Viessmann vitodens 200 low modulation low run time high return temp

[monkeysballs]

ok so now back and the house is a nice 18 - 19 ish on a 0.8 curve but set to 15c!
looking at the curve it should be about 25c flow but im seeing highs of 30 ish in the pipes so that would be the +5 thats been mentioned what do i need to do ? burns have been alot fewer today 1 per hour except when we had showers and excluding showers im looking at 1kw/h but its only on for about 8 minutes but modulation at times hs been better but still mainly 23% ish and ive removed the range rating amd i think everything is back except the curve

 

[monkeysballs]

So it's been a bit better I'm now on a curve of .5 and +5

One thing I don't understand is why there is a dhw schedule on a combi? Also is there anything I can tweak to speed up the switch to dhw? It takes a bit too long for the Mrs to be happy

 

[Johntheo5]

I think you should maybe try and establish what exactly % modulation means as you purchased a fairly expensive boiler that allegedly can get down to 1.9kw output.
The next time its firing at the lowesr possible rate, note the % modulation and take a snapshot of your gas meter, then exactly 60 secs later take it again and you will then know if 23% (or whatever you noted over this one minute period) modulation is 7.36kw or 24.6kw, if it can get the output down to 1.9kw then IMO it should read 5.9% (OR maybe 94.1%).
If it does get down to 1.9kw then the one minute gas consumption should be ~ 0.003m3. If only getting down to 7.36kw the consumption will be ~ 0.011ms.

 

[Deleted member 281264]

I think you should maybe try and establish what exactly % modulation means as you purchased a fairly expensive boiler that allegedly can get down to 1.9kw output.
The next time its firing at the lowesr possible rate, note the % modulation and take a snapshot of your gas meter, then exactly 60 secs later take it again and you will then know if 23% (or whatever you noted over this one minute period) modulation is 7.36kw or 24.6kw, if it can get the output down to 1.9kw then IMO it should read 5.9% (OR maybe 94.1%).
If it does get down to 1.9kw then the one minute gas consumption should be ~ 0.003m3. If only getting down to 7.36kw the consumption will be ~ 0.011ms.

Do you think Viessman are telling fibs about the 1.9kw modulation?

 

[Johntheo5]

No, but if so, it will be reflected/veified by the fuel consumption?, was the commissioning carried out properly, ie minimum gas flow checked etc??.
It will aso show hopefully, what the % modulation actually means.

 

[monkeysballs]

Installer is back in a few weeks to carry out water quality test on the ch circuit as I'm running vdi2035. Will question him more then.

Just looking had 7 minute burn at 12 (5 minutes actually in 30 minute window then another 7 minute
So 12 minute total used 1.35kwh 60/12=5 and 5*1.35= 6.75 if it was actually running for an hour so that's more than I worked heatloss to be at -3 it is now +7

 

[Johntheo5]

So why should it cycle at all if the demanded fuel burn averages 6.75kwh and it can modulate to 1.9kw?.

What is the minimum range(d) output that you can set the boiler to?, is this in kw or % or what?, suggest setting this to its lowest.
It would seem that the boiler isn't modulating down to anywhere near its minimum otherwise it wouldn't cycle as often.

 

[monkeysballs]

Im getting 6-9 min burns usually 7
I agree I'd say I should be getting maybe 30min burns at 2kw if heatloss was as it thinks

 

[Deleted member 281264]

i've got an oversized 630 ecotec on weather comp minimum modulation is 5.8kw ish and it will go into anti-cycle mode 2 to 3 times an hour

your heatloss is 7kw and minimum output 1.9kw, run times should be much longer

With my system I've noticed small changes in settings can make a difference to consumption, I was having the opposite issue and the burner was staying on for too long because I had set a time controlled temperature that was too high and this was causing the boiler to burn constantly and wasting a lot of gas. Now that I've changed it to manual the boiler is cycling at resonable intervals and gas consumption is where I want it to be.

Either figure it out yourself or get an installer in who knows their stuff

 

[Johntheo5]

why not reduce the

Im getting 6-9 min burns usually 7
I agree I'd say I should be getting maybe 30min burns at 2kw if heatloss was as it thinks

Why not range the output to its minimum, this is normally the min boiler output and even if you can set it lower the boiler will still not go below its gas valve minimum setting (to give 1.9kw output), take the gas meter reading as suggested, note the % modulation and you will then have a few facts to give Viessmann, if warranted.

 

[vulcancontinental]

I've seen similar threads elsewhere with the 200's.

Not an expert on Viessmann's but I watch the posts closely. The company have put all their efficiency ambitions into the wide modulation which has considerations of it's own

There is no anti-cycle delay nor heating after hot water delay, it fires on demand.

There is a maximum and minimum fan speed, the % will be between those figures not zero and the max the fan can be used at.

Some boilers fire after ignition at a % of the max htg fan speed before modulating down as there's an improvement in efficiency in doing this, say 75% for 15 minutes then ramp up to max if demand has continued. So boilers do that then modulate ASAP; some boilers fire at the % of the range rated not max fan speed before modulating, that makes a difference

Boilers have to fire at a preset fan speed to ensure safe ignition. That fan speed is not the minimum fan speed but quite a bit higher which means when a system is up to temperature and you need minimum input it gets more than the system can adsorb so cycling occurs as it reaches setpoint quickly.

Personally I don't care about cycling if the run times are decent. The calculation is if longer than 180 seconds the loss of efficiency is around 1.5% dropping to zero the longer the burn.

The Viessmann has combustion control management which means it needs to fire, look at combustion, then adjust fuel and air to optimum mix, this delays modulating because it needs a reading before acting.

∆T is an advantage with high temp systems but less of an advantage with compensated flow temps as you don't have the 'headroom' to drop 20⁰C anyway.

It has a low residual head as the exchanger is quite restrictive which is why they like low loss headers and additional pumps. I think the residual head is around 1.5m, look at the manual, but on a small system that's sufficient mostly.

If the boiler is running hot the minimum output might be 1.9kW but if condensing at low temps it's 2.3 I think as you need to add the latent heat that's harvested.

Is the OP on Facebook?

 

[Johntheo5]

Do you know what the minimum that the boiler, a B2KF, 32kw, (possibly 27kw to CH) can be range rated to (from a menu) and what are the units?.

 

[Deleted member 281264]

I've seen similar threads elsewhere with the 200's.

Not an expert on Viessmann's but I watch the posts closely. The company have put all their efficiency ambitions into the wide modulation which has considerations of it's own

There is no anti-cycle delay nor heating after hot water delay, it fires on demand.

There is a maximum and minimum fan speed, the % will be between those figures not zero and the max the fan can be used at.

Some boilers fire after ignition at a % of the max htg fan speed before modulating down as there's an improvement in efficiency in doing this, say 75% for 15 minutes then ramp up to max if demand has continued. So boilers do that then modulate ASAP; some boilers fire at the % of the range rated not max fan speed before modulating, that makes a difference

Boilers have to fire at a preset fan speed to ensure safe ignition. That fan speed is not the minimum fan speed but quite a bit higher which means when a system is up to temperature and you need minimum input it gets more than the system can adsorb so cycling occurs as it reaches setpoint quickly.

Personally I don't care about cycling if the run times are decent. The calculation is if longer than 180 seconds the loss of efficiency is around 1.5% dropping to zero the longer the burn.

The Viessmann has combustion control management which means it needs to fire, look at combustion, then adjust fuel and air to optimum mix, this delays modulating because it needs a reading before acting.

∆T is an advantage with high temp systems but less of an advantage with compensated flow temps as you don't have the 'headroom' to drop 20⁰C anyway.

It has a low residual head as the exchanger is quite restrictive which is why they like low loss headers and additional pumps. I think the residual head is around 1.5m, look at the manual, but on a small system that's sufficient mostly.

If the boiler is running hot the minimum output might be 1.9kW but if condensing at low temps it's 2.3 I think as you need to add the latent heat that's harvested.

Is the OP on Facebook?

So the Viessman has no anti-cycling mode like the Vaillants do?

When I first got my Vaillant I was unhappy about it being in anticycling mode a few times an hour, but soon realised it boosts efficiency and is not noticable (heating wise) unless watching burner status in the VRC720

 

[JonathanM]

There is a maximum and minimum fan speed, the % will be between those figures not zero and the max the fan can be used at.

Do you mean that the user can set a minimum and maximum fan speed? I'm asking because the OP seems to have a problem where the power never goes below 23% and I'm wondering if inadvertently the minimum fan speed has been set to 23%.

 

[monkeysballs]

Not on FB
All I can see is set Max output as a %

As far as I can tell when I see modulation 100% means no modulation I've seen it when using dhw some times
This is today's modulation history so 19 hours and the burn times