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Viessmann boiler short cycling

Hi,

I have noticed my boiler Viessmann Vitodens 100-W(B1HC-26) makes a series of short cycling before starting to heat. Boiler is controlled by Baxi uSense room thermostat connected via Opentherm. I installed some monitoring equipment to get the better understanding of the problem. Bellow is the illustration of the problem:
Viessmann cycling.png

I will explain the graph from the top to bottom:
  • Green bar indicates heating pump is on
  • Red bar indicates flame is on
  • Black line indicates max relative modulation level (%)
  • Orange line indicates DWH temperature (Celsius) (not relevant in this problem )
  • Red line indicates boiler water temperature (Celsius)
  • Grey line indicates setpoint temperature (Celsius)
The graph reveals that it took 10 times for the boiler to fire up until it started to run. Water temperature inside the boiler reaches ~40 degrees very quickly after boiler fires up and then it switches off.

I would appreciate any ideas of how to solve the short cycling.
 
Can you be a little more specific?
House is based on floor heating, system is all new. Circulation pump is the one that's inside the boiler. The problem I have is that boiler switches on and switches off frequently without making any heat. For example temperature inside the boiler is 25 degrees(Celsius). The boiler switches on and the temperature starts to rise up rapidly. It reaches 40 degrees in ~5 seconds and the boiler switches off. The temperature inside the boiler drops to 25 degrees again. After several minutes boiler switches on again and switches off very quickly. It takes 3-10 cycles for boiler to start working.
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Remove the baxi I sense does it work / stay on for longer ?
Once the boiler starts working it keeps heating for a quite long time, about 2 hours. It short cycles until it gets working. I haven't tried without the Baxi thermostat as monitoring equipment will not work without it.
 
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Re the "relative modulation level".... is 0 = minimum (modulated) boiler output and 100 = maximum boiler output or vica versa?.
 
View attachment 42946

I will explain the graph from the top to bottom:
  • Green bar indicates heating pump is on
  • Red bar indicates flame is on
  • Black line indicates max relative modulation level (%)
  • Orange line indicates DWH temperature (Celsius) (not relevant in this problem )
  • Red line indicates boiler water temperature (Celsius)
  • Grey line indicates setpoint temperature (Celsius)
The black line shows the relative modulation level, not the max level.

0% is the minimum the boiler will modulate down to, e.g. 8kW; and 100% the maximum it will produce, e.g 32kW. This is Opentherm parameter 17, which is a Read parameter, i.e it is just telling you what the current modulation level is.

The max relative modulation (parameter 14) is only used in multiple boiler installations and sets the maximum output of a specific boiler. In single boiler applications it is always 100%, the default.

ShaunCorbs suggestion of disconnecting the uSense thermostat (and connecting the boiler Opentherm terminals togethr or substituting a volt free one) is a good idea as it will tell you if the problem is due to the thermostat or the boiler.
 
So in that single boiler application, above, it should read 100% at all times and not change throughout the firing period?.
 
So in that single boiler application, above, it should read 100% at all times and not change throughout the firing period?.
I assume you are talking about the Max Relative Modulation (parameter 14). If so, yes it reads 100% all the time It is a Write parameter, i.e the required value is sent from the thermostat to the boiler. The Relative Modulation will change during firing as it is just informing you of the currrent boiler output. An Opentherm thermostat sends the boiler the required CH temperature; it is up to the boiler how this is achieved and controlled.
 
The black line shows the relative modulation level, not the max level.

0% is the minimum the boiler will modulate down to, e.g. 8kW; and 100% the maximum it will produce, e.g 32kW. This is Opentherm parameter 17, which is a Read parameter, i.e it is just telling you what the current modulation level is.

The max relative modulation (parameter 14) is only used in multiple boiler installations and sets the maximum output of a specific boiler. In single boiler applications it is always 100%, the default.

ShaunCorbs suggestion of disconnecting the uSense thermostat (and connecting the boiler Opentherm terminals togethr or substituting a volt free one) is a good idea as it will tell you if the problem is due to the thermostat or the boiler.
How the red line (boiler water temperature) should look like in a perfect case? Is spike normal at the moment of ignition? As I know Viessmann Vitodens 100-W boiler starts up with 70% of it's capacity at ignition phase and then modules down if necessary. In my case boiler doesn't get over the spike as temperature inside the boiler gets too high comparing to the setpoint. As the graph shows, boiler gets over the spike only when setpoint temperature goes a little bit over 30 degress.
 
I often think its quite a achievement for a (gas) boiler to ignite @ 70% output and modulate down rapidly before the temperature reaches SP+5C, and burner cut out. If one assumes a Hx content of 3 litres and 10 LPM flowrate and a 10C deltaT before SP+5C is reached then it will only take ~ 12 secs, obviously the Hx metal etc must heat up firstly but even allowing for this, the time probably isn't more than say 20 secs?, if it doesn't then IMO the off or anti cycle time has to be increased to allow a greater deltaT but in your case above, it can hardly go lower than a return temp of 25C so increasing the boiler SP is one possible solution and also, if possible, increase the boiler minimum flowrate.
Also, I see above that the temperature rises from 25c to 40c in 5 secs, theoretically, even with no boiler flowrate this would require a boiler output of ~ 37 kw? so maybe problem with flow temp sensor?.
 
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As the graph shows, boiler gets over the spike only when setpoint temperature goes a little bit over 30 degress.
Can you post a copy of the log? This will show the messages between the uSense and the boiler. Only those sent while the boiler is trying to light are required.
 
Can you post a copy of the log? This will show the messages between the uSense and the boiler. Only those sent while the boiler is trying to light are required.
Pasting the log which was produced at the time of ignition.
18:09:53.108755 T00000300 Read-Data Status: 00000011 00000000
18:09:53.249320 B40000302 Read-Ack Status: 00000011 00000010
18:09:54.108727 T90011EB3 Write-Data Control setpoint: 30.70
18:09:54.264932 B50011EB3 Write-Ack Control setpoint: 30.70
18:09:55.124474 T00110000 Read-Data Relative modulation level: 0.00
18:09:55.265010 BC0110000 Read-Ack Relative modulation level: 0.00
18:09:56.124454 T80190000 Read-Data Boiler water temperature: 0.00
18:09:56.280670 B40191866 Read-Ack Boiler water temperature: 24.40
18:09:57.124458 T101815D1 Write-Data Room temperature: 21.82
18:09:57.139510 R00740000 Read-Data Burner starts: 0
18:09:57.280680 BC074DEA0 Read-Ack Burner starts: 56992
18:09:57.295408 A701815D1 Unk-DataId Room temperature: 21.82
18:09:58.140062 T00000300 Read-Data Status: 00000011 00000000
18:09:58.280538 B40000302 Read-Ack Status: 00000011 00000010
18:09:59.140078 T90011EB3 Write-Data Control setpoint: 30.70
18:09:59.296146 B50011EB3 Write-Ack Control setpoint: 30.70
18:10:00.155698 T00110000 Read-Data Relative modulation level: 0.00
18:10:00.311794 BC0110000 Read-Ack Relative modulation level: 0.00
18:10:01.155751 T80190000 Read-Data Boiler water temperature: 0.00
18:10:01.327453 B40191866 Read-Ack Boiler water temperature: 24.40
18:10:02.171340 T801A0000 Read-Data DHW temperature: 0.00
18:10:02.327493 B401A3533 Read-Ack DHW temperature: 53.20
18:10:03.171340 T00000300 Read-Data Status: 00000011 00000000
18:10:03.343118 B40000302 Read-Ack Status: 00000011 00000010
18:10:04.186950 T90011EB3 Write-Data Control setpoint: 30.70
18:10:04.343179 B50011EB3 Write-Ack Control setpoint: 30.70
18:10:05.187021 T00110000 Read-Data Relative modulation level: 0.00
18:10:05.358760 BC0110000 Read-Ack Relative modulation level: 0.00
18:10:06.202610 T80190000 Read-Data Boiler water temperature: 0.00
18:10:06.358778 BC0191880 Read-Ack Boiler water temperature: 24.50
18:10:07.202738 T80730000 Read-Data OEM diagnostic code: 0
18:10:07.218018 R00780000 Read-Data Burner operation hours: 0
18:10:07.374399 BC07803D4 Read-Ack Burner operation hours: 980
18:10:07.388754 A70730000 Unk-DataId OEM diagnostic code: 0
18:10:08.218257 T00000300 Read-Data Status: 00000011 00000000
18:10:08.374357 BC000030A Read-Ack Status: 00000011 00001010
18:10:08.546166 T00000300 Read-Data Status: 00000011 00000000
18:10:08.780521 BC000030A Read-Ack Status: 00000011 00001010
18:10:08.952282 T00000300 Read-Data Status: 00000011 00000000
18:10:09.170937 BC000030A Read-Ack Status: 00000011 00001010
18:10:09.874255 T90011EB3 Write-Data Control setpoint: 30.70
18:10:10.077310 B50011EB3 Write-Ack Control setpoint: 30.70
18:10:10.890048 T00110000 Read-Data Relative modulation level: 0.00
18:10:11.093177 BC0110000 Read-Ack Relative modulation level: 0.00
18:10:11.890140 T80190000 Read-Data Boiler water temperature: 0.00
18:10:12.108817 B40191E99 Read-Ack Boiler water temperature: 30.60
18:10:12.905783 T90383200 Write-Data DHW setpoint: 50.00
18:10:13.124419 B50383200 Write-Ack DHW setpoint: 50.00
18:10:13.905831 T00000300 Read-Data Status: 00000011 00000000
18:10:14.124485 BC000030A Read-Ack Status: 00000011 00001010
18:10:14.921466 T90011EB3 Write-Data Control setpoint: 30.70
18:10:15.124534 B50011EB3 Write-Ack Control setpoint: 30.70
18:10:15.921425 T00110000 Read-Data Relative modulation level: 0.00
18:10:16.140124 BC0110000 Read-Ack Relative modulation level: 0.00
18:10:16.937030 T80190000 Read-Data Boiler water temperature: 0.00
18:10:17.140234 BC0192500 Read-Ack Boiler water temperature: 37.00
18:10:17.936982 T00050000 Read-Data Application-specific flags: 00000000 0
18:10:18.155773 BC0050000 Read-Ack Application-specific flags: 00000000 0
18:10:18.937100 T00000300 Read-Data Status: 00000011 00000000
18:10:19.155762 BC000030A Read-Ack Status: 00000011 00001010
18:10:19.952619 T90011EB3 Write-Data Control setpoint: 30.70
18:10:20.171328 B50011EB3 Write-Ack Control setpoint: 30.70
18:10:20.952720 T00110000 Read-Data Relative modulation level: 0.00
18:10:21.171315 BC0110000 Read-Ack Relative modulation level: 0.00
18:10:21.968293 T80190000 Read-Data Boiler water temperature: 0.00
18:10:22.186925 BC0192719 Read-Ack Boiler water temperature: 39.10
18:10:22.968328 T001B0000 Read-Data Outside temperature: 0.00
18:10:23.186957 BC01B054D Read-Ack Outside temperature: 5.30
18:10:23.983924 T00000300 Read-Data Status: 00000011 00000000
18:10:24.202570 B40000302 Read-Ack Status: 00000011 00000010
18:10:24.374378 T00000300 Read-Data Status: 00000011 00000000
18:10:24.702489 B40000302 Read-Ack Status: 00000011 00000010
18:10:24.874279 T00000300 Read-Data Status: 00000011 00000000
18:10:25.202548 B40000302 Read-Ack Status: 00000011 00000010
18:10:25.796222 T90011EB3 Write-Data Control setpoint: 30.70
18:10:25.999309 B50011EB3 Write-Ack Control setpoint: 30.70
18:10:26.796388 T00110000 Read-Data Relative modulation level: 0.00
18:10:27.015029 BC0110000 Read-Ack Relative modulation level: 0.00
18:10:27.812033 T80190000 Read-Data Boiler water temperature: 0.00
18:10:28.015128 B40192380 Read-Ack Boiler water temperature: 35.50
18:10:28.812098 T900E6400 Write-Data Maximum relative modulation level: 100.00
18:10:29.046341 B500E6400 Write-Ack Maximum relative modulation level: 100.00
18:10:29.827707 T00000300 Read-Data Status: 00000011 00000000
18:10:30.046349 B40000302 Read-Ack Status: 00000011 00000010
18:10:30.827740 T90011EB3 Write-Data Control setpoint: 30.70
18:10:31.046393 B50011EB3 Write-Ack Control setpoint: 30.70
18:10:31.843414 T00110000 Read-Data Relative modulation level: 0.00
18:10:32.062006 BC0110000 Read-Ack Relative modulation level: 0.00
18:10:32.843416 T80190000 Read-Data Boiler water temperature: 0.00
18:10:33.062062 B40191CCD Read-Ack Boiler water temperature: 28.80
18:10:33.858917 T90101600 Write-Data Room setpoint: 22.00
18:10:33.874089 R00740000 Read-Data Burner starts: 0
18:10:34.077566 B4074DEA1 Read-Ack Burner starts: 56993
18:10:34.092368 A50101600 Write-Ack Room setpoint: 22.00
18:10:34.858934 T00000300 Read-Data Status: 00000011 00000000
18:10:35.077562 B40000302 Read-Ack Status: 00000011 00000010
18:10:35.874595 T90011EB3 Write-Data Control setpoint: 30.70
18:10:36.093320 B50011EB3 Write-Ack Control setpoint: 30.70
18:10:36.874300 T00110000 Read-Data Relative modulation level: 0.00
18:10:37.092811 BC0110000 Read-Ack Relative modulation level: 0.00
18:10:37.889807 T80190000 Read-Data Boiler water temperature: 0.00
18:10:38.092995 B40191980 Read-Ack Boiler water temperature: 25.50
18:10:38.889975 T101815D1 Write-Data Room temperature: 21.82
18:10:38.905778 R00780000 Read-Data Burner operation hours: 0
18:10:39.108631 BC07803D4 Read-Ack Burner operation hours: 980
18:10:39.123521 A701815D1 Unk-DataId Room temperature: 21.82
 
Thanks for the log.

The log shows less than one minute of data, but does not show when the boiler eventually lights. It's impossible to see how long that takes as there is no time information on the graph,

Do you have an external sensor for weather compensated boiler temperature control? The log shows a request for the data and a reply of 5.3C. If so, was it commisioned by the installer so the correct flow temperatures were obtained?

Have you tried our suggestion of disconnecting the thermostat and replacing it with a link? This will then eliminate the boiler or the stat as the cause of your problem.
 
Thanks for the log.

The log shows less than one minute of data, but does not show when the boiler eventually lights. It's impossible to see how long that takes as there is no time information on the graph,

Do you have an external sensor for weather compensated boiler temperature control? The log shows a request for the data and a reply of 5.3C. If so, was it commisioned by the installer so the correct flow temperatures were obtained?

Have you tried our suggestion of disconnecting the thermostat and replacing it with a link? This will then eliminate the boiler or the stat as the cause of your problem.
The log represents one spike of the graph. As far as I understand Read Status command tells what's happening with the boiler.
Status: 00000011 00000010 - means circulating pump is on, flame is off.
Status: 00000011 00001010 - means circulating pump is on, flame is on.
So according to the log, ignition happened at 18:10:08 and boiler went off 18:10:24. During that time temperature inside boiler rose up from 24.50 C to 39.10C.

Yes, there is a weather compensation control that is adjusted to meet house energy requirements for floor heating.
I will disconnect the thermostat this evening and will tell the difference.
 
Status: 00000011 00000010 - means circulating pump is on, flame is off.
Status: 00000011 00001010 - means circulating pump is on, flame is on.
Opentherm has no way of saying if the pump is working or not; this is because the pump is not always controlled by the boiler. Bit 3 (in bold red) indicates the status of the flame (1=on, 0=off), not the pump. Bit 1 (green) just tells you if CH is 'on', i.e available.

monitor.JPG

Looking again at the Monitor, I am thinking that it just doesn't add up.

  1. The room set point is 22.00 but the Room temperature is 22.05. So why does the boiler need to light?
  2. The relative modulation is 0.00 which means the boiler is set to the lowest ouutput. So it can't modulate any lower all it can do is run in on/off mode, which it is doing - initially.
  3. The Control setpoint (required water temperature) is 28.90, but the water temperature is already 33.90. So why does the boiler need to turn on?
The boiler is running in on/off mode, unnecesarily, as the house is already up to temperature. So what happens to make the boiler suddenly decide to ramp up to 100% modulation and start a frenzied dance?

The log from just before the boiler ramps up for a few minutes after may provide a clue. It may be a large file, so zip it up and attach it to your reply.

Second thought: How long have you had the boiler and thermostat?
 
Opentherm has no way of saying if the pump is working or not; this is because the pump is not always controlled by the boiler. Bit 3 (in bold red) indicates the status of the flame (1=on, 0=off), not the pump. Bit 1 (green) just tells you if CH is 'on', i.e available.

View attachment 43009

Looking again at the Monitor, I am thinking that it just doesn't add up.

  1. The room set point is 22.00 but the Room temperature is 22.05. So why does the boiler need to light?
  2. The relative modulation is 0.00 which means the boiler is set to the lowest ouutput. So it can't modulate any lower all it can do is run in on/off mode, which it is doing - initially.
  3. The Control setpoint (required water temperature) is 28.90, but the water temperature is already 33.90. So why does the boiler need to turn on?
The boiler is running in on/off mode, unnecesarily, as the house is already up to temperature. So what happens to make the boiler suddenly decide to ramp up to 100% modulation and start a frenzied dance?

The log from just before the boiler ramps up for a few minutes after may provide a clue. It may be a large file, so zip it up and attach it to your reply.

Second thought: How long have you had the boiler and thermostat?
Thanks for your reply. I'll try to explain:
1. Probably there is some kind of threshold. Boiler doesn't cut of the flame at the moment room setpoint temperature is reached, it goes on for a little bit more.
2. Header at the top of Monitor window shows current values. The most recent part of the graph at that time is not visible as horizontal scroll bar is moved to the left. So visible part of the graph represents historical values that differ from what's at the top.
3. Threshold again. I was told Viessmann exceeds control setpoint temperature by ~7 degrees before switching off. Also lets water temperature to drop ~7 degrees below control setpoint temperature before switching on.

The boiler is 2 years old. At the beginning there was no thermostat at all, I used to set control setpoint manually. Thermostat installed 3 months ago, I think all was working fine. After some time noticed short cycling.

Attaching log file of full day. Graph depicts interval starting at ~18:00.
 

Attachments

  • otlog-20200312.zip
    1.3 MB · Views: 5
There's definitely soething unusual going on.

Did you have the boiler installed? If so, who said you needed a 26kW one?

Did you, or your installer, confirm with Viessmann that your boiler would work correctly with a Weather Compensation Sensor and the Baxi Usense thermostat?

How is the HW controlled? How many motorized valves do you have?

Edit: Did you know that the Usense is designed to be used with a combi boiler?
 
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The boiler was installed by Viessmann installer, the capacity was decided according to the size of house (140 m2) and hot water demand for 2 bathrooms. Installer confirmed that older versions of Baxi/Remeha/De Dietrich OpenTherm thermostats work fine with Vitodens 100-W boilers. Baxi uSense was his first installation with Viessmann.
I adjusted heating curve according to installers comments. These are the values of the curve:

Outside temp - Circuit temperature
-20c - 39c
-15c - 37c
-10c - 35c
-5c - 33c
0c - 31c
5c - 29c
10c - 27c
15c - 25c

House meets "A" energy efficiency class building requirements. Boiler heats separate HW tank(temperature sensor installed) which is fed directly from the mains according to HW temperature sensor. I'm not aware of any motorized valves.
Now I have unplugged Weather compensation sensor and will monitor how the boiler behaves.
 
It seems to me that you found the problem stating in post #11 that the SP has to be > 30C to get the boiler away which indicates that the minimum weather compensated SP temperature should be > 30C, if the SP is say 32C then the boiler shouldn't fire until 25C (SP-7) and must not reach 39C (SP+7) to avoid cut out.
 
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I'm not aware of any motorized valves.
There must be one, or two, somewhere. They control the flow of water through the CH and HW circuits so they only get heated when required. If you dont have them, both circuits would be heated whenever the boiler was alight.

How were hot water times/temperatures controlled before the Usense was installed? There must have been valves to stop CH and HW heating up when not required?

With an Opentherm system it's normal to have a diverter valve which allows either the HW or the CH to be heated, not both at the same time. The reason for this is that HW required the flow temperature to be constant at about 70C, while CH will vary the temperature as required. A diverter valve may have been installed in the boiler (its a Viessmann option); check with the installer.

As I said, the Usense is designed for use with some Baxi combi boilers. Like all combis, they have a diverter valve built in and automatically disconnect Opentherm when a tap is turned on, so the boiler runs at max output. The HW feature on the Usense is there for preheating only.
 
There must be one, or two, somewhere. They control the flow of water through the CH and HW circuits so they only get heated when required. If you dont have them, both circuits would be heated whenever the boiler was alight.

How were hot water times/temperatures controlled before the Usense was installed? There must have been valves to stop CH and HW heating up when not required?

With an Opentherm system it's normal to have a diverter valve which allows either the HW or the CH to be heated, not both at the same time. The reason for this is that HW required the flow temperature to be constant at about 70C, while CH will vary the temperature as required. A diverter valve may have been installed in the boiler (its a Viessmann option); check with the installer.

As I said, the Usense is designed for use with some Baxi combi boilers. Like all combis, they have a diverter valve built in and automatically disconnect Opentherm when a tap is turned on, so the boiler runs at max output. The HW feature on the Usense is there for preheating only.
I think boiler does all the switching as it has separate pipes for CH and HW. It behaves very similar to combi boiler. When the temperature inside the tank drops then boiler stops CH flow and switches to HW circuit. There is an indicator on boiler's display telling which circuit is currently running.

Viessmann boiler has an option to set HW temperature to maintain in tank so before thermostat was installed I simply set 60C HW temperature using boiler's control panel.

Here is how it looks.
91764956_246189669763240_1379886616117510144_n.jpg
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It seems to me that you found the problem stating in post #11 that the SP has to be > 30C to get the boiler away which indicates that the minimum weather compensated SP temperature should be > 30C, if the SP is say 32C then the boiler shouldn't fire until 25C (SP-7) and must not reach 39C (SP+7) to avoid cut out.
It might be true. I unplugged weather compensation sensor yesterday and haven't noticed short cycling since then. Weather is getting warmer and boiler runs not often so need more time to be sure short cycling is gone.
 
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What is the HW SP now?.
If you look at page 78 of the attached you will see the selectable weather compensation curves, yours seems to be set up on a very low curve but doesn't really make a lot of sense as it runs out of settings, if this is your curve setting table would suggest setting somewhere between "15" and "20".
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Should have asked, "what is the CH SP now" above.
 
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yours seems to be set up on a very low curve but doesn't really make a lot of sense as it runs out of settings, if this is your curve setting table would suggest setting somewhere between "15" and "20".
I thought the curve was very flat, but then remembered that the OP said he had underfloor heating (on both floors??)
 
What is the HW SP now?.
If you look at page 78 of the attached you will see the selectable weather compensation curves, yours seems to be set up on a very low curve but doesn't really make a lot of sense as it runs out of settings, if this is your curve setting table would suggest setting somewhere between "15" and "20".
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Should have asked, "what is the CH SP now" above.
CH SP is controlled by thermostat. Monitoring tool shows CH SP is 7C when there is no need for heating. When thermostat decides it's time to start heating, it sets CH SP to 50C (which MAX CH water setpoint) and the boiler starts up at once without short cycling as temperature spike during ignition doesn't exceeds 50C. After that thermostat lowers CH SP and when it's time to stop heating it lowers CH SP more and the boiler cuts off.
The heating curve I've provided is from thermostat which overrides boiler's heating curve. Those heating curves from the manual are quite steep and fit more for radiator based heating.
[automerge]1585768123[/automerge]
I thought the curve was very flat, but then remembered that the OP said he had underfloor heating (on both floors??)
Yes it's all underfloor heating, no radiators.
 
It makes sense to me to kick the SP up initially to get the boiler to stabilize and then reduce it, you said you had removed the weather compensation sensor so on what basis is the boiler now controlling its SP? the curve you gave looks like the boiler is still looking at a outside temperature from somewhere.
Can I assume it doesn't/wasn't doing this (initially increasing the SP to 50C) when running on the outside temperature compensation and have you the curve for this?.

Edit: I'm mixed up I think, the curve you gave is for the outside temp compensation? so can you provide the boiler curve or its basis for changing the SP?.
 
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I'm mixed up I think, the curve you gave is for the outside temp compensation? so can you provide the boiler curve or its basis for changing the SP?
Yes, the curve supplied by the OP is for the weather compensator.

Theree is no such thing as a boiler curve. The boiler is told by the thermostat, via Opentherm, what the required SP is. How this is calculated is known only to the stat manufacturers. I would assume that it will depend on the difference between actual and required room temperatures and take into account the rate of temperature drop.

To quote the Opentherm Protocol:

The slave [boiler] does not need to know how the master [thermostat} has calculated the control setpoint, e.g.
whether it used room control or OTC [outdoor temperature control], it only needs to control to the value. Likewise, the master does not need to know how the slave is controlling the supply.
 
Thanks, that clears it up for me, I had thought of the room stat as just a on/off switch without communication. It is interesting how it starts up at 50C., its a pity that the OTC doesn't seem to do likewise but I suppose that is only a sensor.

I have seen a relations Vokera Vision 20S with a cruder version of this called SARA, if the room stat contacts stay closed for longer than 20 minutes then the boiler SP temperature is increased as long as the "manual" boiler SP is set between certain limits.
 
There was not a single short cycle after disconnecting outside temperature sensor for more than 2 weeks. Without OTC thermostat sets 50C SP when it's time to start heating and boiler overcomes the spike without switching off.
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I had a similar problem, for me it was the flow rate sensor, and this was with 100w as well.
Was the boiler short cycling because of rapidly rising temperature during the ignition phase? What has changed after replacing flow rate sensor?
 
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There was not a single short cycle after disconnecting outside temperature sensor for more than 2 weeks.
I've been working on the 24 hr log you sent me. I have converted it to an Excel file to make analysis easier.

Would it be possible to send me another 24 hr log, without the outdoor sensor, connected, so I can see exactly what happens?

Heres a chart showing how the boiler runs over the 24 hours. The oudoor temperature uses the vertical scale on the left, All other lines use the left hand scale where 100 is either 100C or 100%. The thing I do not understand is why the boiler is switching up and down between modulations which are 20% apart when on heating, but more refined when on hot water. It's as if TPI (from the usense?) was controlling the boiler temperature. The actual room temperature stays very constant at 22C so I haven't bothered to show it.

Usense with Outdoor Sensor.png
 
I have attached the log file containing data for full day when the outdoor sensor was disconnected. I think Baxi uSense thermostat doesn't include TPI software.
 

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  • otlog-20200401.zip
    1.3 MB · Views: 5
I had a similar problem, for me it was the flow rate sensor, and this was with 100w as well.
I was wondering why you mentioned this but I now know (I think). The boiler has a flow switch which will shut down the boiler if the heating flow rate falls below 200l/h (3.3LPM) but with no alarm. I am assuming that this is a heat boiler only and that its not a combi and that they are not referring to the DHW flow? , but they do refer to the "heating" flow rate.
 
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