My Viessman boiler journey

[marshian]

15 minutes ago, John Carroll said:

Even though the targettemp is remotely (weather compensated) set and presumably not jumping around, can/have you set the targettemp manually high enough to ensure the heat demand is slightly higher than the boiler's minimum output to prevent burner cut out and recycling and see does the boiler behave under these conditions, if not, then. IMO, a boiler control problem?.

Can you post any trends?

 

I tried that

 

 

 

 

Weather compensation off,

No Range Rating,

Running a Fixed Target Flow Temp of 27 Deg C.

9.7 Deg Outside Air Temp

00:00:00 to 00:01:10 Start up and modulate down (missed initial start) Flow temp 29.1

 

00:01:10 to 00:03:33 Boiler modulated to min flow temp 29.1

 

00:03:34 Boiler ramps up to 24.8% flow temp increases to 32.1

 

00:05:44 Boiler starts to ramp down to 20.4% flow temp drops from 31.6 to 30.9

 

00:07:59 Modulation drops to 15.5% and flow temp drops to 30.4

 

00:08:41 Showing that the boiler has not been range rated by increasing the flow temp to 43 Deg C - Boiler ramps up rapidly to 44.2%

 

00:09:37 Reducing the flow temp back to 27 showing the boiler modulate down to 10.6% flow temp drops to 29.7

 

00:11:00 to 00:18:03 Boiler running at min modulation flow temps gradually rise from 29.7 to 31.2 Deg C

 

00:18:08 Settings review

 

00:18:33 Boiler ramps up to 15.5% flow temp rises to 31.7

 

00:19:39 Boiler flow temp risen to 32.7

 

00:20:30 Modulates down to 10.8% flow temp drops to 32.2

 

00:23:53 Flow temp 31.7 Modulation 10.8%

 

00:28:18 Boiler shuts down

Edited Wednesday at 10:40 by marshian
formatting mistakes

[John Carroll]

If the heat demand is/was steady and if the boiler is not at minimum modulation then one might expect a very low target/flow temp dT of max 1 or 2C, I've often looked my daughter's Vokera and its almost allways spot on, biggest dT I've ever seen is 1C.  Also, when you were decreasing the targettemp back down to 27C, (from 43C) were you keeping the dT < 5C while adjusting, otherwise the burner should have tripped?.

[marshian]

34 minutes ago, John Carroll said:

If the heat demand is/was steady and if the boiler is not at minimum modulation then one might expect a very low target/flow temp dT of max 1 or 2C, I've often looked my daughter's Vokera and its almost allways spot on, biggest dT I've ever seen is 1C.  Also, when you were decreasing the targettemp back down to 27C, (from 43C) were you keeping the dT < 5C while adjusting, otherwise the burner should have tripped?.

 

No facility to adjust the DT at the boiler - in steady state running it's 5.9 Deg C (Difference between flow and return)

 

When I demonstrated that the boiler wasn't range rated the DT obviously widened because the return temp would not have changed whilst the flow temp ramped up

 

The target temp I changed was the boiler target flow temp so it wouldn't have tripped out on the way up - on the way down it's a possibility hence why it was a short burst up and quick reduce back down - the return temp being stable soon dragged the flow temp down to close to target once the boiler had modulated back down

 

I hope that makes sense

[marshian]

I'm sure I've stated this before but the window where this happens is between 5 deg C OAT and 10 Deg C OAT when running on Weather comp

 

I occasionally see it when there is a big swing in OAT outside that window but under normal circumstances below 5 Deg OAT and above 10 Deg C it's rock solid behave totally normally - Fire up - initial purge at 50% modulation, ramp down to min 10.6 - 10.8% and then steady state running until over temp by 5 Deg.

[John Carroll]

10 minutes ago, marshian said:

I'm sure I've stated this before but the window where this happens is between 5 deg C OAT and 10 Deg C OAT when running on Weather comp

 

I occasionally see it when there is a big swing in OAT outside that window but under normal circumstances below 5 Deg OAT and above 10 Deg C it's rock solid behave totally normally - Fire up - initial purge at 50% modulation, ramp down to min 10.6 - 10.8% and then steady state running until over temp by 5 Deg.

But if you have steady state running and the target/flow dT increases to 5C can only mean that the heat demand is lower than the boiler minimum output?

[marshian]

22 minutes ago, John Carroll said:

But if you have steady state running and the target/flow dT increases to 5C can only mean that the heat demand is lower than the boiler minimum output?

 

Maybe I'm not explaining myself very well

 

DT between Flow and Return at the boiler is 5.9 Deg C 

 

Initially before the start the DT is 0

When the boiler fires up the DT is slightly larger as it throws 58% of max at the circuit

Once it's modulated down to Min the DT stabilises and runs at 5.9 Deg C

 

The house circuit is ~130 Litres in total and the flow thro the circuit is being pumped at 0.6 m3/Hr (10 Litres/Min) so if the rads can't get rid of all the heat then there will be some rise in the return temp.

 

This causes the flow temp to increase because the boiler cannot modulate down any lower.

 

During the cycle the DT remains at 5.9 Deg C until the temp limit is reached when the boiler shuts down

 

The boiler doesn't measure the return temp - it measures just two things Flow temp (at the Hex and at the outlet - Why both I don't know but it does) and the temp of the Flue Gas

[John Carroll]

20 minutes ago, marshian said:

 

Maybe I'm not explaining myself very well

 

DT between Flow and Return at the boiler is 5.9 Deg C 

 

Initially before the start the DT is 0

When the boiler fires up the DT is slightly larger as it throws 58% of max at the circuit

Once it's modulated down to Min the DT stabilises and runs at 5.9 Deg C

 

The house circuit is ~130 Litres in total and the flow thro the circuit is being pumped at 0.6 m3/Hr (10 Litres/Min) so if the rads can't get rid of all the heat then there will be some rise in the return temp.

 

This causes the flow temp to increase because the boiler cannot modulate down any lower.

 

During the cycle the DT remains at 5.9 Deg C until the temp limit is reached when the boiler shuts down

 

The boiler doesn't measure the return temp - it measures just two things Flow temp (at the Hex and at the outlet - Why both I don't know but it does) and the temp of the Flue Gas

My understanding of boiler controls then is fairly radically different to the above, my understanding  is that the boiler looks at the difference between the target flow temp and the actual flow temp, if the flow temp is < the target temp then the boiler will eventually fire flat out until the flow temp is the same as the target temp or slightly exceeds it, the boiler will then modulate down until the target flow temp and the actual flow temp are almost exactly the same, if the heat demand is < the minimum boiler output then the boier return temp will start increasing which in turn means that the flow temperature will increase until eventually it may exceed the target flow temp by 5C, the burner will then trip, the circ ump will continue to run and the burner will then refire when tha anticycle time has elapsed. Boilers like the Valliant do have return temperature monitoring and if the DT  between the flow and return temps exceeds something like 27C/30C will pause the increase in modulation or take some action like this until the dT decreases, their fairly recent system boilers have various pump settings, one of which allows you to set the flow/return dT between 10C and 20C, it achieves this by changing the pump speed which changes the circulation flwrate but does not interfere with the target flow templ/actual flow temp control.

 

 

[marshian]

5 minutes ago, John Carroll said:

My understanding of boiler controls then is fairly radically different to the above, my understanding  is that the boiler looks at the difference between the target flow temp and the actual flow temp, if the flow temp is < the target temp then the boiler will eventually fire flat out until the flow temp is the same as the target temp or slightly exceeds it, 

 

Yep it does all that (unless range rated in which case it can only fire at maximum range rated setting) NB only applies once out of the initial fire up and ramp down

 

5 minutes ago, John Carroll said:

if the heat demand is < the minimum boiler output then the boier return temp will start increasing which in turn means that the flow temperature will increase until eventually it may exceed the target flow temp by 5C, the burner will then trip, the circ ump will continue to run and the burner will then refire when tha anticycle time has elapsed. 

 

No anticycle that I am aware of - my monitoring indicates that the boiler re-fires based on flow temp being target - X Degs (It does vary depending on target Flow temp) - I'll post a table at the end

 

5 minutes ago, John Carroll said:

Boilers like the Valliant do have return temperature monitoring and if the DT  between the flow and return temps exceeds something like 27C/30C will pause the increase in modulation or take some action like this until the dT decreases,

 

Very familiar with all the above - Had a glow worm before that did the same thing (although that had anticycle paramerters that could be tweaked)

 

5 minutes ago, John Carroll said:

their fairly recent system boilers have various pump settings, one of which allows you to set the flow/return dT between 10C and 20C, it achieves this by changing the pump speed which changes the circulation flwrate but does not interfere with the target flow templ/actual flow temp control.

 

The viessmann doesn't have that flexibility - very few parameters that you can play with

 

Basically "Viessmann technical" are saying the behaviour is normal for a heat only boiler where the boiler has no control of the pump.

 

Table of start temp and shut down temp based on WC settings

 

 

 

 

[John Carroll]

At a OAT of 10C the Targettemp is 23C, the end temp is 29.5C, TT+ 6.5C

At a OAT 0f 2C   the Targettemp is 31C, the end temp is 32.7C,  TT+ 1.7C

At a OAT of 10C the burner shuts down at TT+6.5C, is this because its exceeded TT+5C?

At a OAT of   2C the burner shuts down at TT+1.7C, why?

Does the boiler (FT)ever run at the the TT??.

Edited Wednesday at 15:53 by John Carroll

[marshian]

11 minutes ago, John Carroll said:

At a OAT of 10C the Targettemp is 23C, the end temp is 29.5C, TT+ 6.5C

At a OAT 0f 2C   the Targettemp is 31C, the end temp is 32.7C,  TT+ 1.7C

At a OAT of 10C the burner shuts down at TT+6.5C, is this because its exceeded TT+5C?

At a OAT of   2C the burner shuts down at TT+1.7C, why?

Does the boiler (FT)ever run at the the TT??.

 

I think that the start up and shut down signal temps are different as OAT changes.

 

From collected data anyway (and it's remarkably repeatable - those aren't just one occasion but the average of multiple occasions)

 

Yes the boiler FT does run at the target temp until it no longer can due to influence from the return temp creeping up

 

The boiler ran for nearly 10 hrs sold when we had minus temps here in Feb

 

 

[SimonD]

3 hours ago, marshian said:

Basically "Viessmann technical" are saying the behaviour is normal for a heat only boiler where the boiler has no control of the pump.

 

Yes, the Viessmann heat only boilers are a completely different beast compared to the system boilers - no opentherm/room compensation available. Even on the system versions of the 100-W return temperature is not available to Opentherm - or at least not some Opentherm controllers where you can interrogate what information is available for control. Whether it is available to Viessmann ones would be interesting to check.

[marshian]

4 hours ago, SimonD said:

 

Yes, the Viessmann heat only boilers are a completely different beast compared to the system boilers - no opentherm/room compensation available. Even on the system versions of the 100-W return temperature is not available to Opentherm - or at least not some Opentherm controllers where you can interrogate what information is available for control. Whether it is available to Viessmann ones would be interesting to check.

 

The boiler has no sensor on the return to measure temp - only sensors are in the HEX and after it

 

Don't get me wrong here I've no complaints about the boilers ability to run stable flow temps comparable with a heat pump - ie sub 30 deg C - it's just the modulation spikes where I can't understand the reason that drives me slightly crazy......

 

I'm not going to say on a public forum what my next action will be but with a spare MBU and some very very techie friends/work colleagues who do this stuff for a living you can guess

 

My bet is the program/boiler logic is very much IFTTT based and I reckon there is a line of code that is squiffy - 99% of users running more conventional flow temps or not monitoring the boiler in the same level of detail would never see the issue - I'm the 1% that does see it and it annoys me because whenever it does it the boiler plumes and I know that's heat/energy leaving the house having not been passed into the water circuit 

 

[marshian]

18 hours ago, marshian said:

The boiler ran for nearly 10 hrs sold when we had minus temps here in Feb Jan

 

Sorry my error it was Jan when we had 3 days where OAT temp ranged from -6.3 to 2.5 Deg C

 

In the 72 hour period the boiler was running for 68 hours (1.5 hrs was HW) and total number of cycles was 19 (average cycles per day is normally 20 to 24 

 

Hourly flow and return temps for that period are below

 

 

 

 

I hadn't noticed till I looked at the data today but the DT at the boiler ranged from 6.3 to 8.8 (I'm going to assume that the heat loss at colder temps was driving this)

 

Avg DT since 01 Nov 2025 to 31 March 26  was 6.1 Deg C

[SimonD]

12 hours ago, marshian said:

Don't get me wrong here I've no complaints about the boilers ability to run stable flow temps comparable with a heat pump - ie sub 30 deg C - it's just the modulation spikes where I can't understand the reason that drives me slightly crazy......

 

I'm the same, it's like having a splinter that has to be removed. I'm dealing with 2 heat pumps at the moment that look like the bottom panel is hitting a resonant frequency that's making them both very loud. The manufacturer's tests have been rubbish. I'm on a mission with that at the moment....

[marshian]

12 minutes ago, SimonD said:

 

I'm the same, it's like having a splinter that has to be removed. I'm dealing with 2 heat pumps at the moment that look like the bottom panel is hitting a resonant frequency that's making them both very loud. The manufacturer's tests have been rubbish. I'm on a mission with that at the moment....

 

Oh that would be really annoying - I'm the same with rattles or resonance in car 

 

I know flow rate thro heat pumps is normally much higher than boilers (due to very low DT) so good luck with your mission

[John Carroll]

4 hours ago, marshian said:

 

Sorry my error it was Jan when we had 3 days where OAT temp ranged from -6.3 to 2.5 Deg C

 

In the 72 hour period the boiler was running for 68 hours (1.5 hrs was HW) and total number of cycles was 19 (average cycles per day is normally 20 to 24 

 

Hourly flow and return temps for that period are below

 

 

 

 

I hadn't noticed till I looked at the data today but the DT at the boiler ranged from 6.3 to 8.8 (I'm going to assume that the heat loss at colder temps was driving this)

 

Avg DT since 01 Nov 2025 to 31 March 26  was 6.1 Deg C

You had a few readings there early in January of flow/return/dT, 35.4C/29.0C/6.4C, by calculation and assuming 20C room temps at I think you said a flowrate of 0.5m3/hr then the rads are 12.199 deg rads with a output of 15.98% and you require a unlikely 23.28kW of T50 rads which will result in a output of 3.72kW, something not right there. if calcs correct, maybe flowrate??

[marshian]

18 minutes ago, John Carroll said:

You had a few readings there early in January of flow/return/dT, 35.4C/29.0C/6.4C, by calculation and assuming 20C room temps at I think you said a flowrate of 0.5m3/hr then the rads are 12.199 deg rads with a output of 15.98% and you require a unlikely 23.28kW of T50 rads which will result in a output of 3.72kW, something not right there. if calcs correct, maybe flowrate??

 

Flow rate is 0.6 m3/Hr

 

Total House Radiator Rating at T50 is 21.870

 

Room Temps aren't all 20 Deg C - varies by usage 

 

General rule

Kitchen, Dining, Hallway 19

Living room, Main Bed 20

Bathrooms 21

Rest 17

[JohnMo]

Those flow rates are higher than my heat pump, it's generally settling down to about 0.36m³/HR, most the time 

[marshian]

1 minute ago, JohnMo said:

Those flow rates are higher than my heat pump, it's generally settling down to about 0.36m³/HR, most the time 

 

It is all rads - No UFH

 

You are all UFH no rads from memory

 

I've tried slowing the flow rate down but it cycled a lot more

[JohnMo]

Yes all UFH plus one fan coil.

 

I have found raising flow temp 1-2 degs higher than I thought was needed, at higher oat has allowed more controlled modulation, have had to use a high limit thermostat set at target house to kill the heat pump off though. The lower the flow temperature the higher the minimum output. It could be you are at or below min modulation and boiler is protecting itself, instead of switching off.

[John Carroll]

59 minutes ago, marshian said:

 

Flow rate is 0.6 m3/Hr

 

Total House Radiator Rating at T50 is 21.870

 

Room Temps aren't all 20 Deg C - varies by usage 

 

General rule

Kitchen, Dining, Hallway 19

Living room, Main Bed 20

Bathrooms 21

Rest 17

Assuming a average 19.25C room(s) temp and 0.6m3/hr then 21.87kW ofT50s will result in 3.924kW of T13.337s, 17.944% output, but with flow/return/dT of 35.4C/29.77C/5.63C, sounds more reasonable?

 

And for interest if the flowrate is slowed to 0.36m3/hr with 19.25C roomtemps and 35.4C flowtemp then, the output falls to 3.44kW with flow/return/dT of 35.4C/27.19C/8.21C, the difference between 3.924kw & 3.44kW might just be enough to tip the scales into boiler recycle??.

Edited 8 hours ago by John Carroll

[marshian]

1 hour ago, John Carroll said:

Assuming a average 19.25C room(s) temp and 0.6m3/hr then 21.87kW ofT50s will result in 3.924kW of T13.337s, 17.944% output, but with flow/return/dT of 35.4C/29.77C/5.63C, sounds more reasonable?

 

And for interest if the flowrate is slowed to 0.36m3/hr with 19.25C roomtemps and 35.4C flowtemp then, the output falls to 3.44kW with flow/return/dT of 35.4C/27.19C/8.21C, the difference between 3.924kw & 3.44kW might just be enough to tip the scales into boiler recycle??.

 

Gas usage in kWh for the three days by hour

 

Yellow marked cells are where it included 30 mins of HW at an elevated flow temp and modulation rate

 

At a 35 deg Flow temp the boiler is running at 4.05 to 4.1 kWh at min modulation

 

[John Carroll]

If the boiler is continuously running at 4.1kWh gas consumption and the output by my calc is 3.924kW then it adds up fairly well as this gives a boiler efficiency of 95.71%. You mentioned somewhere I think that you thought your boiler's  minimum output should be better as its not achieving its advertised figure of 3.2kW at 50C/30C, which means the gas consumption should be 3.2/0.9571, 3.343kWh?, what do you reckon is the minimum gas consumption when it starts cycling?