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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
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 8pm Mon-Sat (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 3.4 out of 5 by 5.
Rated 1 out of 5 by from Didn't work for me Bought this because of the positive reviews. But boiler pressure still going down.
Date published: 2019-05-28
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 Works really well My pressurised central heating started to lose pressure and this got steadily worse with a top up required every few days. I searched but could not find any leaks as there are a lot of hidden pipes and joints. I added Fernox F4 and within a few hours the pressure loss stopped. One week later, the pressure is still the same, brilliant. Problem solved.
Date published: 2019-04-03
Rated 1 out of 5 by from Never worked for me Had a leak on a buried in concrete elbow joint and this was previously used by an engineer from baxi. He said if you keep on losing pressure then put another bottle in. This time I dug out the concrete to ensure the joint was exposed to air as the product needs air to go off and seal. So I put the bottle in the system after isolating half the system to ensure there was an extra good concentration of the product. Switch heating back on and within 2 hours the leak had stopped. Perfect I thought as I would not have to replace the pipework. Unfortunately once the joint went cold overnight it started to weep again. I let this work for 3 - 4 days before calling it quits and I eventually replaced the pipework. It could be it was too much of an ask for the product. I estimate I was losing quarter pint per day but it did totally stop leaking when hot!
Date published: 2019-02-19
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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