expansion & feed tank working condition

[Tris]

Need some education on my CH/HW system. it's an open vented pumped system with a new potterton 40 boiler.
should the mains feed to the expansion&feed tank be left ON for normal operation? the ballvalve operates correctly.

I get the feeling that it should be, but surely this might introduce fresh, airy water into the system - obviously undesirable(??)

I had scottish gas in 10 months (immediately after moving in) ago to check over my system, and a few days after that, the system started banging violently about 5 mins after fire-up. I traced this to an empty feed tank (cold supply had been shut off). I turned the supply back on, and the tank filled & levelled out. No problems for the ensuing 10 months. (incidentally I am certain SG inadvertently turned the coldfeed off for some reason and forgot to switch it back on)

I have been thinking about it over that period, and decided to experiment last month so I went to the loft, and shut off the cold feed to the tank, and noted the level.

after 7 days of twice daily fire-up, I checked the tank again and found that the level in the tank had dropped considerably. However, the system was still functioning (though the boiler was a little more gurgly, quite noticably at initial fire-up) satisfactorily.
the system ran another week, and this morning I was woken up by the boiler banging away again and sucking air down from the tank.

The question is: where is the water going? surely this cannot be plain evaporation? I am worried there is a leak in the circuit, but I am in a top-floor flat, and I've been round the lower flats and there's no sign of water! (the system has been operating 10 months and no direct evidence of a leak!)

In the meantime, I have shut off the boiler, and I'm going to re-fill the expansion tank and add inhibitor. And possibly run a "control" experiment with a bucket of water to check what the actual evaporation rate is.

as an aside, during the 10 months when the system was OK (with coldfeed ON) I regularly checked the radiators for air and they never needed bleeding.

All comments on my waffling would be gratefully received!


thanks
Tris

 

[oilman]

The water to the header tank should be left on to replenish the tank when water is lost from the system. The boiler may be kettling, which will increase the evaporation as hotter water gets into the vent pipe. You may have this problem with your boiler.

 

[ChrisR]

Feed should be left on. The level should be set low in the tank so there's no chance of overflowing when the water expands on heating. Molst systems "leak " a bit, often around rad valve stems. Thewater often just evaporates so you don't see it. It IS bad to keep adding new water to the system because of the new oxygen and scale compounds. Would be a good idea to keep the inhibitor level up if new water is goung in at a high rate. Opinions vary, but you could put a leak sealer in the system.

 

[Tris]

Thanks for confirming my suspicions!
when inhibitor is added it will circulate around the system and mix nicely with the water, becoming a sort of coolant. Assuming that coolant is lost only by water evaporation from the expansion&feed tank, will I lose inhibitor via evaporation this way also?
If not, then theoretically the inhibitor will "work" on the incoming fresh water to an extent(?)
I have heard about the coolant-test, where you take a sample of coolant and see how corrosive it is with a steel nail. Whereabouts in the system can & should you draw off a sample?

Incidentally, I believe that my system is kettling, as I have occasionally felt warm water dribbling from the vent pipe. Could this be ameliorated?

I note ChrisR is in antarctica...! are you with BAS? how are you linking to the www?

 

[oilman]

The system should be cleaned before the inhibitor is added as it works on whatever it touches, and if it's not the metal of the system, it's wasted. It does not work by stopping corrosion, so much as cause a preferencial chemical reaction which leaves the metal till later. This means the inhibitor will lose it's effectiveness and will need to be replenished.

 

[Tris]

thanks for that oilman... wish I'd known to clean it before I put in fresh inhibitor though! But it's a relatively new system (2 years), and I think I'll leave the cleaning for next year.