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
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Standard delivery: €15

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€19.95 Incl. 20% VAT

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Fernox F4 Central Heating Leak Sealer 500ml is rated 4.4 out of 5 by 50.
Rated 5 out of 5 by from Stops the leak Had a small leak for many months and used this product to plug the hole, it works. Does however need a top up once a year but small price to pay for sorting a leak
Date published: 2019-05-25
Rated 5 out of 5 by from Leaky Had steady drip from CH flow pipe work. Added Fernox F4. Gradual leak reduction over two days till complete cessation. Would def use again.
Date published: 2019-04-06
Rated 1 out of 5 by from Do not use in combi systems Yes it mostly sealed the leaky joint at an annoyingly inaccessible magnetic filter. But shortly afterwards, the line to the expansion vessel in the boiler unit clogged up solid, leading to instant over-pressure. The plumber who sorted it said he'd seen it happen elsewhere and strongly advised not using any sort of leak sealer in a combi system. Unfortunately I'd already recommended it to a workmate, and he ended up with a stuck diverter value in his combi. Be careful about reviews that report instant good results - that might not be the whole story.
Date published: 2019-03-08
Rated 2 out of 5 by from Didn't work for me I have a sealed heating system that's 8 years old and losing pressure, requiring a top-up every couple of days. No visible leaks in the walls or ceiling, recently replaced PRV and pressure vessel, which suggested a small leak somewhere below the downstairs flooring. As the house has only just been decorated I'm very reluctant to rip up the solid wood floors, so I thought I'd give this product a try. There are other leak sealers on the market, but this was my first choice because we use Fernox inhibitor in the system. For 5 days after the treatment there was no pressure loss so I was starting to get very pleased with myself. But after 6 days there was a small drop in pressure and after 7 days the daily drop was back to what it had been before the treatment. I presume the treatment had managed to block the leak, but it was only very temporary. I suppose all leaks are slightly different and I was unfortunate. We have plastic pipes under the floor with copper tails joining the radiators. Perhaps one of the other products is more suitable to my particular problem. I will try Sentinel next.
Date published: 2019-02-20
Rated 5 out of 5 by from Boiler pressure loss Gradual pressure loss on boiler and suspected a small leak. Used sealant f4 today and hope that boiler will now maintain pressure. Product very easy to use.
Date published: 2019-02-14
Rated 5 out of 5 by from Does what it says on the Tub Had an inaccessible leak on my Boiler, thought I would try Fernox F4 before pulling the Boiler out, which would be a big job. Follwed the instructions and the leak seems to have gone which has saved me so much work.
Date published: 2019-02-06
Rated 5 out of 5 by from Good stuff I've used this stuff before, it was recommended by a good plumber mate of mine, i have a towel radiator and i just remove the big but on the top, i use a turkey baster to remove some water, then pore it in, put the nut back,turn the heating on, job done !
Date published: 2019-01-15
Rated 5 out of 5 by from Fernox F4 leak sealer Had constant pressure drop past week on worcester combi boiler and noticed slight damp down wall in lounge. It seemed obvious that i had a leak in the heating pipework and my fear was to remove tiles or floorboards. My gas engineer suggested putting some Fernox F4 liquid sealer which hopefully would seal the leak as it went round the system. Within 3 hours i had noticed no more damp and no pressure drop. What a relief that Fernox did what it said on the bottle.
Date published: 2018-11-03
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