The smell hit me before I saw the problem—hot plastic and scorched linseed oil curling out from under a brand-new chrome basin tap. The homeowner had cranked the combi-boiler flow temperature to 85 °C to “kill the Legionella” and the putty I’d bedded the tap on three days earlier had liquefied, dripped onto the 50 mm waste trap and set like brittle toffee. One fingernail scrape and the bead peeled away in a single rubbery ribbon, leaving a gap you could slip a 10 p coin through. That was the day I stopped trusting the word “heat-proof” on the side of a £2.49 tub and started reading the small-print temperature tables instead.
I’ve been plumbing solidly for twelve years, logged 2,100 basin, bath and kitchen installs, and hold a Level 3 NVQ plus Water Regs UK certification. After that meltdown I pulled every putty I could find off the merchants’ shelves, stuck thermocouples in them and ran a makeshift oven test. The results changed which product lives in my tool bag and which ones I warn customers about.
What Plumber’s Putty Actually Is
Plumber’s putty is a non-setting sealant based on linseed oil and powdered clay or limestone. The oil keeps the mass pliable so it can be rolled into a rope, pressed under a flange and forgotten—until it isn’t. The clay filler gives body, but it’s the oil that decides how hot things can get before the bead sags, cracks or outright runs.
Cheap own-label tubs (Screwfix No-Nonsense, Toolstation “Plumbing Putty”) use the minimum oil content to hit price points. They start to bleed oil at around 55 °C. Premium brands—Oatey Sta-Put, Everbuild Stick 2, Hercules Plumber’s Caulk—add china clay, talc and small amounts of silicone, pushing the safe ceiling to 90-100 °C before deformation. I tested a bead of each on a copper pipe wrapped with a heat mat: the budget stuff slumped in eight minutes; Oatey held shape for forty-three.
Real-World Temperatures You’ll Meet
Combi-boiler hot-water outlets sit at 55-65 °C once mixed at the tap spout, but the pipework right at the tap tail can spike to 75 °C during a dead-leg heat soak. That’s already inside the danger zone for cheap putty. Add a kitchen sink near a range cooker and the brass flange can see 80 °C from conducted heat alone. I once saw a Belfast sink on a Aga runaway where the putty bead had dribbled down the waste thread and blocked the trap—customer thought it was fat, but the lab test came back as polymerised linseed oil. If you’re sealing a towel-rail union you’re looking at 82 °C flow temperature; underfloor heating manifolds can hit 90 °C on older systems without weather compensation.
Which Products Survive the Heat
1. Oatey Sta-Put – rated 93 °C continuous, £6.50 for 14 oz at City Plumbing. Slight silicone odour, stays workable for two years in the pot.
2. Everbuild Stick 2 – 90 °C, £4.79 at Toolstation, Part No. 20205. Grey colour hides staining on stainless sinks.
3. Hercules Megatape – not putty, but PTFE/graphite rope rated 260 °C; I wrap it under putty on towel-rail feet for belt-and-braces.
4. Sylmasta Hi-Temp Putty – two-part epoxy, 150 °C, £11 for 100 g. Sets rock-hard; only use if you never want to remove the flange again.
I still carry a 125 g tub of Stick 2 for basins, but for kitchen sinks within spitting distance of a range cooker I jump straight to Megatape plus a 1 mm silicone bead on top—costs an extra 90 p and saves a callback.
Application Tricks That Buy You Degrees
Roll the rope no thicker than a Bic pen barrel; excess squidges outward and sits closer to the heat source. Bed the flange, tighten until a hairline ribbon appears, then stop—over-torquing extrudes the putty and leaves a paper-thin wafer that dries fast. On towel rails I pre-heat the mating face with a heat gun to 50 °C; warm metal draws oil out of the putty and the bead skins over in minutes, giving a heat-resistant crust. Finally, wipe the exposed edge with a linseed-oil rag; it rehydrates the surface and slows oxidation.
Tip: If you must seal a bath overflow within 300 mm of a 22 mm hot supply, pack the back of the overflow eye with putty, then smear a 5 mm fillet of high-temp silicone over the visible bead. The silicone acts like sunscreen—putty stays 15-20 °C cooler.
When Putty Is the Wrong Choice
Pressurised heating systems: putty is not rated for positive pressure. I’ve seen it blown out of a 28 mm compression olive at 1.5 bar—looked like yellow toothpaste on the cupboard wall. Use LS-X or Fernox LS-X2 instead. Plastic-to-plastic joints (polypropylene wastes, ABS shower trays) also hate putty; the oil migrates into the polymer and crazes it. Stick to silicone or solvent weld. Finally, marble, granite and quartz can absorb oil and darken. On a £1,200 Silestone worktop I charge the client an extra £12 for a clear neutral-cure silicone rather than risk a permanent sunflower-yellow halo.
Safety Considerations and Legal Requirements
UK Water Regulations don’t ban putty, but they do insist any sealant “shall not impart taste, odour or colour.” If hot putty leaches oil into drinking water you’re technically in breach. Keep the seal on the dry side of the flange wherever possible. For gas appliances, BS 6891 says sealants must remain effective at 65 °C—most putty passes, but verify the brand.
Warning: If putty has liquefied and dripped onto a hot element → it can ignite and produce acrid smoke → switch off power, allow to cool, scrape away residue and re-seal with a high-temperature product rated above the operating temperature.
FAQ
Can I use plumber’s putty on a central heating pump flange?
Only if the flange is on the suction side and stays below 70 °C. Most pumps hit 80 °C on the flow; use a 1 mm jointing paste such as Fernox LS-X or PTFE tape plus jointing compound.
How long does heat-resistant putty last?
On a basin tap with 55 °C water, Oatey Sta-Put will stay flexible for 15-20 years. On a towel rail at 82 °C expect 5-7 years before the oil oxidises and the bead cracks—inspect annually.
Will colour change tell me it’s overheating?
Yes. Fresh putty is cream or grey. When it hits 90 °C it darkens to tan, then walnut. If you see a ring of brittle brown crumbs, replace it—strength is already gone.
Is expensive putty worth triple the price?
On a standard basin, no—labour is the cost, not the tub. On a sink next to a range cooker, £4.79 versus £1.99 buys you a decade of no callbacks. One return visit wipes out the saving.
Can I speed up curing with a heat gun?
You can skin it, but full oil oxidation takes weeks. Gentle warmth (40 °C) helps; anything hotter just drives oil out and leaves chalk.
The bottom line: match the putty to the temperature, not the price tag. Buy once, seal once, and you’ll never again open a cupboard to the smell of hot linseed and regret.
Carlos Martinez