Possible leak in heating system

we recently had some work done and have noticed the paint peeling around the radiator that was replaced. It's on an outside wall next to a bay window so there is some thought that it is coming from outside.
The radiator is wall hanging and I cannot see any signs of damp around the skirting under the radiator or the tile floor (grout).
Plumber is suggesting we put fernox in the system to eliminate this. is this ok to do on an oil fired grant boiler?.
The F & E tank in loft seems to intermittently drip (got really old ball valve on it so not sure if there is a leak or not)
Worried we may wreck the boiler as have read some things saying it can damage boilers.

Depends what they mean by 'fernox' as that is a brand applied to a whole range of different products.
In any case, chemicals to seal leaks are a temporary fix at best, at worst they may not work at all.
The only real solution is to locate the leak and repair it properly.

To determine if there is a leak or not, isolate the supply to the F&E tank (or tie the ballvalve up with string), mark the level of the water and check again in a few hours/days - if the water level has dropped, there is a leak.

Ok so i did as to suggested and the and the water level dropped by an inch overnight so clearly there is a leak somewhere. How do I go about finding this as the whole of the ground floor is concrete

Similar issue to me, in a thread I made yesterday. I think your options are either going around all your pipes and looking for signs of leakage, if only a small leak tie a bit of toilet paper around the rad pipes and see if after a few hours there is any staining. Otherwise, as I suspect I will be doing, you can get guys with thermal imaging cameras around to scan for leaks; I have it covered on buildings insurance so might be worth checking your policy (think it is fairly standard these days).

Steve, I think you are right going to have to get the big guns in as whole of downstairs is concrete with either suspended wood floor or tile.
Yep my insurance policy has trace and access cover £250 excess mind but am pretty sure it's gonna cost a whole load more than that to get a man out to survey, find the leak and then fix it.

Ashtara - did you have any joy with your insurance guys? I made an inital call to mine today, didn't give my details as I did not want them making notes before I got my position straight, but they said:

"under the policy wording we will only cover trace and access if there is visable damage to your property, if there are no signs of damage you are not covered". Which they have derived from the following policy wording:

2a Tracing and Accessing Leaks
In the event of damage to the Buildings caused by escape of water from
water tanks, pipes, apparatus or fixed heating systems in the home, we will
pay the reasonable costs, up to £5,000, for removing and replacing any
other part of Buildings necessary to find and repair the source of the leak.

It strikes me as completely unacceptable they take this to mean you need to have visible water damage before they will cover the cost of tracing a leak. If I can see the damage then I am pretty sure I can make an educated guess as to where the leak is!!! I plan to push the insurance guys as hard as I can on this, otherwise it seems they are encouraging me to turn a blind eye to an obvious leak until more damage arises! Anyone else had difficulties with their insurance co's on such stuff?

My insurance company have confirmed they will cover it - sending someone round to take a look tomorrow.

The wording on mine is slightly different though in that it doesn't ask for there to evidence of damage to the building.
As it happens we think we know where the damage is but are not sure.

This morning the f & e tank in loft was totally full to the overflow so think ball cock is faulty or I damaged it when doing the test. This has now been changed but still doesnt explain why when ball lifted I lost over an inch of water so there still must be a leak somewhere

So the update on my heating system problem is the trace and access company came round on Friday and found a leak where a radiator had been moved and the solder on the cap had failed.
Unfortunately, the leak travelled quite a way along the pipes and has done quite a lot of damage.

Last night we tied up the ball cock again to check no more leaks. I went up after about an hour and a half and released the ball cock, I had a little spurge of water enter the tank and then nothing so decided to leave it up overnight. This morning, the water had gone down but very slightly and took a few seconds to fill up. I stood and watched for 10 mins (system was totally cold as heating not on all night) and no water filled in after initial purge. We turned heating on and I could see bubbles forming as something stirred from the cold water inlet.

I then went round and found air in a few of the radiators (they weren't bled after leak was found on Friday)

Just went in loft again (heating been on all morning) and nothing dripping into the tank but there is quite a bit of condensation.

A few questions

Am I panicking thinking there is still a leak and now somewhere else?
Why would tank level drop albeit only by a little (about 1cm) overnight?
Would the air in radiators have affected water level?
Could it just be evaporation (heating had been on when we lifted ball cock)?
I found some of the radiators had black water and some clear is this as a result of the leak and air getting into the system.

Sorry for all the questions but as you can imagine I am stressed as it will take months to get my house back to normal now.

As you bleed rads, fresh water from tank takes its place.
Condensation in cold loft caused by having hatch open, letting up warm moist air.
Nothing to worry about unless you get further activity from ball cock.

Thanks Mazperks, I only bled radiators after I had untied the ballcock and the tank had refilled for a minute with it's initial purge after that tank did not fill at all for the 10 minutes I stood there.

Why would the water level have dropped overnight slightly?

Your water expands as it gets hot raising level. As it cools overnight it contracts.

Thanks that's what our initial thoughts were especially as as soon as we dropped the ball cock in the morning it filled up only slightly and then stopped completely.

What a relief, we have enough damage to contend with from the first leak without having to deal with another (I just went into panic mode when I saw the water level drop in the morning)