The fact you top up at all means you have a leak, and air will be leaking in (quite possibly at a faster rate than the water leaks out). Topping up once a year is not serious enough to rip up all the floorboards to find it, but the leak will be slowly using up the inhibitor.
Do the nail test I mention in my other post.
This is a brief summary of the accumulated knowledge/ignorance (delete as you think appropriate) I have acquired in the course of 20+ years in building services. I'm not a chemist, there's a lot more to corrosion science, but this is adequate for most purposes.
There are 2 main varieties of corrosion which affect heating systems; dissolved oxygen and galvanic corrosion.
DISSOLVED OXYGEN Dissolved oxygen will corrode the inner surface of radiators and reduce them to black, stick magnetite sludge. The oxygen can get into the water by a variety of routes, e.g., the exposed water surface in a feed & expansion tank, or leaks through valve glands caused by the low partial pressure of the oxygen in the water, mentioned by AG above. A water leak will introduce dissolved oxygen in the fresh make-up water. fresh water with dissolved oxygen. 'Pumping over' will introduce large quantities of oxygen into the water and will ruin the system within a few years.
Plastic pipes are permeable to oxygen and some oxygen will enter the system through the pipe walls. This is it relevant with car cooling systems, which generally have a lot of flexible, non-barrier hoses. Pipes with an oxygen barrier will reduce the permeability of the pipe walls to a negligible level, but they do completely stop the oxygen absorption.
The inhibitor solution deals with the dissolved oxygen by having an oxygen scavenger component. The scavenger reacts with the oxygen before the oxygen reacts with the steel radiators. The scavenger can be a number of chemicals, typically sodium sulphite. The sulphite should be maintained at nlt 20 to 50 ppm; titration teat kits are available to monitor the concentration. O2 + 2SO3= 2SO4
The rate of the sulphite depletion indicates the rate at which oxygen is being absorbed
GALVANIC CORROSION
Galvanic/ bi-mettalic corrosion normally takes place between the copper pipes and the steel radiators. A corrosion cell will exist between any two metals that are electrically connected and immersed in a suitable electrolyte. A voltage will be created between the two metals and ions will be stripped from the anode, the less-noble of the two metals, like electro-plating in reverse. Hydrogen is generated at the anode (I think), the steel radiator in this instance. The hydrogen is insoluble and can be identified by collecting it in an upturned glass and igniting it. The oxygen is dissolved and attacks the radiators, see above.
The galvanic corrosion is dealt with by ensuring that the electrolyte/water is alkaline pH>7. Typically this is done by adding caustic soda to neutralise any acids and adjust the pH to 8 or 9.
The rate of galvanic corrosion will be greatly increased if the water is acidic. The main source of the acid, in new systems, is active flux, which contains hydrochloric acid. This should be thoroughly flushed out of the system but often the pre-commisioning flushing is inadequate or is completely omitted. Excessive flux and the careless use of flux will add to the problems.
Another source of acidic compounds is anti-freeze (AH please note). The glycols in anti-freeze will degrade in the presence of heat and dissolved oxygen to form acidic compounds. Bad anti-freeze can cause truly spectacular corrosion. This is very relevant with car cooling systems. Once the residual alkalinity is consumed, the coolant will become increasingly acidic and galvanic corrosion will attack the metal engine components. Usually the head gasket fails.
Chemical corrosion inhibitors are cheap. The steel components of a heating system are costly and internal damage cannot be repaired. Neglecting or omitting the corrosion inhibitors is the mother of all false economies. Inhibitors will not stop existing corrosion problems (leaks, pumping-over). You can overdose with inhibitors, but I don't know what the effect would be.
He is not competent, he should stick to gas.
He may be a technician, he is not a professional.
No. You'd only need to powerflush it if it had been neglected. Powerflushing is unlikely to remove all the corrosion products. You should maintain it and ensure that it never needs flushing. The above strange opinions add to my suspicions about Corgi-registered technicians.
Typo
titration teat kits are available should be titration test kits are available.
My mind must have been elsewhere after writing titration.
That depends on the water in your area. I have seen rads in London last 5 years without inhibitor, and rads still fine after 26 years in softwater areas.
Top it up now.
Please explain the chemistry behind this statement.
Not that I'll hold my breath.
It is easy. Drivel installed the rads in London and used a supersize bucket of active flux. He was called back repeatedly, but they'd all failed within 5 years.
The 26 year old rads were installed by someone competent when Drivel was a lad and are still fine.
Drivel thinks it must have been the water. That clear it up?
;-)
My brother lives in a soft water area - Aberdeen - and has had many of his rads replaced through corrosion. BG service contract and no inhibitor. I'm in London and my system is older - with Thermapanel rads which some say aren't too robust corrosion wise - but no problems at all in 30 years. Inhibitor changed about every 5 years - and on draining down the water looks reasonably ok. Certainly not black. And this with an open vented system.
Radiators seem to have been gradually turned from capital purchases into consumables. My parents' heating system (installed by my dad) ran from late 1950's through to
2000 with no inhibitor. All the original radiators (which look exactly like today's plain stelrads, but no fins) are all still fine, with no rust or leaks whatsoever, so inhibitor-free life is in excess of 40 years. In 1964, two extra radiators were added, and these both failed about 20 years later. In 1970, one extra radiator was added and this failed after 10 years.So we can plot radiator lifetime without inhibitor as a function of year of manufacture something like this:
Year of manufacture Expected lifetime 1958 > 42 years 1964 20 years 1970 10 years
It would seem the industry cleverly worked out how to turn radiators into consumables throughout the 1960's, and create the need for inhibitor (another consumable).
Well it is a view from a 'Professional' and I posted it as many respect the views of Professionals. (and indeed pay for them) ... and the way legislation is going the CORGI man and the NICEIC man are going to have a lot more say in what goes on in peoples houses.
BTW - I did not follow his comments, and although I have a sealed thermal store I did dose with correct Inhibitor.
Rick
I looked at his business card - he is also a member of IDHE (now called IDHEE) - "The IDHEE is still the pre-eminent professional body for the domestic heating engineer. The Institute aims to promote energy efficient domestic central heating components and the installation of safe and efficient systems. "
and also a member of IPHE - Institute Plumbing & Heating Engineers .
So I would say that from a professional Standing point of view he fits what most people would expect as 'meeting the standard' but as I mentioned in previous post, I did not follow his advice and used Inhibitor. I had installed the system myself, and he was commissioning the system and issuing the gas safety certificate.
However somebody who is not well versed with heating systems, may well follow the advice of "the Professional" and not use the inhibitor.
Rick
Richard Cranium, why?
Please hold your breath for a few days.
So the usual don't know?
Stick to quoting brochures.
Did you actually completely drain the system, or would some water have remained?
If so you may still have some inhibitor.
My system survived a number of years without new inhibitor. Certainly much longer than I intended!
There was very little sludge in the bottom of the radiators when I did get around to drains and re-filling the system.
Mr Cranium, will you tell us please? You appear to think you know. Give us your views. Spill the beans. Well I'm sure you spilt them on your cardigan.
It currently doesn't seem to have a leak. I have a hand pump for pressurisation, and a 2 gallon old beer barrel (pressure vesel) as a container for topping up water, and for draining/depressurising for minor work. I top this up occasionally usually after minor work. I've had two leaks in the 13 years, a failed pressure switch P****** water over the electrics, and the auto air bleed in the boiler leaked, so the 5L per year is a maximum guess. I'm not going to rip anything out unless I get a bigger leak.
I'm trying that. The liquid coming out looked like lager (or re-cycled lager!) but possibly slightly darker, certainly no dark sludge. The nail is rust free overnight. How long do I leave it to show up problems?
You want someone else to 'explain' your theories? Figures.
Mr Cranium, will you tell us please? You appear to think you know.
He may be competent about every other aspect of heating systems but I strongly disagree with his policy of not adding inhibitors; it must cause considerable long-term damage to his installations, but the warranty would be long expired before the damage was noticed. A ha'porth of tar. I wonder what the IoP (now IoP&HE) Guide says; I may have to look.
There is ceratinly an element of snake-oil marketing involved. No water treatment company will volunteer to tell you the ingredients of their solutions. I think you could make an effective home-brew inhibitor for less than =A31 a go, IF you had access to the chemicals. They charge an exorbitant amount for their test kits, so most people cannot just top-up as required, but tip in a whole bottle at =A315 a go, to be sure.
Excessive pH (>9) can damage copper pipes I believe, but I've never seen it happen.
You can do an interesting experiment about galvanic corrosion by connecting a multi-meter between a piece of copper tube and steel pipe, with their ends in a bucket of water. Plain mains water will cause a very small PD between the two pipes. Adding a few drops of anything acidic (lemon juice, vinegar) can increase the PD by a factor of 10. Adding anything alkaline (caustic soda, oven cleaner) will reduce the PD to 0.
Active/acidic flux wasn't widely used, probably until sometime around the '70s (?). Until then you had to use a passive flux and thoroughly clean the pipe and fittings before soldering. A couple of old-timers have told me that the raging corrosion, and the need for power flushing to shift corrosion products obstructing the pipes, is something that they rarely/never saw when they were youngsters. I suspect the active flux is a major cause.
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