Short cycling until flow temperature reached - error 2967

[Ccw71]

In the previous post I stated it was running in CC. I just checked the manual, and realise it was running it on CP3. The photo is correct and Ive edited the previous post to avoid any further confusion. Sorry!

 

[Johntheo5]

The photo is showing CP3, if it was on any of the CC curves, either I,ii or iii would be lit on the left.
I don't think much has changed since last year, you were getting a florate of about 10LPM @ a 2M head on PP2 which = 15LPM @ a 4.5M head, so maybe no restrictions really, just change back briefly to PP2 and
post the power watts.

 

[Ccw71]

Thanks so much for all the help. I need to leave the house now to go look after my elderly mother, but I'll be back late tomorrow to continue my schooling

 

[Johntheo5]

Yes, I really don't think you have a flow problem as PP2 readings now arn't much different to that of a year ago and your boiler had no problem with a 10/11LPM flowrate at a @2M head?
I would suggest setting it to CP2 which at 17W (just now) is producing a flowrate of 0.83m3/hr,13.8LPM @ a 3M head and shouldn't present problems to drawing air etc through the vent, so may have to look elsewhere for your problems.


Re your problem with air ingress/vent problems, just put "combined feed and vent" into the search box, above, and you will see some interesting comments, you can start a new thread when the above is sorted out as I am almost 100% certain that my form of combined cold feed and vent (50 years old) will solve it.

 

[Johntheo5]

You might just get away without air ingress, if required, by running in PP3 mode as the head will be 3.9M but will give a flow of 15.7LPM.

Original setting, one year ago, was PP2 which gave a flow of 10/11LPM, Present flow is very close, at 11.2LPM so unlikely to be restricted flow caused by chemical clean?.

 

[JonathanM]

The challenge with all gas boilers, no matter how good the modulation is, is that they fire up with ignitions conditions set to 60 to 70% of max output, some then take 60 secs or so, (Vaillant are "notorious" for this) to modulate down, so requires a reasonably high flow rate to get away without exceeding the SP flowtemp+5C and burner trip + recycle. Range rating will not range rate the ignition settings of 60/70%. What can also help is to have a decent anticycle time as the circ pump remains running and will get the boiler temp well down if the anticycle time is, say, 5 minutes.

I've been thinking about this issue quite a bit recently, because I've come across this issue in a couple of threads, and it might throw a spanner in the works for my plans for a new boiler to run at a very low temperature. It seems common for boilers to cut off at SP+5 and come back on at SP-5. Presumably to then get back to SP+5 takes a 10C rise. I have been quoted for a 24KW regular boiler and 70% of 24KW is 16.8KW. So a burn for 60 seconds at this rate would output 0.28KW. I then worked backwards, and it looks to me like that the volume of water that 0.28 KW will heat through 10C is 24 litres. So does that mean, with a 60 second minimum burn, you would need a flow rate of 24 litres per minute to avoid it tripping again? That seems a bit mad, but I've checked my workings.

 

[Johntheo5]

Pretty spot on but the boiler will not fire up at SP-5C if the anticycle time is set "properly", some like vokera have a 3 min anticycle time, it is user adjustable on others so the boiler temp can/will be way lower than this as the circ pump continues to run so the flowrate can be somewhat lower Again, boilers like Vokera, or at least the one, a Vision 20S, that I regularly observe when it that house, appears to begin modulation within seconds of firing up and doesn't seem to have any particular problems in continuously cycling even with a rad demand of 3kw, its min turndown is around 5.5kw I think (20kw max).

 

[JonathanM]

Pretty spot on but the boiler will not fire up at SP-5C if the anticycle time is set "properly", some like vokera have a 3 min anticycle time, it is user adjustable on others so the boiler temp can/will be way lower than this as the circ pump continues to run so the flowrate can be somewhat lower Again, boilers like Vokera, or at least the one, a Vision 20S, that I regularly observe when it that house, appears to begin modulation within seconds of firing up and doesn't seem to have any particular problems in continuously cycling even with a rad demand of 3kw, its min turndown is around 5.5kw I think (20kw max).

It will definitely be a major factor to take account of when I choose my new boiler. Does the Vaillant you mention have an option to change these settings? I know Worcester Bosch does.

 

[Johntheo5]

The Vaillant does but it appears to have the most problems in cycling mode, possibly because of this 60 sec ignition settings "hold", this may now have been changed, I don't know as I'm not or never was in the domestic gas/oil fired boiler business but spent decades (over 3) looking after a Utilities with 3 x 50MW high pressure steam boilers that could be fired on Nat Gas only or Nat Gas+Bio Gas or, in a emergency, heavy fuel oil, so have a life long interest in all sizes of boilers.

 

[JonathanM]

One of my great uncles had a life long interest in the same, and wrote a book about a revolutionary steam boiler. Wish I could remember its name now! Some boilers like Ideal and Vokera seem to have very few settings you can change. Worcester Bosch seem to have zillions; it looks like you can set a both a minimum -SP temperature and a minimum anti-cycle time. But you can't run weather compensation natively on WB heat only boiler, so that's what's putting me off at the moment. I haven't found the boiler that has everything yet.

 

[Ccw71]

Yes, I really don't think you have a flow problem as PP2 readings now arn't much different to that of a year ago and your boiler had no problem with a 10/11LPM flowrate at a @2M head?
I would suggest setting it to CP2 which at 17W (just now) is producing a flowrate of 0.83m3/hr,13.8LPM @ a 3M head and shouldn't present problems to drawing air etc through the vent, so may have to look elsewhere for your problems.


Re your problem with air ingress/vent problems, just put "combined feed and vent" into the search box, above, and you will see some interesting comments, you can start a new thread when the above is sorted out as I am almost 100% certain that my form of combined cold feed and vent (50 years old) will solve it.

Hi, well I'm back at the house, full of hope that the problem will have gone all by itself, but alas no.
So from your calculations you'd say there in no restriction. Does this apply to the HX also, or could there still be a partial blockage there? Is it still a good idea to attempt to back flush it or would this just be a waste of time?

You also talked about the flow rate at a 2M and a 3M head. Is there anything I can measure / tell you to allow you to confirm this figure or is it something that comes from your calculations?

I've also noticed that rating the boiler down to 50% seems to make no difference as it sounds like the boiler is starting at a very low rate and begins to gradually ramp up it's output, but as it only runs for 2 to 3 minutes, it never reaches the set maximum output.

Any other thoughts as to what may be causing the problem? Are there any settings in the menu that may have got screwed up that could cause this kind of operation?

Thanks.

 

[Johntheo5]

Did you get your thermal gun battery (or was that on another forum?) and monitor the flow and return at the boiler?.

Re circ flow, except the pump has developed some fault or other which is most unlikely, then yes I think your circ rate is the same now as it was before that acid clean, there could be a bit of crap holding the internal bypass open (if the boiler has one installed as supplied) now after the clean, monitoring that flow & return will give a few clues.

"CP2 which at 17W (just now) is producing a flowrate of 0.83m3/hr,13.8LPM @ a 3M" that is the actual flowrate which should just about, IMO, allow the boiler to get away.
"You might just get away without air ingress, if required, by running in PP3 mode as the head will be 3.9M but will give a flow of 15.7LPM." Have you tried this setting??.

 

[Ccw71]

After letting it run for some time at a steady flow of 64.5 (stated on the boiler display) the return temp (possibly not that accurate as it's a cheap aliexpress sensor, but in the ball park) was 48.5. This was when testing it running on CP3 last Thursday?

I've not given PP3 a go yet.

 

[Johntheo5]

Last hursdays numbers added up CP3 @ 28W = 16.33LPM. Boiler dT, (64.5-48.8), 15.7C. By calculation boiler output is
16.33X60X15.7/860, 17.88kw, everything pretty good IMO. The Vaillant fires up at ~70% of max output (irrespective of range rating) so fires up at 40X70% = 28kw, by calculation, dT = 28X860/16.33/60, 24.6C, still OK as I think it may flag S53 at a dT > 30C.
Its now trying to fire up at a pump setting of CP2 which flowed 13.8LPM, so dT now is 28X860/13.88/60, 28.9C, very close to the 30C limit if this is one of S53 triggers which it probably is.
PP3 should give a flow of 15.7LPM with a dT of 28X860/15.7/60, 25.56C , might just get the boiler away, give it a try.

You havn't a Valliant!!! but try the above, anyway.

 

[Ccw71]

Thanks Johntheo5, cubs night tonight, so I'll give that a try tommorow and report back