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Product Details

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

Technical Specification
Brand Fernox
Volume (L) 0.4
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 400ml is rated 4.5 out of 5 by 43.
Rated 5 out of 5 by from Great product Excellent product. Easy to use. Worked first time no problems.
Date published: 2020-05-28
Rated 5 out of 5 by from Worked a treat Apprehensive at first but thought its the same company that makes the filters for my boiler so gave it a go I was losing 1.7 bar to 0 bar in less than a day had two plumbers out neither could find any leaks so used this sealer nothing happened for 48 hours then no pressure loss boiler working as it should radiators working great so to the hot water its been a few weeks now since using and everything worki g fine and no pressure loss whatsoever I have since found where the leak was due to brown stain just needed a couple of compression joints tightening but would recommend fernox leak sealer all day long
Date published: 2020-05-27
Rated 5 out of 5 by from Does what it says on the tin Great product. My Baxi boiler had a solid filling loop rather than a flexi pipe so a little more fiddling than the Fernox Video to connect up but it did the job sealing a weeping leak from a weld on our bathroom towel rail. No draining or other messing about. Much cheaper at Toolstation than others.
Date published: 2020-05-20
Rated 5 out of 5 by from Leak seal Bought this at the weekend worked a treat
Date published: 2020-05-19
Rated 4 out of 5 by from The most popular Not installing yet but will soon
Date published: 2020-05-15
Rated 3 out of 5 by from Will give an update in 2 months Placed the Fernox F4 Sealer in the magnetic filter on the 13th May 2020 due to consistent pressure loss from 1.4 Bar to 0.7 Bar on a new boiler from the well known Company 'B' which was poorly installed. I had a leak detection company come out and they couldn't find anything so I am assuming this is a faulty boiler pressure sensor or a very small weep somewhere under the floor/screed which couldn't be detected. I have an Intelligent Filling System on a Worcester Bosch 8000 and it will auto fill back to 1.4 Bar when it drops pressure but this is programmed to a max of 8 times in 8 weeks. I will update then. 3 Stars because it hasn't been evaluated yet.
Date published: 2020-05-14
Rated 5 out of 5 by from Always helpful Brought this to mend a pressure leak on the central heating,did the job,the staff are always pleasant and helpful.
Date published: 2020-05-05
Rated 5 out of 5 by from Fernox F4 Central Heating Leak Sealer I was consistently losing around 3 bar pressure over a 14 day period and could not find the leak above ground so feared the worst that it was in the ground floor screed. The whole house had been re-plumbed in copper to replace the original badly installed plastic (because that was actually leaking into the screed), so it looked like the original problem had come back to haunt me. Being a loyal user of Fernox products in the past I filled the system with this in desperation as I really didn't want to start digging up an oak planked floor to trace the source of the leakage. Well I'm extremely pleased to report that since adding this leak sealer to the system the pressure has held up with no noticeable drop in over a mont. Needless to say I'm extremely relieved! Footnote: I've now just found what I think was the source of the leak and which is actually in one of the upper floors and not the screed so at some point I'll lift the boards and re-make the joint but in the meantime I'm a very happy chappy : )
Date published: 2020-05-05
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