Corrosion Inhibitor. How good should it be?

[stem]

Three years ago, I had 2 radiators added to my heating system when I had an extension built. For the previous 8 years, it had been treated with Fernox. When it was drained down for the extension, the water was slightly coloured but only to the extent that when it filled a bucket, the bottom was still visible.

After the extension, the system was refilled and a litre of Sentinel added. A second litre was added last month after a leak (loosing approx 1 gallon) to top up the strength. Yesterday, I removed a radiator for decorating with a view to saving the contents to return it to the header tank to avoid diluting the system with fresh water.

When I collected the water, it was dark brown, to the extent that half an inch in the bucket completely obscured the bottom and it looked like gravy. It didn’t seem to be any thicker than water though and not at all sludgy.

I'm not sure what to do now. The system now has 2 litres(ish) in will it be OK at that?

The system comprises of 8 radiators of varying sizes and mixtures of singles & doubles. Fitted to a standard 3 bed house.

 

[JohnD]

it is possible that the sludge you have found was lurking in the bottom of the rad all the time.

modern professionals will frown on this idea, but many years ago Fernox used to recommend it:

drain off enough water to fill a jam jar. Drop a bright steel nail in it and shake it to aerate. Leave it on your garage shelf. look at it once a month and see if it has gone rusty. If not, the water is well-inhibited.

You can dilute the system water to 50% with tap water to get a double measure of confidence.

IMO it does no harm to give an old open system a chemical clean with sentinel X400 every ten years or so, it will loosen old sludge so you can drain it out and refill using fresh inhibitor. You will know it is working when the circulating water goes jet black with loosened sludge and you can leave it in for 4 weeks - it is not aggressive to the metal. Bale out any mud from the F&E tank first and sponge it clean.

If you can afford an extra £100, also fit a Magnaclean or Spirovent or Sentinel or similar filter. You will be pleased to see how much muck it collects. The Magnaclean is very impressive if you have much black magnetite sludge, which is very common in old systems.

I am a householder, not a professional

 

[mogget]

drain off enough water to fill a jam jar. Drop a bright steel nail in it and shake it to aerate. Leave it on your garage shelf. look at it once a month and see if it has gone rusty. If not, the water is well-inhibited.

A couple of years ago I took a couple of glass jars and put three steel nails and a scrap of copper pipe in each. The first one I filled with tap water and the second with a sample from my heating system.

The difference was detectable within a day and after a week the tap water jar was distinctly rusty. The inhibited sample stayed clear.

I still have the jars and the corrosion did appear to stop after a while. I suspect that if I removed the lids, the oxygen in the air would start it going again.

 

[stem]

It is possible that the sludge you have found was lurking in the bottom of the rad all the time.

I don't think so, this was one of the new ones.

Drain off enough water to fill a jam jar. Drop a bright steel nail in it and shake it to aerate. Leave it on your garage shelf. look at it once a month and see if it has gone rusty. If not, the water is well-inhibited.

I'll try that, but I think I would have to remove the nail to examine it, I doubt I would be able to see it through the murk.