UFH Help

I haven't noticed a major issue with short cycling to be honest, so far...

Just so I understand, boilers generally try to stick to a 20 degree delta t, so if you have it set to say 65, it will want the water coming back to be around 40/45? If its lower (35 or so) that would potentially cause it to short cycle as it has to work harder to heat the return back up, and can't modulate its heat output as well so ends up killing the burner on/off?

Thanks

Paul

Most boilers dont care what the dT is as long as it doesnt exceed 30C, most will then trip the burner and go into recycle so ideally try and have a dT of 20/25C, some, now, like the newer Vaillants can try and maintain a selectable dT up to 20C, otherwise its uncontrolled (by the boiler).
The boiler doesn't have to work any harder with a lower return temperature as the boiler demand is still the same, at 10kw in the above example, in fact it has to work a little less hard as the condensing effect is a few % better at the lower return temperature so less fuel required but from a output point of view there is no difference as 4.1*60*(65-30)/860 = 10kw, but so too does 7.17*60*(65-45)/860 = 10kw

Johntheo5 said:

It should also be remembered that when running UFH only with a gas fired boiler that a bypass is almost all ways required to prevent the burner tripping on a excessive dT and constant cycling even if its minimum output is < than the UFH demand.
You can see below with a boiler flow temp of 65C, a UFH flow/return temps of 40C/30C that the boiler would have a return temp of 30C with a dT of 65C-30C, 35C, unless some by pass is used to reduce the dT to say 20C.


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Thanks! So what would this by-pass typically consist of?

I ask, because I have all UFH on the ground floor and normal rads upstairs (on a Y Plan). This was installed by a heating engineer (not me) as part of a refurb 11 years ago - and I am not aware of any components that could be deemed to be a by-pass. Do you mean some sort of additional expansion tank?

Next time you are running on UFH only (ground floor) make a note of the boiler flow and return temperatures, d.40 & d.41, if the difference in these, the dT, exceeds 30C I think, but certainly if it exceeds 35C, then the (Vaillant) burner will trip and will not refire until the anticycle time has elapsed.
The by pass can be as simple as a piece of pipe and a valve (thats throttled) installed between the boiler flow and return or you may have a towel rad thats allways on even if the upstairs zone valve is closed as this (towel rad) will act as a bypass as well and keep the boiler flow/return dT below 30C or so.

Should also have said that all Vaillant system/combi boilers have a internal (automatic) by pass valve which is set very low, even with a all rad system this can lead to very high return temps despite someone spending a lot of time balancing/playing with the flow temperature to get the return down to say 40C only to have this internal by pass increasing it at the boiler by sometimes another 10C or so to say 50C.

Johntheo5 said:

Next time you are running on UFH only (ground floor) make a note of the boiler flow and return temperatures, d.40 & d.41, if the difference in these, the dT, exceeds 30C I think, but certainly if it exceeds 35C, then the (Vaillant) burner will trip and will not refire until the anticycle time has elapsed.
The by pass can be as simple as a piece of pipe and a valve (thats throttled) installed between the boiler flow and return or you may have a towel rad thats allways on even if the upstairs zone valve is closed as this (towel rad) will act as a bypass as well and keep the boiler flow/return dT below 30C or so.

Should also have said that all Vaillant system/combi boilers have a internal (automatic) by pass valve which is set very low, even with a all rad system this can lead to very high return temps despite someone spending a lot of time balancing/playing with the flow temperature to get the return down to say 40C only to have this internal by pass increasing it at the boiler by sometimes another 10C or so to say 50C.

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Oh thank you so much for this!

It's an EcoTec Plus 837 and yes, I have an upstairs Bathroom towel rail that is always on - even though, as you say, the zone valve is closed (only opens when the timer calls for upstairs heating in the evening.

Now, there is some weirdness - which will probably make you spit out your coffee.. All the settings below, work fine with the V-Smart turned OFF. The only thing that turning it on seems to do, is to make bonkers calls for Flow Target temp (D9).

D40 and D41 are between 5-10C apart - but neither go much over 50C, which figures because I have it set like this:

CH Target temp (physical dial on front): 60C (but note that the burner normally turns off at about 64-65C)
D0: 12
D1: 11 (in order, to pull as much residual heat out of the exchanger AFTER anti-cycling has kicked in)
D2: 10 (see D1)
D9: 86 (right now - I mean, WTF V-Smart - particularly as I programmed you for a UFH profile..)
D17: 0 (Yes, I know the advice - even from Vaillant - is that this should be 1 (return) but set at 1, the boiler hardly ever fires
D18: 0
D47: 999 (again, what is going on here? - when V-Smart is meant to be providing weather compensation)
D67: I have never seen it go much higher than 5 - so that indicates that D2 is set about right?
D19: 0 - Until about two weeks ago, this was 2, but all of a sudden, upstairs rads stopped heating up and I was getting a F22 and F73 - not hard errors though, this didn't stop the boiler, but were kind of soft errors sufficient to alert V-Smart and show on the error list. Apparently, the pump was spinning fine (and flow pressure still showed a 0.1 - 0.2 bar increase when heat was called), but just to be on the safe side, I had the pump start capacitor replaced. The errors went away, but the upstairs rads still wouldn't get hot though - so I whacked D19 up to 2 - and the upstairs rads are fine now - but I think I probably have just covered up the symptoms, rather than getting the underlying cause fixed.

creamola said:

Oh thank you so much for this!

It's an EcoTec Plus 837 and yes, I have an upstairs Bathroom towel rail that is always on - even though, as you say, the zone valve is closed (only opens when the timer calls for upstairs heating in the evening.

Now, there is some weirdness - which will probably make you spit out your coffee.. All the settings below, work fine with the V-Smart turned OFF. The only thing that turning it on seems to do, is to make bonkers calls for Flow Target temp (D9).

D40 and D41 are between 5-10C apart - but neither go much over 50C, which figures because I have it set like this:

CH Target temp (physical dial on front): 60C (but note that the burner normally turns off at about 64-65C)

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The target temp should change if WC in use and the OT (outside temperature) is reading correctly, see D.47 comment, but if the actual target temperature is 60C then the burner will trip when the actual flow temperature, D.40, reaches 65C which indicates that the heat demand is lower than the minimum output of the boiler

creamola said:

D0: 12

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Output is restricted to 12kw, only on CH.

creamola said:

D1: 11 (in order, to pull as much residual heat out of the exchanger AFTER anti-cycling has kicked in)

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D.1 is the pump overrun time and should only operate when CH stat is satisfied or (I think) the DHW demand.

creamola said:

D2: 10 (see D1)

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See table, below, this is the anticycle time, the actual anticycle time depends on the target flow temperature and this setting, if the target temp is say 60C and the burner trips (at 65C, like above) then the actual anticycle time is only 3.5 minutes, far too low IMO, I would set that to its default setting of 20 minutes.

creamola said:

D9: 86 (right now - I mean, WTF V-Smart - particularly as I programmed you for a UFH profile..)

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Thats the target temperature set remotely (Ebus) depending on the OT d.47 and because this is reading a crazy 999 then D.9 is looking for max target temperature.

creamola said:

D17: 0 (Yes, I know the advice - even from Vaillant - is that this should be 1 (return) but set at 1, the boiler hardly ever fires

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D.17 normally is set to its default setting of 0, Flow temperature control.

creamola said:

D18: 0

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this says Comfort or Eco mode, if settable, should be in comfort mode IMO

creamola said:

D47: 999 (again, what is going on here? - when V-Smart is meant to be providing weather compensation)

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D:47 You said the V smart is giving this crazy number?.

creamola said:

D67: I have never seen it go much higher than 5 - so that indicates that D2 is set about right?

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D.67 is the actual elapsed anticycle time so should read the (elapsed) time since the burner tripped and the anticycle time started.

creamola said:

D19: 0 - Until about two weeks ago, this was 2, but all of a sudden, upstairs rads stopped heating up and I was getting a F22 and F73 - not hard errors though, this didn't stop the boiler, but were kind of soft errors sufficient to alert V-Smart and show on the error list. Apparently, the pump was spinning fine (and flow pressure still showed a 0.1 - 0.2 bar increase when heat was called), but just to be on the safe side, I had the pump start capacitor replaced. The errors went away, but the upstairs rads still wouldn't get hot though - so I whacked D19 up to 2 - and the upstairs rads are fine now - but I think I probably have just covered up the symptoms, rather than getting the underlying cause fixed.

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D.19 says two stage pump, not sure what this means but setting 2 is probably the correct setting since the rads heat up?.

@Johntheo5 – my big apologies, I should have been clearer that I was the one that set these.

D0: 12 – yes, 12K otherwise I find that when the burner fires, it heats up faster than it can modulate back down – so I get a burst of about 15 secs and then nothing until..

D2: 10 – because if I set it at 20 (default), I get a burst of heat (see D0) and then zip for 20 mins – and when you are trying to heat 4 UFH zones at about 40C, well, it never heats up of course..

D1: yes – I have it at 11 – so as to be sure that it covers the entire anti-cycling lockout, and I still get (residual) heat from the exchanger otherwise, not only do I not get any heat (see D0 and D2) I don’t get any flow either LOL!

D9: yes – exactly, so what is the V-Smart even thinking of to set this at 86 – when it is a UFH (profile AND a CH target hard set from the dial of 60C). It simply makes no sense to me, that V-Smart should call for 86C – unless I am missing something? (If V-Smart is disconnected, this number is sensible in the 40-50 range).

D17: yes, that’s my point, it works fine when set to 0 (default) BUT both Vaillant and a number of folk in this forum say that for UFH, it should be set to 1 (return) which kind of makes sense to be testing the return flow (low) temps from a UFH circuit – except that it just doesn’t seem to work if I set it for return – i.e. the boiler hardly ever fires, like there is no call for heat (but when there actually is of course).

D18: on this boiler, 0 is Return, 1 is non-stop and 2 is winter – with 0 as default.

D47: always shows 999 (whether V-Smart is connected or not) so I guess that 999 means no weather compensation – except, that I would expect V-Smart to identify itself as weather compensation surely?

D67: agreed – and my point about it rarely going over 5, indicates that D2: set at 10 (rather than 20) is probably sufficient, because anti-cycling time is naturally less than 10 in any case.

D19: default is 2, which means that speed is dependent on part load in D0 and as D0 is below 60% of the full load, then pump speed is 1, otherwise speed 2) . Setting to 0, means that heating pump speed is permanently 2 – but why this has suddenly become a requirement (in order to get heat to the upstairs rads in the last two weeks) is beyond me…

creamola said:

@Johntheo5 – my big apologies, I should have been clearer that I was the one that set these.

D0: 12 – yes, 12K otherwise I find that when the burner fires, it heats up faster than it can modulate back down – so I get a burst of about 15 secs and then nothing until..

D2: 10 – because if I set it at 20 (default), I get a burst of heat (see D0) and then zip for 20 mins – and when you are trying to heat 4 UFH zones at about 40C, well, it never heats up of course..

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There's something not adding up here because a anticycle time of 20 minutes only means that the actual anticycle time is 20 minutes if the target flow temperature was a very unlikely 20C (see table), if the target temp was/is 60C then the anticycle time is only 6 minutes, its where the 20C vertically downwards intersects the 60C (target temp) going horizontally right, your 10C setting only gives a anticycle time of 3.5 minutes and there is no way that the boiler will get away in one firing after that short time as the flow/return temps will be far too high, its more than likely that the boiler is anticycling multiple times IMO, eventually, it will/may get away after 20 minutes or so of this constant multiple anticycling which is what you are seeing?.

creamola said:

D1: yes – I have it at 11 – so as to be sure that it covers the entire anti-cycling lockout, and I still get (residual) heat from the exchanger otherwise, not only do I not get any heat (see D0 and D2) I don’t get any flow either LOL!

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D.1 is the (pump) overrun time when the boiler is shut down or the CH roomstat is satisfied and the setting is in stone, ie a setting of 11 minutes means the pump will run for exactly 11 minutes but this has nothing to do with the anticycle settings,

creamola said:

D9: yes – exactly, so what is the V-Smart even thinking of to set this at 86 – when it is a UFH (profile AND a CH target hard set from the dial of 60C). It simply makes no sense to me, that V-Smart should call for 86C – unless I am missing something? (If V-Smart is disconnected, this number is sensible in the 40-50 range).

D17: yes, that’s my point, it works fine when set to 0 (default) BUT both Vaillant and a number of folk in this forum say that for UFH, it should be set to 1 (return) which kind of makes sense to be testing the return flow (low) temps from a UFH circuit – except that it just doesn’t seem to work if I set it for return – i.e. the boiler hardly ever fires, like there is no call for heat (but when there actually is of course).

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I have asked in numerous forums for anyone who uses this return setting and have had no takers.

creamola said:

D18: on this boiler, 0 is Return, 1 is non-stop and 2 is winter – with 0 as default.

D47: always shows 999 (whether V-Smart is connected or not) so I guess that 999 means no weather compensation – except, that I would expect V-Smart to identify itself as weather compensation surely?

D67: agreed – and my point about it rarely going over 5, indicates that D2: set at 10 (rather than 20) is probably sufficient, because anti-cycling time is naturally less than 10 in any case.

D19: default is 2, which means that speed is dependent on part load in D0 and as D0 is below 60% of the full load, then pump speed is 1, otherwise speed 2) . Setting to 0, means that heating pump speed is permanently 2 – but why this has suddenly become a requirement (in order to get heat to the upstairs rads in the last two weeks) is beyond me…

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Johntheo5 said:

There's something not adding up here because a anticycle time of 20 minutes only means that the actual anticycle time is 20 minutes if the target flow temperature was a very unlikely 20C (see table), if the target temp was/is 60C then the anticycle time is only 6 minutes, its where the 20C vertically downwards intersects the 60C (target temp) going horizontally right, your 10C setting only gives a anticycle time of 3.5 minutes and there is no way that the boiler will get away in one firing after that short time as the flow/return temps will be far too high, its more than likely that the boiler is anticycling multiple times IMO, eventually, it will/may get away after 20 minutes or so of this constant multiple anticycling which is what you are seeing?.

D.1 is the (pump) overrun time when the boiler is shut down or the CH roomstat is satisfied and the setting is in stone, ie a setting of 11 minutes means the pump will run for exactly 11 minutes but this has nothing to do with the anticycle settings,

I have asked in numerous forums for anyone who uses this return setting and have had no takers.

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@Johntheo5 thanks so much for this!

My apologies for not being sufficiently clear:

If I set D2 to default time of 20, you are correct, that according to the table, with a target of 60C, actual anti-cycling time will be 6. That's what I get in any case with D2 set to 10. HOWEVER, if I set D2 to 20, anti-cycling is rarely 6 (at a target 60C flow temp) - often as not, it is "nope, not firing ever".. Now while this seems bonkers, the firmware on this board has been reported as being freaky before: https://vaillantcyclingproblem.blogspot.com/2012/01/anti-cycling-mode-and-d2-parameter.html?m=1

Bottom line: With D2 set at 10, the result, is as if it is set at 20 (for my flow target temp of 60C).

Now, the only reason why I said my D1 was set at 11 and was connected to D2, is simply so that the pump continues running for the entire duration of anti-cycling (at around a max of 6 mins) just to ensure that when the boiler isn't firing, I am sucking as much residual heat out - and still pumping it around the UFH manifold as possible. Now, this might be a really stupid approach but if I leave this at default and D2 at 20 (default) then the resulting flow of warm water though the manifold is about 60 secs worth every once in a blue moon..

Thanks for checking for D17 being set to Return for UFH, similarly, I also can't find much mention of it now - I wonder if I am (incorrectly) recalling the Urban Plumber (youtube)?

That comment above where it says "pump is off and zone valve(s) are closed during the anticycle time" certainly IMO is not how it works, if the flow temperature exceeds the target temperature (example) by 5C then the burner stops firing but the pump stays running and the zone valves stay open, the burner will then refire when the flow temperature is at target temp -5C and the anticycle time has elapsed, the bit I'm not sure about is whether the anticycle time starts immediately the burner trips or whether it doesn't start until the temp is at
target temp -5C. I can understand weird things happen if the target temp changes due to weather compensation etc during a anticycle although one might say the correct logic should base the anticycle time on the time as determined by the target temperature at the start of the recycle period?

Johntheo5 said:

That comment above where it says "pump is off and zone valve(s) are closed during the anticycle time" certainly IMO is not how it works, if the flow temperature exceeds the target temperature (example) by 5C then the burner stops firing but the pump stays running and the zone valves stay open, the burner will then refire when the flow temperature is at target temp -5C and the anticycle time has elapsed, the bit I'm not sure about is whether the anticycle time starts immediately the burner trips or whether it doesn't start until the temp is at
target temp -5C. I can understand weird things happen if the target temp changes due to weather compensation etc during a anticycle although one might say the correct logic should base the anticycle time on the time as determined by the target temperature at the start of the recycle period?

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Ah, I see what you are saying - but that's not me saying that. it's the dude who wrote that blog LOL!

I agree with your analysis though, HOWEVER, while I can't comment on the zone valve closing during anti-cycling (simply because I have never experienced that) my pump DOES stop during anti-cycling if I leave D2 at the default 5. It seems like this is a total 5 mins - rather than target temp - 5C (or whatever) so in my case, while anti-cycling, if the D2 threshold is reached, then the pump stops - hence, why I set the time to be a minute more than D1 i.e. if heat is still being called for, the effective result, is that the pump will be always running.

One additional weird thing - which others might have experienced - is that even when the pump is running while the anti-cycling period expires - and hence the burner is about to re-fire - the boiler always stops the pump, and then restarts it when the burner fires - why?

Additionally, to clarify my Comments about V-Smart: it is SUPPOSED to be weather compensating AND capable of automatically controlling other ebus parameters - in order to make this more efficient. The blurb suggests, that this includes (but is not limited to) adjusting the ebus call for specific flow temperatures. i.e. Let's just say that there is a room AKA V-Smart set temp of 20C, but in 30 mins time, the set temp will be changed to 17C, rather than go all guns blazing and then, 30 mins later, completely stopping until room temp drops to 17C, it should gracefully adjust flow temps so as to not waste residual heat with a hard heating call switch-off. Similarly, it should do weather compensation - but even though it is programmed for the precise location, I see absolutely no evidence of that weather comp - and a D47 of 999 seems to indicate that "999" is default for "no weather compensation" as it shows 999 whether the V-Smart is connected or not. Frankly, the V-Smart is about as smart as a programmable room stat right now - with the exception that it does render thus in Home Assistant - which looks more impressive than it is..