Posted 6 March 2023

Fernox F4 Express Leak Sealer 400ML - £12.99 + Free click and collect @ Screwfix

johnnytwohatz's avatar
Shared by johnnytwohatz
Joined in 2022

About this deal

For use in domestic heating systems for sealing small inaccessible leaks. Quick and convenient, doses in only 30 seconds. Works best in warm / hot circulating water. Once dispersed evenly in the system water it is effective within 1-24 hours. The sealing efficiency has been formulated to prevent blockage of safety devices, pumps, air vents and more. Compatible with all Fernox Protectors.

1 Year Guarantee
Non-Toxic, Environmentally Friendly Formulation
100% Recyclable Can
Compatible with All Fernox Protectors
Screwfix More details at Screwfix
Community Updates
Edited by a community support team member, 6 March 2023
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  1. dwaindigital's avatar
    I've got a pressure leak it think.
    Topped up my pressure about 2 weeks ago to 1.2bar and today it's again at 0.9bar
    I've checked all my rads on the circuit but cannot pin point the issue
    I'm wondering if this could repair it
    A lot of my piping is under floors or chased into walls (edited)
    creek's avatar
    Charge your expansion vessel to the correct pressure first and check it again in a few weeks. Had the same problem, boiler kept losing pressure, turned out the expansion vessel needed changed due to an air leak
  2. jazzman466's avatar
    Good price for this. Also went for some Fernox F1 too as this is on offer.
    Note - if you've been needing to regularly top up your boiler pressure over winter then you will need to make sure you've put some more inhibitor back in as it will have diluted somewhat.
  3. raitandco's avatar
    The temperature can also effect the pressure,as it does with tyres.
  4. sully01's avatar
  5. edwardo1973's avatar
    Be careful with this, whilst I’m not in the trade I’m sure I heard that it can void your boiler guarantee.
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