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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
Order value: Under £10 £10 & over
Express usually Next Business Day before 6pm £5 Free
Turbo usually Next Business Day before 12 noon £10 £5
Remote areas standard delivery (2-4 days) £5 Free
Order Value: Under £25 Over £25 but less than £75 £75 & Over
£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
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 9pm 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 4.2 out of 5 by 15.
Rated 5 out of 5 by from Easy Dead simple to use. Seems to do exactly what it says on the website
Date published: 2020-02-11
Rated 4 out of 5 by from good product Easy to use - so far so goo. Too early to tell if it stops the problem
Date published: 2020-02-11
Rated 5 out of 5 by from Fernox F4 The excellent reviews on this product seem well justified. I had air entering the system, I assume due to a pinhole leak that I couldn't locate, and which meant I needed to bleed two rads every couple of days. It's now five days since dosing with no sign of any air entering the system.
Date published: 2020-01-28
Rated 5 out of 5 by from NEW PRODUCT I have used the Fernox F4 central Heating leak Sealer before and it is a great product. However, collecting another can from Toolstation makes it even more pleasurable, because it was cheaper than most other suppliers and so easy to collect. Well done
Date published: 2020-01-24
Rated 4 out of 5 by from Does what it says Would have given it 5 stars but when something does what it says but I knocked off 1 star due to the 15 minute wait for click and collect when you have to join the ordinary queue . What's the point of that?
Date published: 2020-01-21
Rated 4 out of 5 by from Leak sealer I hope should be working..
Date published: 2020-01-12
Rated 5 out of 5 by from It really works! Had a slow pressure drop in the central heating system, about 1 bar over 3 months, after having a ground floor radiator replaced which may have disturbed a fitting under the floor. Not wishing to rip up the tiles in the kitchen to try and find the leak. I used one 500lm bottle of Fernox F4 , which I put into the system via the heated towel rail in the bathroom, and after running the heating for 24 hours, the pressure has remained rock steady.
Date published: 2020-01-11
Rated 5 out of 5 by from Fernox 4 express Had a small leak on my central heating boiler after changing the PRV a few weeks ago. Applied the Fernox 4 via the filling loop. Didn't even have to drain down the system and applied within thirty seconds. Job done and leak fixed.
Date published: 2020-01-10
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