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Fernox Central Heating Leak Sealer F4 500ml (45902)

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

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£16.99INC VAT (£33.98/Ltr)

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Rated 4.5 out of 5 by 15 reviewers.
  • 2016-08-31 T09:28:41.448-05:00
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Rated 5 out of 5 by 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. 13 Jun 2016
Rated 5 out of 5 by 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! 14 Jan 2016
Rated 5 out of 5 by 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. 2 Jan 2016
Rated 2 out of 5 by Unable to sort leak The boiler engineer has put my boiler pressure vessel on test,but believes I have a system leak. Tried a 1litre bottle of a competitor's leak sealer and it didn't work.Thought I'd try a 0.5 litre bottle of Fernox several day's ago.This hasn't worked either so I haven't found it to be the "magic bullet" that others speak of.Will still have to investigate under the floorboards! 12 Nov 2015
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5 Questions  6 Answers

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We have 11 radiators, but several of those are large. Would you use 2 whole bottles? Thanks.
22 Jul 2016
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Huddersfield
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Answer: 
The Fernox F range of products are designed to treat 10 single panelled radiators, always count any double panels as 2 and always round up to the next complete bottle. So in your case you would need 2 cans (assuming that your 11 are all single panel?). After application ensure that the system is left circulating as the product reacts faster with warmth and needs to circulate to get to the source of the leak.
25 Jul 2016
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13 Jul 2016
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kingswinford
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Hello Redecorating1. For a system this size you should use 2 bottles of F4 Leak sealer to adequately dose.
14 Jul 2016
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My plumber added 500ml of Fernox F4 to my CH system to stop a leak and loss of boiler pressure. This worked for about 3 weeks, now we have a very slight loss again. We have 15 radiators and I note that 500ml is designed for up to 10 rads. Should I add another 250ml, or start again and add 750ml ??
16 Mar 2016
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Portsmouth
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Answer: 
Definitely. One bottle per 100 litres of water in system or roughly 10 radiators. It only workd on my 26 radiators after the 3rd bottle. The manufacturer says it won't clog pumps. I never had any problem.
28 Apr 2016
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London
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Answer: 
Hello Twodogs,

Thanks for your question. Unfortunately, as you have a larger (15 radiator) system, using 500ml has under dosed your system, which is why the problem has come back. In order to fix this you should re-dose using two 500ml bottles of the F4, in order to cover your whole system.
17 Mar 2016
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9 Nov 2015
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southminster
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Answer: 
Hello Edwardy. If you have no expansion tank the best option would be to dose using the F4 Express (61837) as this is designed to go straight into your radiator, rather than an expansion tank.

If you have already purchased the 500ml version, you can either through go through a system filter, such as the TF1 Total Filter, or you can dose using a small funnel to pour into a radiator bleed valve.
10 Nov 2015
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10 Sep 2015
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east sussex
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Hello Diy4Den, these two products are compatible.
3 Nov 2015
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  • 2016-08-31 T09:25:51.693-05:00
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Specification
Brand Fernox
Colour Green
Manufacturer Guarantee 1 Year Manufacturer's Guarantee
Model No F4
Pack Size 1
Pieces in Pack/Case 1
Product Type Central Heating Leak Sealer
Sealant Adhesive Volume 500 ml
Total Product Weight 572 kg

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Safety Data Sheet (COSHH)