Fernox F4 Central Heating Leak Sealer Express 400ml

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


Suitable to be used throughout a central heating system to seal small leaks and weeps which may cause pressure loss and boiler breakdown. Once dispersed within the system it will be effective 1-24 hours after application. The unique sealing efficiency of this proven product has been specifically developed to prevent blockages or restrictions within the boiler or vulnerable system components such as pumps and safety devices.

• Seeks out and seals inaccessible, internal microscopic leaks fast
• Quick and convenient - doses in 30 seconds - no need to drain down the system
• Non-toxic, environmentally friendly formulation, compressed air propellant and 100% recyclable can
• One 400m canister treats up to 130 litres of system water (16 radiators) and the equivalent of 250m2 of underfloor systems.

DELIVERY OPTIONS (UK mainland only)

Please be aware that delivery of your order may take a little longer over the festive period. Check our Christmas and New Year delivery times page for more information.

 

Next Day Delivery – £5.00 or FREE for orders over £40

• Order before 9pm Monday to Thursday for next business day delivery
• Order Friday to Sunday before 5pm for Monday delivery
• Orders placed after Sunday at 5pm will be delivered the following Tuesday

 

Pre-12pm Delivery – £10.00 to arrive before 12pm on the scheduled days above

Saturday Delivery – £10.00 or £5.00 for orders over £40 & FREE for orders over £75 (order by 6pm Friday)

Sunday Delivery – £10.00 or £5.00 for orders over £40 & FREE for orders over £75 (order by 4pm Saturday)
Times may vary for items being delivered directly from the supplier. For more information, please visit our delivery information page below.

Items over 30kg can only be delivered from Monday to Friday. If ordered before 6pm, please allow up to two business days for delivery. Orders placed after 6pm Thursday will be delivered after the weekend.

 

Supplier Delivery - Items delivered directly by our suppliers may take up to 15 business days.

 

More about delivery

Collection Details

Please be aware that store opening hours may differ from our standard times over the festive period. Check our Christmas and New Year opening hours page for more information.

 

Our standard opening times are 7am-8pm Monday-Friday, 7am-6pm Saturday and 9-4pm Sunday. Please check your local store is open on the store locator page, before you visit.

 

Once you have placed your order online, we’ll have it ready for you at the store you have selected in as little as 5 minutes. We will hold your order for 14 days, after which we will automatically generate a refund to the payment type you used to place the order.

 

Don’t forget your mobile number – Make sure your mobile number is added so that we can text you when your order is ready to collect. Please wait for your text or email before coming to collect your items.

 

If an item is eligible for Next Business Day Collection in store, the order must be placed by 9pm Monday to Thursday for collection the next day. Orders placed after 9pm on Thursday, and up to 5pm on Sunday, will be available for collection on Monday. For larger items over 30kg, please allow two business days.

Please note: Many suppliers will operate reduced services or close between 17 December 2025 and 8 January 2026. Deliveries and responses may take longer during this period.

This item will be delivered directly by our supplier. Lead times are shown on the product and trolley pages and are always advertised in working days (Monday to Friday).

If your order contains items stocked by us and items that come direct from the supplier, your order will be automatically split and processed accordingly.

We will pass the details you give us, including delivery details, your email address and your phone number, to the relevant supplier.

Most of our suppliers can deliver to any UK address. Delivery to remote or offshore postcodes may take a little longer than the advertised lead time. If the supplier cannot deliver to your postcode or we believe your order may take longer, we will try to tell you beforehand.

Items that come direct from a supplier will be delivered by the courier chosen by the supplier. Where possible, suppliers will update you with delivery information. This may be by email, phone call or text message.

Items delivered direct from suppliers cannot be returned through a store. You would need to return these items direct to the supplier, unused and in their original packaging, at your expense. You can arrange for the items to be collected, for a fixed fee of £9, by contacting our Customer Services team. The fee will be taken off your refund. There is no charge for faulty items being collected.

Please note, you cannot return bespoke or made-to-measure items unless they are faulty.