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Fernox F4 Express Leak Sealer 265ml (61837)

You must be over the age of 18 to purchase this product.

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.

  • 1 Year Guarantee
  • Treats up to 10 Radiators

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£25.99INC VAT (£98.08/Ltr)

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Fernox F4 Express Leak Sealer 265ml is rated 4.8 out of 5 by 9.
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Rated 5 out of 5 by from Fernox F4 Express As they do, central heating boiler developed a leak (heat exchanger I think) 2 days before Christmas!! Purchased this product more in hope than expectation but low and behold it works!! Leak stopped within 15 minutes and so far so good. Only downside was that I had to drain and remove a radiator from the wall to get the stuff into the system by injecting it into the radiator while it was on its side. But hey, no big deal. Got the heating on again way higher than necessary just because I can.
Date published: 2016-12-24
Rated 5 out of 5 by from Fantastic stuff! Recently spent a LOT of money on a new kitchen and extension, rewire and new Karndean flooring throughout the ground floor. A few weeks a ago I noticed the pressure on my two year old Worcester Bosch boiler kept on dropping. Of late, however, the pressure drop became alarmingly rapid. I hunted high and low to find any leaks but to no avail, which was worrying as soon you shall see. The ground floor of my home is all concrete and had recently been covered with rather expensive Karndean (stuck to the floor) so I prayed that the leak was not beneath it. Past experience of some years ago when I had the misfortune to experience just such a situation made me rather concerned. A bit of googling lead me to Fernox F4 Express; I decided to give it a go after watching a video on the Fernox website gave me confidence. I was loosing about 1 bar of pressure in a day or so, so suspected that it may be a leak to big to handle. I am pleased to say, however, that it managed the job and the pressure drop disappeared overnight. I administered two canisters via the filling loop on my 9 radiator system at 6pm one evening and topped the system up to 1.5bar. The following morning at 7am I was pleased to find the needle on the pressure gauge still sitting at 1.5bar. It is now a few days after the procedure was carried out and I am delighted to say that it appears to be holding out. The expense, mess and misery of digging up the new floor seems to have been avoided.
Date published: 2016-12-23
Rated 4 out of 5 by from Leak fix Combi boiler central heating system losing pressure over several days. Acccording to my Plumber it was not in the boiler so probably a leak on the system. I did not want to lift floorboards at this time of year - he suggested a leak seal product as a first fix. I chose this product because of its delivery system, unfortunately the filling loop threads are smaller on my boiler but used the smaller injection nozzle provided and connected to a radiator bleed nipple as per instructions. Done in less than 5 minutes 2 days ago, no pressure loss as yet so touch wood, job done. Little bit messy when refitting bleed nipple but otherwise an easy product to use. Too early to comment on reliability but website asks for a score hence the 1 star.
Date published: 2016-12-22
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
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Kettling after addition of F4, is product known to cause foaming?

Small boiler weep. F4 stopped. If I drain the whole system will the leak remain fixed, is this product known to causing foaming? It certainly seemed very 'bubbly' when injecting in the rad.
Asked by: PZCustomer
Hello PZCustomer. No, the F4 Express is not known to cause foaming, in fact as a Leak Sealer it will discourage foaming in the system. Please call our technical team on 0330 100 7750 for more information.
Answered by: Fernox Client Services
Date published: 2017-04-22


Is this product safe to use on a mixture of 15mm copper pipe into 8mm microbore and 10mm microbore pipe? Fed from a combi boiler system. I have a small weep in an inaccessable area. Thanks
Asked by: PeterVenkman
Hello Peter. Leak sealer is ok to use with micro-bore but it needs access to air to work, so if your pipework is buried in solid concrete or is lagged it will not work. Hope this helps
Answered by: Fernox Client Services
Date published: 2017-04-15

Magna clean

Can this be added via the magna clean type filter or does it have to be added via a radiator. I am not that competent in plumbing but I can clean out the filter
Asked by: Amature1985
Hello Amature1985. The Express can be dosed via a Filter, as long as the filter you are using has a bleed valve and has the capacity to take the volume of the express product (265 ml). If can't use the filter you can dose the F4 Express via a radiator or a filling loop. You can find video instructions on how to dose using the following link; Hope this helps.
Answered by: Fernox Client Services
Date published: 2017-03-12

Air Source Heat Pump

Will this work with an Air Source heat pump, i am losing pressure over about 2 months and have no idea where from. the Heating system is loaded with Glycol anti freeze so will this mix in with that?
Asked by: Dmmm
Hello Dmm,. Unfortunately F4 will not work when used with Anti-freeze, so in this case no.
Answered by: Fernox Client Services
Date published: 2017-02-05

Can you overdose a system with this product?

We have 11 radiators, but some of those are large, so I was wondering whether 2 bottles would be far too much. Thanks.
Asked by: Whaley
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.
Answered by: Fernox Client Services
Date published: 2017-05-09
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Brand Fernox
Manufacturer Guarantee 1 Year Guarantee
Model No F4 Express
Pack Size 1
Pieces in Pack/Case 1
Product Type Leak Sealer
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)