How to reduce corrosion in vented heating systems

[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.