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
Download more information
Safety Data Sheet
Additional Information
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

Click here to see information on our delivery charges and times



Hopefully you won't need to return an item, however, if you do, it is our aim to make this as simple as possible. By following the guidance below you can help us to help you when a product is not what you expected, or is faulty. Your statutory rights are not affected by this guidance.


Click here to see information on returns and cancellation

€19.95 Incl. 20% VAT

Add to Wishlist

More Items to Consider

Loading... Loading...
Fernox F4 Central Heating Leak Sealer 500ml is rated 4.6 out of 5 by 10.
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
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 5 out of 5 by from Does Exactly What It Is Suppos I have an oldish CH system (about 40 years old) and have been bothered for ages by a variety of hard-to-trace weeps. I recently had a new pressurised boiler fitted and the system was losing pressure at the rate of around 0.1 - 0.2 bar per week (more than 0.5 litres per week).I added the fernox and the system has lost no noticeable pressure over a period of a month. Better than expected result!
Date published: 2013-07-23
Rated 5 out of 5 by from Does exactly what it says ! Having had a fairly bad leak from a large vertical radiator (poorly welded joints) I decided to give Fernox F4 a go by adding the whole bottle to my central heating header tank in my loft...and then draining a few pints off from one of my radiators...to get the stuff into the system & circulating. Amaizingly after only 2 hours of running the central heating...the leak had stopped completely !. Fantastic product.- does exactly what it says !.
Date published: 2013-01-31
Rated 5 out of 5 by from it did the trick my central heating system had suffered from a persistent, inaccessible, weeping leak for several months. A large plumber's bill for pipework replacement was feared. A friend in the know recommended Fernox F4. Within 24 hours the leak had stopped completely. The directions for application on the container were easily followed and are spelled out slightly more specifically on the Fernox web-site. Problem solved.
Date published: 2012-04-09
Rated 5 out of 5 by from Problem solved After trying and failing for almost a year to find leak causing pressure drop - and topping up every few days , I put Fernox into the system via radiator using ' no nonsence filling kit' - easily done. Since then after 3 or 4 weeks no pressure loss at all. So far -so good I would recommend this to anyone.
Date published: 2011-02-12
Rated 5 out of 5 by from Does what it is supposed to do Recommended by a plumber mate. Talked me through doing a partial drain down of my system and poured this in, a day or so later the leaky radiator valve has been sorted out. Does the job, what more can I say. Have patience, it didn't work straight off but 24 hours or so later and having run the central heating a couple of times seems to do the job.
Date published: 2009-12-02
Rated 5 out of 5 by from Great product Both isolation valves on my Gloworm Combi were leaking. Expensive and difficult to replace. I was disappointed at first but after 48 hours the drips stopped. No pressure loss over 1 year later. Recommended.
Date published: 2009-11-27
  • y_2020, m_2, d_19, h_17
  • bvseo_bulk, prod_bvrr, vn_bulk_3.0.5
  • cp_1, bvpage1
  • co_hasreviews, tv_1, tr_9
  • loc_en_GB, sid_45902, prod, sort_[SortEntry(order=SUBMISSION_TIME, direction=DESCENDING)]
  • clientName_screwfix-ie