Viessmann vitodens 200 low modulation low run time high return temp

[monkeysballs]

Hi brand new install of a 30kw 200 combi
And it isn't doing what I'd expect or hoped
It's set with a 50c Max temp range rated to 23% heatloss is less than 7kwh
But it is having very short burn runs of about 5-7 minutes
And I'm only getting about a 10c drop between flow and return yet I designed the rads to give me 20c
I've turned min and max pump speeds down to 5/20 yet hasn't made a difference

I'm using preset rad valves so flow through the rads should be as designed
Any thoughts

 

[Johntheo5]

What exact model Vitodens 200 have you got, some models can modulate down to < 2.0kw but other models are > 5 to 7kw.
What do you mean by pump speeds down to 5/20?.

This model goes as low as 1.8kw and max of 35kw.

 

[monkeysballs]

B2kf is what I have it's been in 3 days

You can set min and max pump speeds so it's set to 5% min 20% max to try and get the return lower I only need max of 285l/h in theory to satisfy my rads

 

[monkeysballs]

My biggest issue though is the low run times I'd have thought it would run longer than 7 minutes

 

[Johntheo5]

B2kf is what I have it's been in 3 days

You can set min and max pump speeds so it's set to 5% min 20% max to try and get the return lower I only need max of 285l/h in theory to satisfy my rads

That model is 1.9kw to 32kw, so very unlikely that the heating demand is less than 1.9kw.
taking you literally that the pump speed is set to 5% then assuming a 7M pump head at 100% speed means that the pump head is only 1.57M which will scarcely circulate the rads, 20% speed is a pump head of 3.1M which should be OK, also remember that a flowrate of 285LPH or 4.75LPM will still only result in a boiler dT of only 5.7C at min output of 1.9kw, you have range rated the boiler to 7kw which would result in a dt of 21C and 7kw output at 4.75LPM.

Suggest watching the boiler flowtemp on its next cycle, if, for any reason, it reaches 55C (SP+5C) then it will trip the burner, the circ pump will remain running and the burner will refire based on the anticycle time, the flowtemp will have fallen to 30c or less during this period.
Presume there is a continuous (CH) call for heat while this cycling is occuring and that the boiler controls/smart thermostats arn't causing this?.

 

[monkeysballs]

It's running weather comp and the flow im seeing from reading from the flow pipe is 33c

 

[monkeysballs]

Ok I just asked it to heat the house to 19 from 18
And I've altered the curve to 1.0

 

[Johntheo5]

Can you post your heat curves and the present outside temperature?, we can then see what the flowtemp SP is and what the actual flowtemp is.
You might also increase the required roomtemp to 21C temporarily, if you have a digital roomstat then the stst will be switching the boiler in/out once the room temp changes by only 0.1C, mine does this with a oil fired boiler, the boiler might fire for 10 minutes and can be off for 15 minutes or longer.

 

[monkeysballs]

5.5c external temp. No room stats I'm just trying to maintain desired temps by restricting rad flow using the ras-b2 valves but I do have temp and humidistats in all rooms logging

 

[JonathanM]

It's running weather comp and the flow im seeing from reading from the flow pipe is 33c

And I'm only getting about a 10c drop between flow and return yet I designed the rads to give me 20c

You won't get a 20C drop if your flow is only 33C. That would mean the return would have to be 13C, which is lower than the temperature of the house, so that isn't possible. On low temperature systems, Viessmann I think (maybe somebody else), suggest a different measure for the drop in temperature between flow and return. I've forgotten what it is exactly, but it's either 30% of the flow temperature, or 30% of the difference between the flow temperature and the room temperature; something like that anyway.

 

[monkeysballs]

Ah makes sense I suppose it was 20c at 50c flow that it was worked out at

 

[Johntheo5]

Generally, with low temp systems you might work with a relatively high flowrate to give a low dT so that the rads can output their greatest output with a low flowtemp of say 33C to give a reasonably quick room warm up from a cold start in the morning.
So a OT temp of 5.5C on curve 1 means a boiler flow SP of ~ 33C and a burner trip if the flowtemp is > 38C and so on, again, if boiler modulating properly then the heating demand must be < 1.8kw?.

 

[monkeysballs]

Last 3 hours of modulation attached.

I do think it's getting too hot as looking at my flow (supplied by the boiler rather from my sensors on the pipes also attached)

So what should I do to get it to run longer/modulate more?

 

[JonathanM]

Last 3 hours of modulation attached.

What is the second graph "C.distinct"?

In the OP did you say the boiler is range rated to 23%?

I don't understand the modulation graph. Is this downloaded from the boiler, or is it a plot of your own readings. Are the dots your own readings. This boiler should modulate down to about 6%, but it never seems to go under 23%. Is the modulation level the % of the full boiler output or the % of the range rated output.

 

[monkeysballs]

C distinct is what the boiler is saying is flow temp

Yes range rated to 23 on basis that I only need less than 7kwh at -3

This modulation is from the boiler I assume it's 23% modulated of 100% so 77% output

This the last 2 days since it's been actually "working,"

It got to 93.5 this morning