How to reduce corrosion in vented heating systems

[siricosm]

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?

 

[John.g]

Because, IMO, almost entirely due to air ingress through the vent, pump over etc, a combined F&E is one of the best ways IMO of preventing this, I don't think the open F&E tank has much effect, my 40 year old system is absolutely spotless after > 40 years with only the very odd drop of inhibitor added, also, of course no leaks whatsoever, I keep the C&F tank (b/cock) feed isolated and open it a few times/year and have never seen as much as a drop of make up.

 

[siricosm]

Because, IMO, almost entirely due to air ingress through the vent, pump over etc, a combined F&E is one of the best ways IMO of preventing this, I don't think the open F&E tank has much effect, my 40 year old system is absolutely spotless after > 40 years with only the very odd drop of inhibitor added, also, of course no leaks whatsoever, I keep the C&F tank feed isolated and open it a few times/year and have never seen as much as a drop of make up.

The expansion of the water will push/pull water to the F&E tank with each heating cycle. This will depend on the difference between the hot/cold temperatures of the system and the total volume of water. With a 30C differential for a system with 100 litres of water, 30 x .0002 * 100 = .6 litres will be pushed/pulled from the F&E tank with each cycle. The water in the F&E tank will always have oxygen in it, and it doesn't really matter if it has a lid or not, .6 litres of air will be exchanged with each cycle, even with a lid.

May I ask how long, and what diameter is your F&E pipe? and a guesstimate for the total water volume?

 

[John.g]

The water will move up and down in the F&E tank due to expansion but only by a few ins and will act like a piston moving up and down IMO with the air cushioned by the water surface(s)
The expansion (3/4") is a continuation of the HW cylinder coil feed and extends ~ 3 or 4 meters above this and bends over into (above) the F&E tank, the feed is literally a few inches long, see attachment.
Total water volume ~ 75 litres.

 

[siricosm]

The water will move up and down in the F&E tank due to expansion but only by a few ins and will act like a piston moving up and down IMO with the air cushioned by the water surface(s)
The expansion (3/4") is a continuation of the HW cylinder coil feed and extends ~ 3 or 4 meters above this and bends over into (above) the F&E tank, the feed is literally a few inches long, see attachment.
Total water volume ~ 75 litres.

I was thinking that if the volume of water in the F&E pipe was larger then the volume of expansion, then no water that was ever in the F&E tank would enter the system, it would just go up and down in the F&E pipe, as you say, like a piston. If the F&E pipe volume is too small, then it would pull water from the F&E tank into the system water in each cycle, and that would be bad.

 

[SJB060685]

As above by these guys, the level in the F & E will rise and fall respectively to waters thermal expansion coefficient, this is where water expands at different ratios depending on the temperature. Typically speaking when water is heated to these system temperatures it will expand roughly 1.5% of original value. As above there is always microscopic bubbles of air in water which wont all be eradicated and a certain amount of sludge will happen over time

 

[siricosm]

As above by these guys, the level in the F & E will rise and fall respectively to waters thermal expansion coefficient, this is where water expands at different ratios depending on the temperature. Typically speaking when water is heated to these system temperatures it will expand roughly 1.5% of original value. As above there is always microscopic bubbles of air in water which wont all be eradicated and a certain amount of sludge will happen over time

I think the point here is that if the volume of expansion exceeds the volume of the F&E pipe, then oxygenated water enters the system in each heating cycle. If this is the case, then it is a design flaw.

 

[John.g]

Well, the proof of the pudding is in the eating, sorry, I mean the drinking. I am now going to make a hot toddy (with Paddy whiskey of course) using this glass of water drawn just now from the bottom of one of my 40 year old rads...Slainte.
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I think the point here is that if the volume of expansion exceeds the volume of the F&E pipe, then oxygenated water enters the system in each heating cycle. If this is the case, then it is a design flaw.

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I think the point here is that if the volume of expansion exceeds the volume of the F&E pipe, then oxygenated water enters the system in each heating cycle. If this is the case, then it is a design flaw.

The water in both the C&F and the vent pipe should in a perfect world act as a U tube and as the area of the C&F is ~ 100 times at least greater than the vent pipe area any imbalance can cause huge movement in the vent, thats why the close coupled system says that there should not be > 150mm distance between them and its why the combined F&E is so successful as there is practically ZERO distance between them.

 

[siricosm]

The water in both the C&F and the vent pipe should in a perfect world act as a U tube and as the area of the C&F is ~ 100 times at least greater than the vent pipe area any imbalance can cause huge movement in the vent, thats why the close coupled system says that there should not be > 150mm distance between them and its why the combined F&E is so successful as there is practically ZERO distance between them.

I was only considering thermal expansion, if there is a problem in this area, it would only make it worse.

 

[SJB060685]

I think the point here is that if the volume of expansion exceeds the volume of the F&E pipe, then oxygenated water enters the system in each heating cycle. If this is the case, then it is a design flaw.

I am not with you. The height of header tank must be a minimum of pump head devided by three above the pump. The F and E pipe will always be full of water, with about 2 inches in bottom of header tank, that 15mm pipe is what pushes and pulls the expanding and contracting water. Do you not mean the vent?

 

[siricosm]

I am not with you. The height of header tank must be a minimum of pump head devided by three above the pump. The F and E pipe will always be full of water, with about 2 inches in bottom of header tank, that 15mm pipe is what pushes and pulls the expanding and contracting water. Do you not mean the vent?

Assuming the vent is not discharging with each cycle, we can forget about it, water just sits in it at the tank level. As the water expands, it pushes up into the F&E tank, mixing with the oxygenated water there. As the system cools the oxygenated water gets pulled down the F&E pipe. If the pipe volume is less than the total expansion, then oxygenated water gets pulled into the system water, causing corrosion.

Hypothesis: Sludgy systems have short F&E pipes, and clean system have longer ones, compared to system volume.

 

[SJB060685]

Assuming the vent is not discharging with each cycle, we can forget about it, water just sits in it at the tank level. As the water expands, it pushes up into the F&E tank, mixing with the oxygenated water there. As the system cools the oxygenated water gets pulled down the F&E pipe. If the pipe volume is less than the total expansion, then oxygenated water gets pulled into the system water, causing corrosion.

Hypothesis: Sludgy systems have short F&E pipes, and clean system have longer ones, compared to system volume.

Ah I get what you're saying now

 

[John.g]

Assuming the vent is not discharging with each cycle, we can forget about it, water just sits in it at the tank level. As the water expands, it pushes up into the F&E tank, mixing with the oxygenated water there. As the system cools the oxygenated water gets pulled down the F&E pipe. If the pipe volume is less than the total expansion, then oxygenated water gets pulled into the system water, causing corrosion.

Hypothesis: Sludgy systems have short F&E pipes, and clean system have longer ones, compared to system volume.

Very very interesting hypothesis indeed: Ok I know you mention 0.6 litre expansion somewhere but I reckon that maybe 2.0 litre may be more realistic, if so, then a 3/4 ins (19MM ID) pipe will accomodate that 2 litre in a 2M length of (3/4") pipe which in a lot of cases means that the hot water will not reach the C+F tank (assuming HW cylinder coil as the benchboard). So, just maybe, that that's why the combined (3/4") F&E results in excellent "results" v/vis the more conventional 1/2" (12.7mm ID) cold feed (+separate vent) which will/would need 7M to accomodate that same 2 litres of water, obviously this will result in mixing with the "oxygenated water in the header tank".

 

[SJB060685]

I did some maths earlier John and I agree with what you say. It's an Interesting hypothesis but there are a few variables like you say, ie. system volume, final temperature and size and length of the f and e. Example. System with 60 litres and average system temp of 70°c will be roughly 1.3 litres extra volume, a 2m high 15mm f and e contains 0.29 litres, so that .29 will rise into tank plus roughly another 720ml of system water so this will mix with any oxygenated water. Yes water is a solvent and air will dissolve in it but not before some black iron oxide can form, this obviously happens every boiler cycle.

 

[John.g]

There must be plenty of plumbers out there with experience of maintaining both sealed and gravity CH systems, it would be interesting to hear their views/opinions on which system has performed better over say the last 10 years.

 

[siricosm]

Very very interesting hypothesis indeed: Ok I know you mention 0.6 litre expansion somewhere but I reckon that maybe 2.0 litre may be more realistic, if so, then a 3/4 ins (19MM ID) pipe will accomodate that 2 litre in a 2M length of (3/4") pipe which in a lot of cases means that the hot water will not reach the C+F tank (assuming HW cylinder coil as the benchboard). So, just maybe, that that's why the combined (3/4") F&E results in excellent "results" v/vis the more conventional 1/2" (12.7mm ID) cold feed (+separate vent) which will/would need 7M to accomodate that same 2 litres of water, obviously this will result in mixing with the "oxygenated water in the header tank".

I was basing this on 100 litres total system water x 30C differential x .0002 thermal expansion of water = .6 litres of expansion. I figured 20C cold and 50C average hot for the temperature differential.

At 19mm internal diameter, there would be about .2835 litres per meter of 22mm pipe, so .6 litres would be around 2.1 meters. You would probable want at least one .5-1.0m extra to prevent mixing of the F&E water with the system water.

It is interesting that this happens to be in the right range where one could envision mixing if the pump is in an airing cupboard under the tank, and no mixing if the pump was more remotely located nearer to the boiler. It also confirms the idea that a combined F&E at 2.5m x 22mm that holds .7 litres (no mixing) is better than a a 15mm pipe which would only hold (.1327 x 2.5) = .33 litres which would have significant mixing for the same distance.
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There must be plenty of plumbers out there with experience of maintaining both sealed and gravity CH systems, it would be interesting to hear their views/opinions on which system has performed better over say the last 10 years.

Indeed, I would be very much interested in hearing about any field experience others may have, and whether the volume of the F&E pipe is a design consideration in vented systems.

 

[Chuck]

People discussing this topic will find this paper:

P. Munn, Corrosion Science (1993) Vol. 35, Nos 5-8, pp. 1495-1501:

http://www.ewta.eu/assets/Uploads/corrosion-inhibitors-en.pdf

interesting. The ingress of oxygen via the f/e tank driven by expansion/contraction is referred to and a couple of other paths are also given. The author doesn't cite a source that quantifies this mechanism, which is rather frustrating but suggests that at the time the article was written it was 'well-known in the profession'.

 

[siricosm]

People discussing this topic will find this paper:

P. Munn, Corrosion Science (1993) Vol. 35, Nos 5-8, pp. 1495-1501:

http://www.ewta.eu/assets/Uploads/corrosion-inhibitors-en.pdf

interesting. The ingress of oxygen via the f/e tank driven by expansion/contraction is referred to and a couple of other paths are also given. The author doesn't cite a source that quantifies this mechanism, which is rather frustrating but suggests that at the time the article was written it was 'well-known in the profession'.

Very interesting indeed. And you are correct, they write this statement without reference as though it was common knowledge: "Oxygen ingress occurs, in open-vented systems, through the feed and expansion tank, especially as the water expands and contracts during the heating cycle."

I am unable to find any references exploring the effect of a F&E pipe with a volume less than the expansion itself, which would lead to direct mixing of F&E tank and system water. If it matters for system life, you would think someone would have written about it.
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In the paper below they discuss the location of the pump. They reference this paper in that context, but I don't have access to it: https://doi.org/10.1179/000705971798324152

 

[John.g]

I was basing this on 100 litres total system water x 30C differential x .0002 thermal expansion of water = .6 litres of expansion. I figured 20C cold and 50C average hot for the temperature differential.

At 19mm internal diameter, there would be about .2835 litres per meter of 22mm pipe, so .6 litres would be around 2.1 meters. You would probable want at least one .5-1.0m extra to prevent mixing of the F&E water with the system water.

It is interesting that this happens to be in the right range where one could envision mixing if the pump is in an airing cupboard under the tank, and no mixing if the pump was more remotely located nearer to the boiler. It also confirms the idea that a combined F&E at 2.5m x 22mm that holds .7 litres (no mixing) is better than a a 15mm pipe which would only hold (.1327 x 2.5) = .33 litres which would have significant mixing for the same distance.
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Indeed, I would be very much interested in hearing about any field experience others may have, and whether the volume of the F&E pipe is a design consideration in vented systems.

I have been rethinking all this again and am getting a bit confused now because no matter what the expansion volume is, does this not have to be accommodated in the F&E tank?? (or 99% of it anyway due to huge differences in the surface areas of the tank and exp pipe and the U tube effect) so whether your expansion is 0.6 litres or 2 litres or whatever then almost all this water is moving in/out of the tank during the cold/hot cycling.

OR are we saying that the vent/feed above the highest point in the system is effectively a dead leg, if so then this is how my system works.

I redid my calcs, my system runs with a average water temp of 67.5C (75+60)/2, so the expansion based on my 75 litre is 1.5 litres, as above, it will require, 1.5/0.285, 5.3 M to accomodate this in a 3/4" pipe so yes, even my system should be flowing ~ 0.5/1.0 litres OF SYSTEM WATER??? in/out of the F&E tank based on the above and taking vent/feed (combined F&E in my case) distance as 3M from the cylinder coil entry to the bottom of the F&E tank, I would need, theoretically anyway to raise my F&E tank by another ~ 2/2.5 M ??

 

[siricosm]

I have been rethinking all this again and am getting a bit confused now because no matter what the expansion volume is, does this not have to be accommodated in the F&E tank?? (or 99% of it anyway due to huge differences in the surface areas of the tank and exp pipe and the U tube effect) so whether your expansion is 0.6 litres or 2 litres or whatever then almost all this water is moving in/out of the tank during the cold/hot cycling.

Yes the water has to be accommodated in the tank, if it is .6 litres total expansion, then .6 litres will move in/out of the tank with each cycle.

Now if the F&E pipe volume is less than .6 litres, then water which has been in the tank will be drawn in to mix with the system water when it cools. If the pipe volume is larger than .6 litres, then it just goes up and down in the pipe, and should mix much less with the system water.

 

[SJB060685]

Assuming we're all on the right path we have to remember that whatever the total expansion is effectively only have will rise up into the header tank and half the vent as we know the level in both is equal. Like John said he would have to raise his tank roughly another 5 metres to avoid this problem. The average system has a f and e pipe roughly 2 metres which when 15mm holds 0.29 litres, so with a system volume of 100 litres that's roughly 2 litres of added volume. Even with a 22mm f and e (which is rare in the average domestic systems) that only accommodates .64 litres, so there will always be some system water rising into the tank, especially when you consider that the hotter less dense water will rise to the top and the cooler more dense will fall. We're all bang on with the maths, I just hope we're not missing something.

 

[siricosm]

Assuming we're all on the right path we have to remember that whatever the total expansion is effectively only have will rise up into the header tank and half the vent as we know the level in both is equal. Like John said he would have to raise his tank roughly another 5 metres to avoid this problem. The average system has a f and e pipe roughly 2 metres which when 15mm holds 0.29 litres, so with a system volume of 100 litres that's roughly 2 litres of added volume. Even with a 22mm f and e (which is rare in the average domestic systems) that only accommodates .64 litres, so there will always be some system water rising into the tank, especially when you consider that the hotter less dense water will rise to the top and the cooler more dense will fall. We're all bang on with the maths, I just hope we're not missing something.

If my calculations are correct, 100 litres of water would only expand .6 litres with a 30C temperature rise. I was thinking 20C cold, and 50C on average hot. If the rads were much warmer than that, you would be burning yourself on them. I wonder how much water is in your average system, maybe 100 litres is too much?

If the vent pipe is separate from the F&E, one could run the vent directly, and put an anti thermosiphon hook in the F&E pipe, also increasing its length.

 

[John.g]

So again, if I accept that there is some F&E tank water entering my system at least some of the time (ie when the u/stairs rads are on as well as d/stairs) then why is my system in such a clean condition after > 40 years?. I installed a bigger hall rad ~ 10 years ago and the 30 year old one had no sludge in it, only slight discolouration of the water in the bottom.

 

[siricosm]

So again, if I accept that there is some F&E tank water entering my system at least some of the time (ie when the u/stairs rads are on as well as d/stairs) then why is my system in such a clean condition after > 40 years?. I installed a bigger hall rad ~ 10 years ago and the 30 year old one had no sludge in it, only slight discolouration of the water in the bottom.

Didn't you say your expansion pipe was 3-4m long in 3/4?

 

[John.g]

If my calculations are correct, 100 litres of water would only expand .6 litres with a 30C temperature rise. I was thinking 20C cold, and 50C on average hot. If the rads were much warmer than that, you would be burning yourself on them. I wonder how much water is in your average system, maybe 100 litres is too much?

If the vent pipe is separate from the F&E, one could run the vent directly, and put an anti thermosiphon hook in the F&E pipe, also increasing its length.

Semi sealed with a E.vessel would be my solution if I was unduly worried about my present system.
Re your water heating from 20C to 50c, my calcs would show a 1.03% increase in vol, (density of water at 20c=998.253 and at 50C=988.079) so 100 litres will expand by 1.03 litres but if my 75 litres (with oil boiler) is typical then my expansion would only be ~ 0.8 litres under the above conditions.

 

[SJB060685]

So again, if I accept that there is some F&E tank water entering my system at least some of the time (ie when the u/stairs rads are on as well as d/stairs) then why is my system in such a clean condition after > 40 years?. I installed a bigger hall rad ~ 10 years ago and the 30 year old one had no sludge in it, only slight discolouration of the water in the bottom.

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.

 

[siricosm]

Semi sealed with a E.vessel would be my solution if I was unduly worried about my present system.
Re your water heating from 20C to 50c, my calcs would show a 1.03% increase in vol, (density of water at 20c=998.253 and at 50C=988.079) so 100 litres will expand by 1.03 litres but if my 75 litres (with oil boiler) is typical then my expansion would only be ~ 0.8 litres under the above conditions.

I think you are correct, it should be around 1% for 30C as you write.

 

[John.g]

Didn't you say your expansion pipe was 3-4m long in 3/4?

Yes, I did but it takes, by my calcs, 5.3 m to accommodate my 1.5 litres of exp water and I have "only" say 3.5 M from the HW cylinder to the bottom of the C&F tank so I fall short by ~ 2m which means I am expanding ~ 0.6 litres of system water?. at least some of the time but definitely borderline so maybe that is why my system is in good condition.

 

[SJB060685]

Yes, I did but it takes, by my calcs, 5.3 m to accommodate my 1.5 litres of exp water and I have "only" say 3.5 M from the HW cylinder to the bottom of the C&F tank so I fall short by ~ 2m which means I am expanding ~ 0.6 litres of system water?. at least some of the time but definitely borderline so maybe that is why my system is in good condition.

Yes John the higher the f and e pipe and or the size will affect this. Assuming I'm getting it right

 

[siricosm]

Yes, I did but it takes, by my calcs, 5.3 m to accommodate my 1.5 litres of exp water and I have "only" say 3.5 M from the HW cylinder to the bottom of the C&F tank so I fall short by ~ 2m which means I am expanding ~ 0.6 litres of system water?. at least some of the time but definitely borderline so maybe that is why my system is in good condition.

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.

 

[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.