How to reduce corrosion in vented heating systems

[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.
[automerge]1578242962[/automerge]
Here we are.

 

[SJB060685]

Thanks John. Now all I need to do is delete the duplicates for these files
[automerge]1578243838[/automerge]
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

 

[John.g]

Try this one, rem you can just change the values in all the yellow boxes to make your own actual rad but only change the "Manuf" rated output to either ONLY 50(C) or 60 (C) depending on whether you are dealing with very old rads (60C) or the European now and for a good few years standard of 50C.
[automerge]1578247506[/automerge]

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

Is a PH measurement now available with the test kits for boiler water as it must be very important with the mix of materials now in mainly gas boilers ie SS or aluminium hxs.

 

[SJB060685]

John. Would this revised sheet work for calculating Delta 40 rads? I know delta 50 is UK standard but lower Delta rads work well on lower temperature systems ie. Heat pumps

 

[centralheatking]

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

Of course the mixture on systems is toxic, pressed steel radiators or even cast ones, copper pipes and
fittings of any heritage...mix in acidic flux mains water of dubious source and bluntly some but not all
mickey mouse chemicals. Anybody can and do mix up nice smelly water with WD 40 whatever and stick up for sale in fancy plastic containers... there was a case in East Lancs last year ...centralheatking

 

[John.g]

John. Would this revised sheet work for calculating Delta 40 rads? I know delta 50 is UK standard but lower Delta rads work well on lower temperature systems ie. Heat pumps

As you are well aware no doubt, HPs give the best COPs when they are run at the lower temps, so for example if you have a system with a rad flow temp of 40C and return of 35C (this would be best case IMO) and you require a room temp of 20C then you will have a 17.5 deg rad.....((40+35)/2 - 20)) based on your 50 deg rad this will give you 45.5%, (17.5/50)^1.3 output so you will have to select a 50 deg rad 219% size than if rated for 50 deg. or over twice the size, however its more than likely that the flow/return temps might realistically be 40/30C in which case you will have a 15 deg rad with a output of 39% so a rad requirement of 256% or over 2.5 times the 50 deg rad size.
I have never heard of 40 deg rads, it doesn't really matter what they are based on AS LONG AS the manuf gives you the output based on a particular temperature ie 50 or 60 or maybe even 40C, just be very aware which I am sure you are that they are NOT referring to rad temperature, they are referring and always were to the mean rad temperature MINUS the required room temperature, you then correct to your requirements.

 

[SJB060685]

Yes I am more than aware of what you say mate. I'm not an installer, quite frankly I couldn't care less about installing, I am however passionate about further my knowledge

 

[John.g]

Apoligies, I made a mess of the calculations above, I used a X instead of a ^ so the correct numbers are as below. I was wondering how the rads had such a high output with those very low flow/return temps, you can see that normally the existing sized rads just don't work very well if one installs a Heat Pump. Fan boosted heat emitters are far better if UFH cannot be installed.
One may get away with the existing rads if they have been oversized originally (as most were) and with the HP running 24/7.

Corrected Numbers.
If you have a system with a rad flow temp of 40C and return of 35C (this would be best case IMO) and you require a room temp of 20C then you will have a 17.5 deg rad...((40+35)/2 - 20)) based on your 50 deg rad this will give you 25.5%, (17.5/50)^1.3 output so you will have to select a 50 deg rad 392% size than if rated for 50 deg. or nearly four times the size, however its more than likely that the flow/return temps might realistically be 40/30C in which case you will have a 15 deg rad with a output of 20.5% so a rad requirement of 488% or nearly 5 times the 50 deg rad size.

 

[alistair42]

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.
[/QUOTE

I am confused. Before the start of the heating cycle (system cold, pump off), the water level in the Fill & Expansion tank is exactly the same as the water level in the vent pipe (why would it not be?).
At the end of the heating cycle - the sytem is hot and the pump is off. Again, the water level in the Fill & Expansion tank is exactly the same as the water level in the vent pipe. Only difference is that the water level is slightly higher in both tank & vent pipe than it was at the start of the heating cycle because the water has expanded because it is now hot.
Hence the majority of water exchange (in the ration of Xsection area of F&E tank to vent pipe Xsection area) due to expansion will be with the water in the F&E tank.
The only reason there can be a difference in the level in the vent & the F&E tank is due to the pump - and this will be off (eventually) before and after the heating cycle. So having a longer vent pipe will not minimise corrosion from the point of view of minimising water exchange with possibly oxegenated water in the F&E tank.
The only advantage I can see from a longer vent pipe is that there is less chance of the pump pushing water out of the vent back into the F&E tank, say when all the TRVs close. Seems to me that the only benefit of close coupling feed pipe into the vent is that it minimises the risk of pump over, or am I missing something?

 

[John.g]

Assuming you don't get pump over like I don't with a combined cold feed & vent there is still a advantage in having the F&E tank as high as possible above the highest circulation point (or have large dia vent&feed pipes) to avoid any "active" system water reaching the expansion tank and absorbing oxygen, see post #13 onwards. I had never thought of this before but it does make some sense I think?.

 

[alistair42]

Assuming you don't get pump over like I don't with a combined cold feed & vent there is still a advantage in having the F&E tank as high as possible above the highest circulation point (or have large dia vent&feed pipes) to avoid any "active" system water reaching the expansion tank and absorbing oxygen, see post #13 onwards. I had never thought of this before but it does make some sense I think?.

OK I think I get it - the length of pipework starting from its lowest point where the F&E pipe joins the circuit near the pump up to the point where the feed comes in from the F&E tank in the loft has a volume. If that volume is larger than the change in volume of the whole system from cold to hot, then the water from the F&E tank never gets drawn down into the circuit, so oxygenated water is never drawn in.