’s not a magic chemical. It’s mechanical filtration — circulating the entire tank volume through a multi-stage filtration rig until the fuel runs visibly clean.
Pre-polish sample
Sample drawn from the bottom of the tank using a weighted bacon bomb. Visual condition check. Dip test for water using water-finding paste. Sample retained for the before-and-after comparison.
Rig connection
Polishing rig sited and protected with spill matting. Suction line into the tank, return line back into the tank. Fresh filter cartridges per job — never re-used, because that just moves contamination from one customer to the next.
Three-stage filtration
Stage 1: water separation. Stage 2: coarse particulate removal (10 micron). Stage 3: fine polish (1 micron). Full tank volume circulated multiple times. Continued until the post-filter sample runs visibly clean.
Tank inspection
While the level drops during circulation, the tank walls become visible. Inspected for internal corrosion, sediment buildup, fitting damage. Bund condition checked (where bunded). Photos taken of anything notable.
Post-polish sample
Second sample drawn after polish. Side-by-side comparison with the pre-polish sample — you’ll see the difference yourself. Water volume removed is measured. Both samples retained for your records.
Documentation & disposal
Written condition report issued — tank state, water/sludge volume removed, recommended next polish interval. Waste removed for licensed disposal, not tipped down a drain. Photos of pre/post samples emailed with the invoice.
Cheaper than the repairs you’re avoiding.
A polish costs less than a single emergency callout. Less than a new burner. Less than a tank replacement. Preventative maintenance done properly always beats reactive repair.
Tank Polish Only
- Pre-polish fuel sample & dip test for water
- Full tank circulation through filtration rig
- Water, sludge & particulate removed
- Tank wall inspection during polish
- Post-polish sample & written condition report
- Waste removed for licensed disposal
Premium Service Bundle
- Full OFTEC annual boiler service
- Burner strip, clean & rebuild
- New nozzle, fuel filter & pump screen
- Tank polish included (as standalone)
- Pre/post fuel samples & condition report
- Single visit, single invoice, both jobs done
Why the bundle is the smart pick: if you’re due an annual service anyway, the Premium bundle adds the polish for just £30. The standalone makes sense only if you’ve recently been serviced and just need the tank sorted.
Tanks over 1000L: standard polish covers up to 1000L. Larger tanks (typical for older steel tanks and rural properties) are quoted at +55p per litre. Quoted up-front before any work starts.
The right time isn’t always now.
If you’ve never polished and your tank is over 5 years old — book it. Otherwise, here’s the proper calendar.
Before winter
August or September is the sweet spot. Get the polish done before the heating season starts. Clean fuel through winter means fewer lockouts in the worst weather, when callouts cost most.
After a tank refill
A delivery can stir up settled sludge from the tank floor. The first few weeks after a refill are when sludge-related lockouts spike. Polishing 2–4 weeks after a delivery catches the disturbed contamination.
After visible contamination
You’ve dipped the tank and seen water. The fuel looks cloudy. You’ve smelled something off from the vent. Don’t wait — book the polish, sort the cause before it sorts the boiler.
Preventatively · every 3 years
If everything’s working fine, every 3 years is the sensible default. Less if you have an old steel tank or low fuel usage. More if you’ve got a new plastic bunded tank and you cycle the fuel reasonably quickly.
Tank polishing across North Yorkshire.
Knaresborough hub, 25-mile radius included on every job. Most rural North Yorkshire villages sit inside the included zone — exactly where oil heating still does the heavy lifting.
Common questions about polishing.
The questions people ask when researching whether their tank actually needs a polish, or whether they’re being sold something they don’t need.
How do I know if I actually need this? +
If your boiler’s been locking out, you’ve had repeat nozzle failures, or you’ve physically seen water in the tank — yes, almost certainly. If everything’s working fine and your tank is under 3 years old, probably not yet. Pre-polish sample is the honest test — if the sample comes back clean, we’d tell you the polish isn’t needed and just charge for the visit. We’re not in the business of selling unnecessary work.
Does it work on plastic tanks? +
Yes. The polishing process is the same on plastic, steel, single-skin or bunded tanks — we connect to the existing supply and return lines, not to the tank itself. Plastic tanks usually have less contamination than old steel tanks (less condensation, no internal rust) but they still build up sludge over time, especially the bunded ones with internal baffles.
How much fuel will you remove as waste? +
Almost none. Polishing circulates fuel through the rig and back into the tank — clean fuel goes back in, contaminated fuel stays in the rig filters. What leaves the property as “waste” is the water, sludge and sediment captured in the filters, plus any water separator drainage. Typically 0.5–5 litres total. You keep your fuel; you just lose the gunk.
What if my fuel is so old it’s beyond saving? +
Rare but it happens — usually in tanks that haven’t been cycled for 18+ months. The pre-polish sample tells us this. If the fuel itself is beyond polishing, we’ll be honest about it and recommend draining and refilling instead. We won’t run hours of polish on fuel that’s never going to come back to spec.
Do you take the waste away? +
Yes — all waste is removed from site and disposed of through a licensed waste oil handler. We’re not allowed to tip it down a drain, into a soakaway, or onto the ground. You’ll see the waste oil consignment note details on your invoice — proper paper trail, properly handled.
How long does the polish take? +
Standalone polish: 2–3 hours on site for a typical 1000L domestic tank. Bigger tanks proportionally longer. If we’re doing it bundled with a service, count on half a day. The polish itself runs in the background while we crack on with the boiler.
Will the polish stop my boiler locking out? +
If the lockouts are caused by fuel contamination — yes, almost always. If they’re caused by something else (worn electrodes, dirty photocell, blocked flue, faulty pump) the polish won’t help, but a full service might. The honest answer: book the Premium bundle and we’ll address both possibilities in one visit.
Is polishing the same as filtering? +
Not really. The fuel filter on your boiler is a single-pass filter — fuel passes through once on its way to the burner. Polishing recirculates the entire tank volume multiple times through a much more aggressive filtration system, lifting contamination that’s settled at the bottom of the tank and would never normally reach the boiler’s filter. Different scale, different purpose.
Cleaner fuel.
Fewer lockouts.
Send an enquiry with your tank size if you know it, when it was last polished, and any symptoms you’ve been seeing. We’ll come back with a quote and a date the same working day.
If you’re due an annual service anyway, the Premium bundle adds the polish for £30 over the polish-alone price. One visit, both jobs done, sample evidence before and after. Smart pick for most homes.