Viessman Vitodens 100-W Low-rate CO₂ Problem

Viessmann Vitodens 100-W CO2 combustion content check
Checking CO₂ content at high and low combustion rates

We serviced a Viessmann Vitodens 100-W B1KC combi boiler. The boiler was less than 2 years old. It was flued vertically with two 135° bends. The flue was bracketed and screwed and stable.

We carried out a flue duct integrity check which showed no problems. This test is done by checking for elevated levels of CO and CO₂ at the inlet air sample port, with the boiler running on maximum and the boiler outer case closed. Levels for both CO and CO₂ stayed at baseline and O₂ levels were also unchanged.

After a number of visual checks, including checking that the condensate trap was not blocked, we connected a combustion analyser to the flue gas test port and set the boiler to maximum. Viessmann call this the upper heating output , 100%.
Combustion analysis was very clean and within the manufacturers’ tolerances.

The Installation and servicing manual says that, at maximum output, the Vitodens 100-W should have CO₂ between 7.5% and 10.5%. The measured figure was 9.1%.
O₂ should be between 7.5% and 3.2% and the measured figure was 4.8%.
Ratio should be less than 0.004 and the measured figure was 0.0006
So there were no problems with combustion at maximum (upper heating output, 100%).

Problems with Vitodens 100-W low rate combustion

The problem was with low rate combustion (lower heating output, 20%).
Combustion was, once again, very clean. CO₂ was 7.9%, CO was 7 parts per million and O₂ was 7.1%.
However, the CO₂ figure was too low. The servicing instructions require the CO₂ figure at low rate to be between 0.5 and 0.9 percentage points below the CO₂ figure at high rate.
In our case that would require the low rate CO₂ figure to be between 8.2% and 8.6%.
Our low rate CO₂ figure was 7.9%.

The servicing instructions say that if the low rate CO₂ content is not within the required range, the flue system must be checked for tightness and any leaks remedied. Failing that, the instructions say “Replace the gas train if required.
The “gas train” means any part through which the gas passes, including the gas valve.

No problems with the flue

The flue looked to be correctly installed and we’d done a flue duct integrity check, which showed no problems, so we did not suspect the flue.
That left the gas valve and, given the clean-ness of the combustion figures, we doubted the gas valve was faulty.

Viessmann Technical telephone support

The boiler was less than 2 years old we wanted to talk to Viessmann Technical Support. We guessed that the boiler might still be under warranty, though our customer couldn’t confirm that.

We rang Viessman’s number and went through the menu system to be connected to Technical Support. All we got was a recorded message which said “During these challenging times, and to ensure that we can continue to offer you a 1st class service, we recommend that all of our customers should use the email support option rather than the telephone.

Back in the office later (Friday late afternoon) we emailed Viessman Technical Support:

Dear Technical Team

I serviced a Vitodens 100-W B1KC boiler today.
I have attached the combustion results as a pdf file.

The boiler was installed by someone else, between 1 and 2 years ago.
It is vertically flued with two 135° bends. The flue is bracketed, screwed and stable.

I ran a flue duct integrity check, with the boiler set to maximum and the outer case closed.
I sampled via the incoming air sample point and the point was well sealed round the analyser probe.
The incoming air showed no reduced level of O₂ and no elevated levels of CO or CO₂

I set the boiler to maximum rate (100) and tested via the flue sample point. The combustion was clean and stable.
I set the boiler to minimum rate (20) and the combustion was again clean and stable.
However, the Installation and Service Instructions [Page 38, Checking the CO₂ content (cont.) point 5] say “Check the CO2 content for the lower heating output (20 %). The CO2 content must be between 0.5 and 0.9 % below the value of the upper heating output.”
My CO₂ reading for the upper heating output was 9.1% and my CO₂ reading for the lower heating output was 7.9%, giving me a difference of 1.2 percentage points.
Point 6 (on the same page) says “If the CO2 content is not within the indicated range, check the balanced flue system for tightness; remedy any leaks. Replace gas train if required.

I do not think the air/flue duct is wrongly installed or faulty.
That leaves me with the option of replacing the gas train.
Are my results acceptable and, given the results, is the boiler safe to run?
If it is not safe, and repair work is required, is the boiler still under warranty (I think it was fitted January 2019)?

I’d be grateful if you could get back to me as soon as possible.
Thank you

Kind regards

Viessmann replied early on Monday morning:

Good Morning Paul,
Apologies for the delay in replying.
At 7.9% it is a very clean and safe combustion. Although it is slightly out of the 0.5-0.9 it is the safer side of the line so we would not be overly concerned with this reading.   

Kind Regards
Viessmann Technical Support

We’d hoped that Viessmann would take ownership of the discrepancy and tell us definitively that 7.9% was an acceptable figure for low-rate CO₂. That way, responsibility for passing the boiler as safe to use would be theirs. They opted for a fudge instead, only implying that the figure was OK.
It’s interesting that the line in their email reply was formatted differently, as if it had been cut and pasted. We’d guess they’ve been asked this question several times before.

And if we had gone ahead and changed the gas valve, would the low rate CO₂ figure with a new gas valve have been any different? We doubt it!