How to reduce corrosion in vented heating systems

[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.