How to reduce corrosion in vented heating systems

[SJB060685]

He has a combined f and e and vent in 22 though mate?

Scratch that I thought you said 15mm lol

 

[John.g]

If you read siricosm article link then it suggests it takes roughly 75 days for the oxygen saturated water content to dissolve the oxygen, perhaps your layout is helping things? Have you been regularly dosing your system to protect it? I imagine someone as well informed as you has done a lot to maintain the system quality.

I throw the odd drop in via a rad now and then (but never have tested it) I'm really not convinced of the efficacy of these additives which themselves will form a type of sludge when they combine with the dissolved oxygen, I spent over 30 years maintaining high pressure (45 bar) 52 MW boilers and while we had deaerators to remove the bulk of the dissolved oxygen we had to add a oxygen scavenger to keep the levels as close to 0 as possible but also had to keep a very small constant blowdown to stop sludge forming in the water drum, you would always get a bucket or two of sludge on opening up the boilers every 16 months or so.
Also there are dozens of systems here around me exactly like my own (combined Feed&Vent) and I have changed a few TRVs on my neighbours over the years who have never added anything to their systems, most still have some of their original rads.
I am very particular about any, even tiny leaks in my system and that is why I have a isolating valve on the b/cock make up.
But having said all that is most certainly a very good practice to keep additives in your system.

 

[SJB060685]

That's some big systems you worked on mate. Personally I'd like to get experience with commercial and industrial oil boilers

 

[John.g]

At .2835 litres per meter for your 19mm ID pipe, your 3.5m gives you around a litre, so comfortably over your .75 litre expansion for your 75 litre system. Unless I missed something.

I keep my flow/return at 75C/60c so my expansion is ~ 1.5 litres for 75 litre water content, not 0.75 litre. (see post#28, above)
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That's some big systems you worked on mate. Personally I'd like to get experience with commercial and industrial oil boilers

Yes, very interesting indeed, they could burn (heavy fuel) oil or (normally) natural gas or natural gas + bio gas which we generated as a result of a manufacturing fermentation process and it supplied between 10 and 15% of our energy needs. We also generated our own power (5MWe) with a aero derivative gas turbine generator which exhausted into another 55 MW boiler fired on nat gas only. happy days indeed.

 

[siricosm]

I keep my flow/return at 75C/60c so my expansion is ~ 1.5 litres for 75 litre water content, not 0.75 litre. (see post#28, above)

You are using 70C vs 20C to get 2% (1.5 litres), correct? Perhaps it is not often that the average temperature of your entire system reaches 70C? All the rads would have to be very hot. I was guessing at an average of 50C for all the water in the system, but maybe it should be higher. At my place not all the rads get super hot, only the ones that are a bit undersized for the room they are in, or in the kids rooms when they crank the TRVs. I also have my boiler set to max temp, but it will cut out before the room stat cuts demand, probably because the cylinder return and/or bypass increases the return temperature.

And FWIW my system has an F&E pipe of around 1.7m of 22mm (or 3/4?) in an airing cupboard upstairs just below the F&E tank. It is a 3 bed detached house with 11 radiators, one of which has rusted through. We just bought the house, so I have no idea how it was maintained, I'm guessing it wasn't.

 

[John.g]

Yes, I am using 70C (67.5) vs 20C to get 2% (1.5 litres), my (oil) boiler has its set point set to 75C and its return is ~ 60C.

I keep my boiler at the above temps as I need my rads to run at their rated output, all modern rads are based on a "50 deg" rating which is the (mean rad temperature) - the required room temperature (often taken as 20C). Older rads, maybe your ones, are based on a "60 deg" rating.
So, in my case, the mean rad temp is (75+60)/2 or 67.5C, (67.5-20) = 47.5C so my rads should produce 93.5% of their 50 deg rating. You can get correction factor tables but I find it far easier by just using excel, the rad output is the (present rating/the "rated" rating)^1.3
in my case above this is (47.5/50)^1.3 or 93.5%. In your case if the rads are 60 deg rated then the output based on a 50C mean rad temperature is only 40.6% (30/60)^1.3 OR if 50 deg rated, 51.4% (30/50)^1.3. That will make your boiler cycle on/off more frequently.
Oil boilers (which cannot modulate) will cycle throughout their life but gas boilers can modulate (and prefer it).

 

[SJB060685]

Yes, I am using 70C (67.5) vs 20C to get 2% (1.5 litres), my (oil) boiler has its set point set to 75C and its return is ~ 60C.

I keep my boiler at the above temps as I need my rads to run at their rated output, all modern rads are based on a "50 deg" rating which is the (mean rad temperature) - the required room temperature (often taken as 20C). Older rads, maybe your ones, are based on a "60 deg" rating.
So, in my case, the mean rad temp is (75+60)/2 or 67.5C, (67.5-20) = 47.5C so my rads should produce 93.5% of their 50 deg rating. You can get correction factor tables but I find it far easier by just using excel, the rad output is the (present rating/the "rated" rating)^1.3
in my case above this is (47.5/50)^1.3 or 93.5%. In your case if the rads are 60 deg rated then the output based on a 50C mean rad temperature is only 40.6% (30/60)^1.3 OR if 50 deg rated, 51.4% (30/50)^1.3. That will make your boiler cycle on/off more frequently.
Oil boilers (which cannot modulate) will cycle throughout their life but gas boilers can modulate (and prefer it).

Do you have a link for this excel sheet as well John?

Also non domestic oil boilers do modulate and its achieved by a few ways but domestic are a fixed rate pump and don't modulate, just thought I'd add that in.

 

[centralheatking]

Some vented heating systems seem to sludge up, and yet other remain remain remarkably clean after many years service.

Why?

Corrosion is caused by oxygen. Eliminating leaks and inhibitor helps, but the F&E tank is open to the air, so oxygen can freely get in there. Is it possible to design a system that minimizes oxygen entry from the F&E tank?

This was never a problem until recently, the water pathways thro older cast iron boilers were wider than the M6 ...you could stick your main digit in. Now the water pathways are less than a rollup ciggy
paper...of course the hex will block. So always treat the system and check each year. Open vented needs the pump speed balanced to avoid pump over ....the colour of the water is crucial
Black just needs treatment, Brown naughty as there is a constant introduction of air etc.
I know ...my company were very involved in this stuff years ago with Sentinel and others
...CHKing

 

[John.g]

I don't have one right now but will add another sheet to the one for expansion above and post it shortly.
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Here we are.

 

[SJB060685]

Thanks John. Now all I need to do is delete the duplicates for these files
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I don't suppose you have sheets for Delta 60 and 40 as well John? So much easier using your sheets than doing the math manually lol