Adding more fernox

5 Apr 2006
Reaction score
United Kingdom
I cleaned and added new Fernox inhibitor to my C/H 18 months ago, I have now replaced an old rad in the Bathroom and the new one is plumbed in and waiting to be filled, would it be advisable to add more fernox into the header tank or just fill the rad with plane water. If I add fernox shall I tip the whole contents or part? main concern is, will too much fernox harm the system ?
Sponsored Links
Over and under dosing with inhibitors
Manufacturers will specify an ideal, or recommended, dose rate for their product. But it is important to understand how the product performs when over or under dosed. Anodic passivating products will require a sufficient dose to "coat" the internal surfaces of the system plus some excess to maintain this film. If the dose is markedly below the level required to achieve protection, exposed areas of metal will continue to corrode. This means that the protection by anodic inhibitors is either, to use the analogy of a light bulb, On or Off. There is little room for partial protection. On the other hand, over dosing with anodic inhibitors is unlikely to have any detrimental effect. Oxygen scavengers and cathodic inhibitors will not be fully effective if under dosed but will have a partially protective effect. ;)
So, without sounding fick, will it be ok to add more with a risk of over doing it a bit?
Sponsored Links
In English, I think he's saying chuck another bottle in, it may do good but it won't do any harm!

If you're concerned, drain down the whole system and refill but as long as you use the same brand and chemical as before, I doubt it will make any difference.

My system (three-bed house and combi) has a system volume of 40 litres, when the guy came to comission my boiler he said to chuck the whole bottle in. Thats 2.5x overdosing already, since the bottle was good for a 100 litre system. As you appear to have an open system, your system volume will probably be a lot higher.

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local