Viessmann boiler question

[siricosm]

surely 1.9kw is enough it’s like two rads ?

Two rads on a very cold day. It is a very good question how low the "infinitely modulating boiler" would need to go to completely eliminate cycling in a small energy efficient house. My guess would be less than .5kW.

 

[ShaunCorbs]

--> (Current) BF 75 , room temp~21.x ,Boiler cuts off at around 2.4 mins and starts after 3 mins
--> BF75, HW ~60, room temp ~18 with rads cold room stats - 25 - Boiler cuts off at around 8 mins .
--> BF 75 ,HW ~60 ( unchanged), room stats - 35 ( max ) , HW ON - Boiler cuts off around 35 minutes ,All rads heat up to uniformly to around 65 . all TRV's fully opened

Also tried with BF reduced to 65 and no change in the short cycling i,e 2 mins .

something isn’t right sorry to say time to get the installers back
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Two rads on a very cold day. It is a very good question how low the "infinitely modulating boiler" would need to go to completely eliminate cycling in a small energy efficient house. My guess would be less than .5kW.

weather comp would adjust the temp to suite

 

[John.g]

If manufacturers offered boilers with high modulation ratios then IMO this would cover most demands without the added expense of a buffered system, manufacturers like Vokera offered 10:1 modulation years ago I think, if you had a heat only boiler with a heat requirement of say 15kw then a 10:1 ratio will give a minimum output of 1.5 kw, some manufacturers as well basically fit the same burner to a range of boilers to keep manufacturing costs down and because the minimum output is governed by the burner size/characteristics then one might only get a effective modulation ratio of say 3:1 at the lower lower end of their power range but a effective modulation ratio of say 6:1 at the higher end of their power range. Combi boilers present a bigger challenge as the HW demand can be well over twice the CH demand and buffering may be the only realistic solution here but sort of defeats the purpose of the combi. I know two people who changed from oil to gas who found it very hard to understand/accept that their constantly cycling oil boiler gave better temperature control than their new gas boiler, at very low heat demands.
Having said that it does seem that some makes of gas boilers can perform better than other makes when cycling, all seem to perform fine as long as operating in their modulation range.
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--> (Current) BF 75 , room temp~21.x ,Boiler cuts off at around 2.4 mins and starts after 3 mins
--> BF75, HW ~60, room temp ~18 with rads cold room stats - 25 - Boiler cuts off at around 8 mins .
--> BF 75 ,HW ~60 ( unchanged), room stats - 35 ( max ) , HW ON - Boiler cuts off around 35 minutes ,All rads heat up to uniformly to around 65 . all TRV's fully opened

Also tried with BF reduced to 65 and no change in the short cycling i,e 2 mins .

Your first "bullet" point: is your room temperature at its SP temperature, with cycling?
Point2 & 3 seem to be stating that with the room stats at 25C that the boiler only runs for 8 minutes and with the room stats at 35C that the boiler runs for 35 minutes, this is very strange if one assumes that the room stat hasn't been satisfied in either case.

Excuse my ignorance, but I assume BF75 refers to the boiler flow temperature and HW60 refers to the hot water cylinder stat SP?

 

[mars_man]

If manufacturers offered boilers with high modulation ratios then IMO this would cover most demands without the added expense of a buffered system, manufacturers like Vokera offered 10:1 modulation years ago I think, if you had a heat only boiler with a heat requirement of say 15kw then a 10:1 ratio will give a minimum output of 1.5 kw, some manufacturers as well basically fit the same burner to a range of boilers to keep manufacturing costs down and because the minimum output is governed by the burner size/characteristics then one might only get a effective modulation ratio of say 3:1 at the lower lower end of their power range but a effective modulation ratio of say 6:1 at the higher end of their power range. Combi boilers present a bigger challenge as the HW demand can be well over twice the CH demand and buffering may be the only realistic solution here but sort of defeats the purpose of the combi. I know two people who changed from oil to gas who found it very hard to understand/accept that their constantly cycling oil boiler gave better temperature control than their new gas boiler, at very low heat demands.
Having said that it does seem that some makes of gas boilers can perform better than other makes when cycling, all seem to perform fine as long as operating in their modulation range.
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Your first "bullet" point: is your room temperature at its SP temperature, with cycling?
Point2 & 3 seem to be stating that with the room stats at 25C that the boiler only runs for 8 minutes and with the room stats at 35C that the boiler runs for 35 minutes, this is very strange if one assumes that the room stat hasn't been satisfied in either case.

Excuse my ignorance, but I assume BF75 refers to the boiler flow temperature and HW60 refers to the hot water cylinder stat SP?

Yes thats correct .

The boiler is normally cutting off in cycles of 2.5 mins when the room temp is set anywhere between 21 and 25 . However as suggested when I ran the heating from completely cold , I did notice that the boiler cut off was much longer that is around 8 minutes both upstairs and downstairs .

If manufacturers offered boilers with high modulation ratios then IMO this would cover most demands without the added expense of a buffered system, manufacturers like Vokera offered 10:1 modulation years ago I think, if you had a heat only boiler with a heat requirement of say 15kw then a 10:1 ratio will give a minimum output of 1.5 kw, some manufacturers as well basically fit the same burner to a range of boilers to keep manufacturing costs down and because the minimum output is governed by the burner size/characteristics then one might only get a effective modulation ratio of say 3:1 at the lower lower end of their power range but a effective modulation ratio of say 6:1 at the higher end of their power range. Combi boilers present a bigger challenge as the HW demand can be well over twice the CH demand and buffering may be the only realistic solution here but sort of defeats the purpose of the combi. I know two people who changed from oil to gas who found it very hard to understand/accept that their constantly cycling oil boiler gave better temperature control than their new gas boiler, at very low heat demands.
Having said that it does seem that some makes of gas boilers can perform better than other makes when cycling, all seem to perform fine as long as operating in their modulation range.
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Your first "bullet" point: is your room temperature at its SP temperature, with cycling?
Point2 & 3 seem to be stating that with the room stats at 25C that the boiler only runs for 8 minutes and with the room stats at 35C that the boiler runs for 35 minutes, this is very strange if one assumes that the room stat hasn't been satisfied in either case.

Excuse my ignorance, but I assume BF75 refers to the boiler flow temperature and HW60 refers to the hot water cylinder stat SP?

If manufacturers offered boilers with high modulation ratios then IMO this would cover most demands without the added expense of a buffered system, manufacturers like Vokera offered 10:1 modulation years ago I think, if you had a heat only boiler with a heat requirement of say 15kw then a 10:1 ratio will give a minimum output of 1.5 kw, some manufacturers as well basically fit the same burner to a range of boilers to keep manufacturing costs down and because the minimum output is governed by the burner size/characteristics then one might only get a effective modulation ratio of say 3:1 at the lower lower end of their power range but a effective modulation ratio of say 6:1 at the higher end of their power range. Combi boilers present a bigger challenge as the HW demand can be well over twice the CH demand and buffering may be the only realistic solution here but sort of defeats the purpose of the combi. I know two people who changed from oil to gas who found it very hard to understand/accept that their constantly cycling oil boiler gave better temperature control than their new gas boiler, at very low heat demands.
Having said that it does seem that some makes of gas boilers can perform better than other makes when cycling, all seem to perform fine as long as operating in their modulation range.
[automerge]1578687456[/automerge]


Your first "bullet" point: is your room temperature at its SP temperature, with cycling?
Point2 & 3 seem to be stating that with the room stats at 25C that the boiler only runs for 8 minutes and with the room stats at 35C that the boiler runs for 35 minutes, this is very strange if one assumes that the room stat hasn't been satisfied in either case.

Excuse my ignorance, but I assume BF75 refers to the boiler flow temperature and HW60 refers to the hot water cylinder stat SP?

Sorry that I am typing my response all in one go as I am logged in with a device with a rather small screen at the moment and cant seem to get the multi quote working .

.The boiler is normally cutting off in cycles of 2.5 mins when the room temp is set anywhere between 21 and 25 . However as suggested when I ran the heating from completely cold , I did notice that the boiler cut off was much longer i,e around 8 minutes both upstairs and downstairs .
When the SP temp was 25 , I did measure with an infrared thermometer the temp on the wall facing the upstairs thermostat and it was 25.7 ( the thermostat is on a stairway directly facing the radiator on the stairs (bad design)) .

I really dont know why the boiler cuts off when the SP is set to max as with the old OFCH the heating would have gone on till eternity .

Yes the BF is the boiler flow temp

 

[Brambles]

The sad fact is these issues are not about TRV’s , weather compensation, boiler modulation or smart controls ( most of which are excellent). The issue is that the system has been installed without being properly designed - or in some cases not designed at all.

With the extensive variety of products ( both boilers and control systems) on the market, there is no excuse for “getting it wrong”.

I will await the onslaught!

 

[John.g]

Thanks, I am interpreting this (1) that the boiler is maintaining the SP temperature when in cycling mode so not a problem really apart from loss of boiler efficiency?
(2) that the 35 min run time is from cold rads but both zones on (all rads in service) and then cuts out, this is puzzling as it will take ~ 25/30 minutes to heat the water contents from cold to 75C, but once the BF reaches 75C then it should modulate down to maintain this temperature and shouldn't cut out (at 80C) because the total radiator output (~12/15kw) is way above the minimum modulating output of the boiler which is around 5kw.
I can only think of one or two reasons for this and that is that the boiler is not modulating or is too slow to modulate down before it reaches its cut out temp of 80C. The only reason that I can think of re no modulation is that someone has set the low limit output to the same as the hi limit output but this would be very very unusual, the boiler hi limit output is often adjusted (range rating) but the low limit is normally always set to its lowest limit.
I presume that the boiler then cuts back in again after this 35 mins? and does it then run in continuous modulating mode or does it start cycling again.

What does HW60 refer to?.

 

[mars_man]

Thanks, I am interpreting this (1) that the boiler is maintaining the SP temperature when in cycling mode so not a problem really apart from loss of boiler efficiency?
(2) that the 35 min run time is with cold rads but both zones on (all rads in service) and then cuts out, this is puzzling as it will take ~ 25/30 minutes to heat the water contents from cold to 75C, but once the BF reaches 75C then it should modulate down to maintain this temperature and shouldn't cut out (at 80C) because the total radiator output (~12/15kw) is way above the minimum modulating output of the boiler which is around 5kw.
I can only think of one or two reasons for this and that is that the boiler is not modulating or is too slow to modulate down before it reaches its cut out temp of 80C. The only reason that I can think of re no modulation is that someone has set the low limit output to the same as the hi limit output but this would be very very unusual, the boiler hi limit output is often adjusted (range rating) but the low limit is normally always set to its lowest limit.
I presume that the boiler then cuts back in again after this 35 mins? and does it then run in continuous modulating mode or does it start cycling again.

What does HW60 refer to?.

Hi John
I will have the house to myself tomorrow and would redo the test with with room thermostats set to 35 degrees .
I wouldn't know how to read the low limit output in the viessmann as its not a part of the display screen .
Will keep you all posted tomorrow ...

 

[mars_man]

I presume that the boiler then cuts back in again after this 35 mins? and does it then run in continuous modulating mode or does it start cycling again.

It starts cycling again although the cycles seem to be longer with higher thermostat temperature set .

Tested again from cold with temperature set to max on the thermostats and HW on.
The boiler cuts off after around 35 mins .It does though kick in again fairly quickly and then stay on for . I could not check how long it stays on for after that as I switched it off after 30 minutes .

What does HW60 refer to?.

Hi John
I will have the house to myself tomorrow and would redo the test with with room thermostats set to 35 degrees .
I wouldn't know how to read the low limit output in the viessmann as its not a part of the display screen .
Will keep you all posted tomorrow .

As above , the boiler does cut off .
.Installer came around today and thinks that this is an issue with the position of the thermostats .
He did spend time balancing the rads and adjusted the flow in the rads so that the rads closer to thermostat ask for more heat in a way and keep the heating on for longer . Not sure if it has made a difference .

I asked him to explain on the reason for the boiler cutting off at max temperature and he thinks that we are comparing a firebird which was probably set at a very high temperature ( 85, which I am not sure as we never did ) to a condensing boiler which will modulate down .

Wondering if should already start contacting Viessmann about it .Can they help in doing their own checks and suggest or that have to go through the installer ?

 

[John.g]

As above , the boiler does cut off .
.Installer came around today and thinks that this is an issue with the position of the thermostats .
He did spend time balancing the rads and adjusted the flow in the rads so that the rads closer to thermostat ask for more heat in a way and keep the heating on for longer . Not sure if it has made a difference .

I asked him to explain on the reason for the boiler cutting off at max temperature and he thinks that we are comparing a firebird which was probably set at a very high temperature ( 85, which I am not sure as we never did ) to a condensing boiler which will modulate down .

Wondering if should already start contacting Viessmann about it .Can they help in doing their own checks and suggest or that have to go through the installer ?

I had and have a feeling that this boiler is not in fact modulating at all!
you should fairly clearly hear the fan speed ramping down once the boiler reaches its SP, ie if your SP is 70C then once it hits this it should ramp down, as the burner cut out is, or should be SP+5C, if you get a chance you might try this again with everything opened up but you can reduce the SP to 50/55c temporarily to carry out this test. If you think that it is in fact not ramping down then I would immediately contact Viessman.
The Firebird, like all oil fired boilers will always reach its cut out temp because it cannot modulate and will spend its whole lifetime cycling.

 

[eagleeye]

Wondering if there are plumbing gurus here who can suggest what could be the issue .Recently changed to gas heating and have a viessmann vitoden 100 , 26 kw condensing boiler .The old oil boiler was a firebird 26 Kw and I have almost already started missing it .

The house is well insulated and has 12 rads /3 zones . I've noticed that the with the thermostats set to 21 deg as before , the rads are lukewarm and dont heat up quickly . The room temp slowly ramps to the set temperature as I can see that the thermostat keeps cutting off frequently .

I've changed the boiler flow temp from the initial setting of 71 deg to 76 and then 78 , but that dosen't seem to have made a big difference.
The only way to heat up to the same temp as before seems to be by setting the thermostat to around 25 , but even with that it seems that the boiler keeps cutting off and when its back on , its not ON for long enough to heat up all the rads .I read the boiler pressure and the boiler pressure is around near the notch near 1 bar .

Is it and old open vent system ? if so did the system get flushed before new boiler installed ? could be sludge build up causing the problem. Was a magnetic system filter fitted with new boiler ?

 

[mars_man]

I had and have a feeling that this boiler is not in fact modulating at all!
you should fairly clearly hear the fan speed ramping down once the boiler reaches its SP, ie if your SP is 70C then once it hits this it should ramp down, as the burner cut out is, or should be SP+5C, if you get a chance you might try this again with everything opened up but you can reduce the SP to 50/55c temporarily to carry out this test. If you think that it is in fact not ramping down then I would immediately contact Viessman.
The Firebird, like all oil fired boilers will always reach its cut out temp because it cannot modulate and will spend its whole lifetime cycling.

Will try that once more
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Is it and old open vent system ? if so did the system get flushed before new boiler installed ? could be sludge build up causing the problem. Was a magnetic system filter fitted with new boiler ?

eagleeye its a condensing Viessmann boiler and yes it was power flushed prior to install very recently .There was hardly any sludge build up in the system as the system was cleaned last year and the plumbing is 3 yrs old .
Magna filter installed
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Will try that once more
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I stand corrected on the boiler switching off at max thermostat setting as the boiler didn't cut off today . We came into a cold house today as I had kept the CH switched off since last night and its a tad nippy and the house felt cold around 17 degrees.
Cranked the heating to 32 degrees both upstairs and downstairs and the boiler hasn't cut off even once since the last 2 1/2 hours.

Spoke to Viessmann over phone earlier and they have asked to call again when I am next to the boiler as they wanted to go through a few "settings " from the display . Something about putting the boiler in "service mode" to do some checks if the boiler is turning off in that mode as well .

 

[John.g]

That's encouraging although a bit of a puzzle as to why it cut out yesterday with practically the same heat demand, if its still running you might just consider slowly turning down the boiler stat and you should hear it modulating down, it will be interesting to see what Viessmann find during the visit.

 

[Charles]

Wondering if there are plumbing gurus here who can suggest what could be the issue .Recently changed to gas heating and have a viessmann vitoden 100 , 26 kw condensing boiler .The old oil boiler was a firebird 26 Kw and I have almost already started missing it .

The house is well insulated and has 12 rads /3 zones . I've noticed that the with the thermostats set to 21 deg as before , the rads are lukewarm and dont heat up quickly . The room temp slowly ramps to the set temperature as I can see that the thermostat keeps cutting off frequently .

I've changed the boiler flow temp from the initial setting of 71 deg to 76 and then 78 , but that dosen't seem to have made a big difference.
The only way to heat up to the same temp as before seems to be by setting the thermostat to around 25 , but even with that it seems that the boiler keeps cutting off and when its back on , its not ON for long enough to heat up all the rads .I read the boiler pressure and the boiler pressure is around near the notch near 1 bar .

Hi I have had a similar problem with a similar boiler that was caused by the boiler always trying to achieve a temperature differential of no more than 20 degrees between the flow and return. If greater than 20 degrees the boiler simply cuts out. As crazy as it seems it was designed to do this. Viessmann recommended replacing the pcb since they have been made aware that this is causing problems for many users and have changed the internal boiler control algorithm on the new pcb. Yours might be dping a similar thing. I would suggest contacting Viessmann who I'm sure would be happy to help. One additional note - if this is the cause of your rads not reaching temperature be sure to check you have a decent flow through the boiler first, since restrictions in flow will cause a greater temp differential. Best of luck and please post omce you get to the bottom of it so we can all learn.

 

[John.g]

That implies that the boiler would be cycling continuously and might never reach its SP temperature. For example, a 25kw boiler with a flowrate of < than 18 LPM (1.1 M3PH) would cut out even from virtually cold at say 20C once the flow temp reaches 41C. Was your boiler behaving in this manner?

 

[mars_man]

Hi I have had a similar problem with a similar boiler that was caused by the boiler always trying to achieve a temperature differential of no more than 20 degrees between the flow and return. If greater than 20 degrees the boiler simply cuts out. As crazy as it seems it was designed to do this. Viessmann recommended replacing the pcb since they have been made aware that this is causing problems for many users and have changed the internal boiler control algorithm on the new pcb. Yours might be dping a similar thing. I would suggest contacting Viessmann who I'm sure would be happy to help. One additional note - if this is the cause of your rads not reaching temperature be sure to check you have a decent flow through the boiler first, since restrictions in flow will cause a greater temp differential. Best of luck and please post omce you get to the bottom of it so we can all learn.

Viessmann hasn't come back yet . They did offer to troubleshoot over the phone which I didnt want .
This boiler is now 2 months old , how old was your when you had this problem ?

 

[Joe.R]

A bit late to the party, but I have had a similar issue in my own house. The Vitodens replaced a 15 year old suprima in December.

The heating took quite a lot longer to reach temp, and I put this down to the weather compensation curve, however, the hot water cylinder went from 40 min reheats to 2 hours + (with the hot water relay for weather comp it’s target temp becomes 80 degrees)

Turned out it needed a low loss header. System works exceptionally well now, no cycling and quicker to heat (sub 25 min reheats on hot water, and 3 hours less on heating to reach 20 degrees - with flow temp that seems to average 55 degrees). I thought it was a little overkill for a 1930s semi, 20 rad, 1 unvented cylinder system...but...it cured the issue. Also, there are three heating zones on this system too - ground, 1st & loft.