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Fernox F4 Express Leak Sealer 265ml (61837)

Internal leak sealer, stops small leaks. Treats up to 10 radiators. Quick and convenient, doses in only 30 seconds. Ideal for reducing pressure loss from sealed systems. Seeks out and seals most weeps and leaks.

  • Treats up to 10 Radiators
  • Treats up to 10 Radiators

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£19.99INC VAT (£75.43/Ltr)
Was £25.99 Save £6.00 (23%)

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Fernox F4 Express Leak Sealer 265ml is rated 4.9 out of 5 by 6.
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Rated 5 out of 5 by from leak sealer purchased this product after reading other reviews, it is as good as other people found, it sealed a leak on a soldered joint., it took 3 days for it work but was well worth the wait
Date published: 2016-11-17
Rated 5 out of 5 by from Fernox F4 I have used this product for years. Every 2/3 years I add it to ensure that the operating bar pressure is maintained. Its biggest advantage for me is that it will seal leaks in the system which you cannot reach or see. Last purchase, 2 years ago, was the can you had to use with a sealant gun, very easy, so was a little apprehensive about how the pressurised can would work. No need to worry, clever design of 'trigger' removed any concerns about possible spurting. Another review mentions difficulty adding this product via a radiator bleed valve, saying that the inserter came free from the bleed valve. That tells me simply that the plastic nozzle just needs to be tightened up a bit more, keep turning until it will turn no more and when cutting the tip off the nozzle, do not be too greedy, only a small snip is needed - you will get a good grip unless there is a fault with the thread in the radiator bleed valve, in which case use a different radiator. The applicator has a very short clever flexible section so that the handle on the applicator acts as a trigger. If adding more than one canister, as directed open one rad valve, load one canister, close rad valve again, then gentle unscrew the canister - you will get a little bit of leak from the top of the canister - a towel and basin will deal with that (only a few spoonfulls). Then screw in next canister, open rad valve and off you go. Close rad valve, unscrew entire device and get that bleed valve screw back in place. Open both sides of radiator fully and let system run for a good 30 minutes with valves wide open to effect good distribution quickly. Then set thermostatic valve (if you use them - good idea to) to usual position. Forget about the system till next time you notice the bar pressure is dropping even with the system hot. Next morning check your bar pressure again, just to be sure! Brilliant, reliable product and now easier to use since gun not required.
Date published: 2016-11-02
Rated 5 out of 5 by from It works! This is the best product I have ever used and saved me buying a new boiler, as the heat ex changer in it had a small leak. Not now, it took around 3hrs to work and its been drip free ever since. The only point I would raise are the instructions, if your not very DIY. Go online for further details if your unsure, though the process is very simple. Takes only a couple of minutes to complete. Would highly recommend.
Date published: 2016-10-04
Rated 4 out of 5 by from Weeping Joints My central heating had developed a number of annoying leaks over the summer, that didn't want to seal up. If you've got assistance injecting into an ordinary radiator might be possible but I found you need too many hands as injector doesn't stay in under pressure! I injected into a towel rail radiator that let me use the large screw in adaptor. This worked great with just two hands, useful when alone :) Had some green leaks first night but seems to have the job perfectly. If you know where leaks are put something underneath to catch anything as the green is a bit messy. It found one I didn't know I had which resulted in a pool of green on the tiles, it cleaned up OK but carpet might not so beware!!
Date published: 2015-10-01
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1 Question  1 Answer

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We have 11 radiators, but some of those are large, so I was wondering whether 2 bottles would be far too much. Thanks.
22 Jul 2016
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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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  • 2016-11-18 T05:38:57.255-06:00
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Brand Fernox
Colour Green / Blue
Manufacturer Guarantee 1 Year Guarantee
Manufacturer Guarantee 1 Year Manufacturer's Guarantee
Model No F4 Express
Pack Size 1
Pack Size 1
Pieces in Pack/Case 1
Pieces in Pack/Case 1
Product Depth 50 mm
Product Type Leak Sealer
Sealant Adhesive Volume 265 ml

pH (concentrate): 5.0-7.0.

Hazardous - Delivery unavailable for any offshore location.

Product contents

Radiator and filling loop adaptors.


Safety Data Sheet (COSHH)