Viessmann Vitodens 100 WB1A System Boiler Modulation Fault

jrling · 15 Apr 2019

I have a Viessmann Vitodens 100 WBIA open vent system boiler (not combi) 24KW Natural Gas running CH only with a dozen rads, so a high heating load. About 10 years old. Until recently it worked perfectly and the system is unchanged.

I run the boiler thermostat at just above the 60C mark on the dial and an external room stat controls the temperature in the conventional way. Nothing clever.

Recently, it only fires at two bars max on the modulation scale even from cold and therefore the system is taking a long time to heat up to temperature, but it does reach 62C eventually. Even if I turn the boiler dial up to max 80C, it only goes to three or four bars for a short time and then back down to 2 bars. It is condensing fine but only operating on low power.

The Grundfos 15-60 circulating pump is new. I have a MagnaClean filter fitted with Fernox X-100 Inhibitor in the system and it does not trap much on the magnet when I clean it. I have flushed the system with Fernox.

The boiler temperature thermostat tests at correct resistance on multimeter. I have tried pushing the ‘Reset’ button on the boiler, but no change.

There are no Fault Codes being displayed.

Any advice what to look for or how to test for fault would be appreciated.

Many thanks

DavidLewis · 23 Apr 2019

I too have a WB1A, and I see the same symptoms. My experience is that to fix it, every year or two I have to remove and flush through the rubber hoses because they become blocked with black "granules". Known as "crunchy hose syndrome" I believe. This leads to much reduced circulation, causing the boiler to modulate down.

JohnD · 23 Apr 2019

My WB1A doesn't have rubber hoses.

I've heard that they can collect scaly deposits on the sharp bend in the metal return pipe that enters the heat exchanger, top left as you look at it.

DavidLewis · 23 Apr 2019

JohnD said:
My WB1A doesn't have rubber hoses.

Depends on the age, I suppose. On mine, parts 019 and 020 on the diagrams on the below page are rubber.

https://www.viessmannspares.co/AA_W...ick_on_images_to_magnify/p917437_8839287.aspx

JohnD · 23 Apr 2019

ah, that one's not open vented, and the pipe layout is different

from part 21 I see it is also a combi.

DavidLewis · 23 Apr 2019

JohnD said:
ah, that one's not open vented

True, mine is sealed. I'd overlooked that the OP's was open vented. I hadn't realised the boilers were so different.

from part 21 I see it is also a combi.

Mine's a system boiler, not a combi, but the rubber pipes are those in the diagrams I linked.

JohnD · 23 Apr 2019

here we are

this is an 11-year-old photo.

I believe it's the brassy looking bend LHS of the round canister that holds the burner and coiled HEX. It does look surprisingly tight and small.

View media item 4839
and here's a wider shot

View media item 3394

jrling · 23 Apr 2019

Thank you both JohnD & David Lewis.

As you say, I have the system boiler (actually with the CH pump external not in the case) and it does not have the rubber hoses, but all copper tubing. See attached picture. I have read the flow sensor (black plastic fitting top left), can get gunked up and slows the flow and the boiler modulates to compensate. But I drained the boiler and took out the sensor and it was clean as a whistle. I had used Fernox flushing solution in the boiler a few weeks before. So I don't think the copper tubing is the cause.

In the second picture - the back of the control dials etc. which folds down, there is an adjustable pot (marked R14 and the left of the two); it was adjusted with the arrow fully clockwise at 5 o'clock. I had read that it could control modulation and wound it a little anti-clockwise to 2 o'clock as in the picture. When I turned on the boiler, it immediately fired up at 4 bars and kept there until the temperature was in the 50s when it backed of down to 2 bars as it approached target 60C setting. Magic!

I don't know what that pot controls. Do you? Anyhow up, so far so good.

JohnD · 23 Apr 2019

Seems a bit rash to be opening it up and adjusting things without knowing what they are. You should be using an RGI with Viessmann experience.

AGAS · 23 Apr 2019

jeez

ianmcd · 24 Apr 2019

jrling said:
As you say, I have the system boiler

You dont have a system boiler, you have a heat only boiler, they are not the same

JohnD · 24 Apr 2019

Perhaps you have the Compact model. Is it 350mm wide?

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Viessmann Vitodens 100 WB1A System Boiler Modulation Fault

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