Fernox F4 Central Heating Leak Sealer 500ml Product Code: 45902

Used throughout domestic heating systems for sealing small, inaccessible leaks. Does not block air vents and pumps.


  • 1 Year Guarantee
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Additional Information
Technical Specification
  • Brand : Fernox
  • Sealant Adhesive Volume : 500 ml
  • Product Type : Central Heating Leak Sealer
  • Volume : 500 ml
  • Model No : F4
  • Pack Size : 1
  • Manufacturer Guarantee : 1 Year Guarantee
  • Pieces in Pack/Case : 1
Product Contents
Delivery & Returns


Standard delivery service up to 3 working days: €8.95

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€20.45 Incl. 23% VAT

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Fernox F4 Central Heating Leak Sealer 500ml is rated 4.6 out of 5 by 27.
Rated 1 out of 5 by from Boiler Presure Droping Had a leak in heating system thought this would be ideal way to fix it, 5 radiator 1 t/rail 24hrs later, still leaking USELESS STUFF and its not the cheapest Did not work on the last leak either
Date published: 2016-11-20
Rated 5 out of 5 by from saved from a mighty mess. Having developed a leak on some underground skimmer swimming pool pipework and not keen on digging up tiles/floor/ground for 50 metres to find it decided to give this a try.blocked the pipe return at the pool and dumped it into the other end,filled with water and crossed fingers,where was losing 80mm a day in height,now dropped that to under 10...happy days only 4 stars for reliability as its only been 2 weeks but heres hoping
Date published: 2016-11-19
Rated 5 out of 5 by from Miracle Cure Had a tiny leak on a 10mm Microbore joint to a Towel Rad in the Downstairs toilet,losing @ half a cup per day. Didn't really want to be draining full system down etc at this time of year. Read reviews on this product which @95% seemed positive. I have 22 Rads so needed 2 Bottles which did seem dear but worth a try. Very easy to use, drained @ 2 litres from Kitchen Rad via drain off , and then poured contents of both bottles into Bathroom Towel Rad. Because its quite thick didn't need funnel, poured straight in.. Cautiously switched Heating on for 15 mins & worked immediately. I keep checking everyday , been a week now and still bone dry. Don't know how it works but it does , well worth giving it a try.
Date published: 2016-11-18
Rated 4 out of 5 by from Good as always but............ This is the second time I have used this product. 1st two years ago and worked perfectly. The problem of loosing pressure from my combi boiler has now reappeared now winter is here. I have used two 500ml this time and so far so good and await to see if it works long term. if you have a leak that's big the inevitable will be a full repair, F4 can not work wonders!
Date published: 2016-11-10
Rated 5 out of 5 by from fernox sealer had a small leak on the boiler / central heating. drained it down added this stuff refill and bleed rads...that simple . realy works well and gives you piece of mind
Date published: 2016-11-05
Rated 5 out of 5 by from Excellent product Recommended by my plumber and did not disappoint, was loosing central heating pressure overnight, filled through magna filter and 4 days now and pressure has not budged so worth every penny.
Date published: 2016-06-13
Rated 5 out of 5 by from Brilliant Didn't think this would work or be easy to use, it was both, sealed my leaks, my combi boiler has now started holding pressure - don't think it's ever done that!
Date published: 2016-01-14
Rated 5 out of 5 by from Does what it says ion the tin As a retired plumber with a back injury, i was not too keen to have to drain down my combo boiler system. I had developed 3 leaks and was encountering pressure drops. One from an iso valve gland, one from a lock shield valve and one from the boiler heating feed pipe. I had never used F4 sealer before, and was a little reluctant. I isolated my bathroom towel rail, and removed the top bleed point fitting. I then used a hand siphon to remove just over half a litre of water. I then added the F4 and topped the towel rad back up to the brim. Bleed plug back in and valves re-opened. Less than 5 minutes to complete......DID it work ? Yep, i was well impressed no sign of a leak anywhere after a couple of weeks and pressure is maintained. Recommended.
Date published: 2016-01-02
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