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.