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: €15

Free delivery on all orders over €70.00

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€19.95 Incl. 20% VAT

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Fernox F4 Central Heating Leak Sealer 500ml is rated 4.6 out of 5 by 30.
Rated 5 out of 5 by from It works like a charm! I was worried when I saw my pressure dropping down from 1.5 bar to 0.25 bar within 24-48 hours. With no visible leak or discolouring of the plasterboards I was considering the prospect of digging up the floor. I added 2x500ml to my system and hey presto leak fixed!. Pressure on the expansion vessel is now holding firm at 1.25 bar. What great product, can't recommend highly enough.
Date published: 2017-10-18
Rated 5 out of 5 by from Excellent stuff! I recently got my combi boiler replaced but did not change the radiators and pipe work as they were in good condition. The pressure used to drop to 0.8-1.0 bar (from 1.5bar) every 4-5 days and I had to keep topping it up. The company that installed the boiler said the only way to fix it would be to upgrade the microbore pipework as they suspect an issue with the manifold. I dind't want to do this as I have laminate flooring. I decided to give this a shot and it has fixed the leak. It's just been 2 weeks now and the pressure has not dropped at all since adding this. Highly recommended and worth the price.
Date published: 2017-10-07
Rated 5 out of 5 by from Absolutely brilliant I had a pin hole leak on a radiator seam, I had just completely redecorated the room, plastering, new skirts and so on....so was quite upset thinking I had to replace the radiator. Partially drained the system and added 2 bottles to my header tank (circa 18 radiators in my house), then filled up and switched the heating back on. Within an hour the leak has stopped, hopefully it will solve it permanently...
Date published: 2017-09-22
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
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