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Product Details

Seals small leaks and weeps which may cause pressure loss and boiler breakdown.

Technical Specification
Brand Fernox
Safety Data Sheet click here
Volume (L) 0.265
Delivery details

Covid-19 Update

  • Free Next Business Day delivery for orders over £25 to home and site is available for orders placed before 5pm
  • Saturday and Sunday delivery is now available for orders placed before 4pm on Friday for Saturday delivery and 4pm Saturday for Sunday delivery, see here for details
  • We have temporarily suspended our Pre-12 delivery service
  • Once your order has been dispatched you will receive an invoice email with the courier details and your tracking number
  • Alternatively go to MyAccount, click on your Order History, click on the TRACK icon to link through to the carrier website and the tracking details for your delivery
  • If your order contains more than one item it may be split into two deliveries as we manage volume across our network
  • Due to carrier constraints it is not currently possible to deliver age restricted items. Please use our Click & Collect service for these products
  • Delivery direct from suppliers is now available and specific delivery times will be displayed on the product page
Order Value: Under £25 Over £25 but less than £75 £75 & Over
Next Business Day Delivery (Mon-Fri) usually before 6pm £5 Free Free
Pre-12 Delivery before 12 noon Next Business Day £10 £5 £5
Remote areas standard delivery (2-4 days) £5 Free Free
Saturday Delivery order by 4pm Friday £10 £5 Free
Sunday Delivery order by 4pm Saturday £10 £5 Free

Please note items delivered by our suppliers are not available for the options above. Delivery times may vary.

More about Delivery & Returns
Click & Collect details

Covid 19 Update

Click & Collect

  • Safe contactless collection is available 7 days a week at our branches
  • Our branches are open 7am-5pm Mon-Sat and 9am-4pm Sun
  • Keeping our colleagues and customers safe and well and respecting stringent social distancing requirements is our number one priority – 2 metre rule enforced
  • Our available product range will not require more than 1 person to lift
  • Please see our guide to click and collect below

  • Please ensure that your mobile number is added so that we can text you when your order is ready to collect

Call & Collect

Due to current trading conditions we have suspended our Call & Collect service until further notice.

If you are using our Click & Collect service please note the following:
  • Order online or over the phone and collect in branch from 5 minutes later.
  • Ensure you check stock is available for collection at your chosen branch.
  • Check branch opening hours to ensure you have sufficient time to collect.
  • For items not available for same day collection, place order before 7pm Mon-Fri (4pm Sun) and you can collect Next Business Day (Mon-Fri) after 3pm.
  • If we have your mobile number, we’ll send you a text when your order is ready to collect.
Fernox F4 Central Heating Leak Sealer Express 265ml is rated 4.3 out of 5 by 8.
Rated 1 out of 5 by from Fernox F4 Express Although i cant see a leak in system anywhere this did not work for me--pressure still drops,although i will say it was very easy to get into the system.
Date published: 2020-03-02
Rated 5 out of 5 by from Central heating leak stop Easy to use, comes with several Connectors did the job.
Date published: 2020-01-31
Rated 5 out of 5 by from It Works Had a small leak in the central heating system somewhere & the boiler pressure kept dropping. Was unable to find the leak so decided to use Fernox F4 sealer express.Turned off in & out valves on an upstairs rad,removed bleed valve & fired in the Fernox. 2 weeks later & still have no problems.Would recommend.
Date published: 2019-12-28
Rated 3 out of 5 by from leak sealant hard to get into radiator, bought another type
Date published: 2019-12-12
Rated 5 out of 5 by from Absolutely brilliant product and a great price. Following a radiator removal/replacement we had a small pressure drop of about 1 bar over 5/6 days which required regular top up from filling loop. Careful investigation of visible joints, connections and valves etc failed to identify any perceptible leaks and the fear was that maybe an inaccessible joint had possibly loosened below a newly tiled floor, of course there was no way of being sure without pulling the whole floor up! I decided to give Fernox F4 a try as I had nothing to lose. It was very easy to use, adding it to the system via a radiator bleed valve without even having to drain down or depressurise. I am pleased to say that it has been 100% successful and the pressure has now remained stable and constant with no loss over several weeks. As a bonus, we have since found that a small amount of the product residue has hardened just below the nut on one of the rad valves thus indicating where the seepage was occurring so I will be able to carry out a permanent repair in the future if need be, though I may just leave it alone all the time it remains leak free. All in all this is a brilliant product which does exactly what it claims, is very easy to use and was a very good price from Toolstation. Highly recommended.
Date published: 2019-12-02
Rated 5 out of 5 by from Works ! Had a slow leak somewhere in my heating system, losing pressure, used Fernox and everything is now fine !
Date published: 2019-11-02
Rated 5 out of 5 by from Easy to use. Excellent product, Fernox speaks for itself. Like the adaptor, makes it so much easier to use.
Date published: 2019-05-19
Rated 5 out of 5 by from Brilliant stuff! I have a Vaillant Combi boiler which is only around 2-3years old and just recently it started to lose pressure and I was having to top it up every other day and sometimes every day. It was really worrying as I seemed to be putting quite a bit of water into the system each time I topped it up and the contractor who installed the boiler seemed a little reluctant to get involved! As I couldn't find a leak anywhere I thought I had nothing to lose by giving Fernox F4 a go and put two cans into the system via the filling loop, I ran the boiler with all the radiators open fully for a full day and hey presto the leak has stopped. It's been a month now and the pressure hasn't budged.... fingers crossed! Great product.
Date published: 2019-02-17
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