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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Technical Specification
  • Brand : Fernox
  • 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 40.
Rated 5 out of 5 by from Stopped leak in a few days Losing pressure in 1 loop system over a few days but pipes in concrete floor. Had used other Fernox products successfully so gave it go. Combi boiler needed to maintain pressure to continue obtaining hot water in this hot summer so heating off. Wondered if it would seal the leak. Circulated round the radiators after inserting solution via a radiator. Waited to see if pressure would remain. Looked uncertain but after a few days all was well. Leak stopped and three months later still OK. This is a 5* product downgraded to 4 for reliability since waiting to see if OK when radiators on.
Date published: 2018-07-13
Rated 5 out of 5 by from It really does work! My system was losing 0.1 bar every day so shut itself down after about 5 days. A right pain during our winter holiday... coming back to a very cold house. No obvious leak so assume it's under the floor somewhere. Tried this without any great expectations but IT WORKED. I put one bottle in my system eight weeks ago and the pressure has not moved a fraction. (Turned off both valves on bathroom towel rad, syphoned some out through top, poured product in, topped up and opened valves.. simples) Could not be more pleased with this product.
Date published: 2018-06-02
Rated 5 out of 5 by from It Really does work. I was sceptical about whther this would work on my old central heating boiler leak. It did and immediatly as well. I can now limp the boiler on for a few more months until it gets replaced.
Date published: 2018-05-24
Rated 1 out of 5 by from Did not work I had a leak that lost 17 litres in 9 days, that equates to 1 ml/min or I drip every 3 seconds. I put two bottles in a large domestic system and the leak stopped for two weeks - then it started again. So, in desperation, I put in another two bottles, and it stopped for one week - then started again. If Fernox regard 1 drip every 3 seconds as too big a leak to seal then they should say so. Otherwise, in my view, this product does not work. I am now suspicious of all the good reviews.
Date published: 2018-05-10
Rated 5 out of 5 by from Annoying small leak Had a small leak where it was impossable to reach under kitchen cupboard. Used Fernox Leak Sealer F4 Sealed in seconds. Great
Date published: 2018-04-26
Rated 5 out of 5 by from Saves money and time! Used this a few times for a combi boiler dropping in pressure numerous times. No leaks on pipes upstairs so presume its under floorboards. Didn't want to have to rip up the floorboards so added this via towel rail in bathroom and left heating on full for couple of hours so it went round the system. Works perfectly and does the job! No boiler pressure loss anymore.
Date published: 2018-04-13
Rated 5 out of 5 by from Does what it says on the bottle I had a constant tiny leak that couldn't be found.Always losing pressure on the underfloor heating system and no sign of wet patches anywhere.I used to have to top up system every week or so.I decided to give this a try.I needed 7 or 8 bottles as I have a large system with a thermal storage buffer tank..Its been about 2 years now and still working a treat.I would highly recommend.Just make sure you use enough bottles to cover the volume of water in your heating system..
Date published: 2018-03-02
Rated 4 out of 5 by from Good product Does exactly what it says ,,great product ,easy to you ,,highly recommended
Date published: 2018-02-24
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