Viessmann Vitodens 100 - Water Temp Modulation Query

[dimension999]

Hi there, I have recently had a sevice on my boiler following the following faults:

F4 error - got this resolved with Viessmann which was great.
F5 error - got this resolved in the service and saw the flap that got "stuck"....what a design!

Now after these annoying "issues" that happen after a few years of use are concerned being resolved, I have now found that whilst no fault codes are being reported, the hot water when on (all round the house) is heating up and then at temp 86 degrees the boiler stops to minimum water heat then starts back up at approx 72 degress back to 86 approx.

I have contacted the chap who serviced the boiler and he says that I need to throttle back the taps as the flow is too much for the boiler to cope with.......however I cannot see this as correct as I have tested with the taps at 1/4 turn which surely has the same effect....? It did the same thing with the 1/4 turn, we never open the taps fully anyway.

This did not happen before the faults I had reported and fixed and the service to resolve the flap problem.

I was wondering if anyone had any advice, and could confirm if the plumber is correct and I need to restrict the water flow through the taps!?

Can anyone shed any light on the issue and or if the plumber is correct?

Is this a part at fault or is this correct behaviour and down to water flow rate?

Cheers

Andy

 

[JohnD]

how much flow are you getting? Fill a bucket and time it, calculate how many litres per minute of hot water you are getting when it happens.

What is the temperature of the water out of the hot tap?

 

[D_Hailsham]

the hot water when on (all round the house) is heating up and then at temp 86 degrees the boiler stops to minimum water heat then starts back up at approx 72 degress back to 86 approx.

Are we talking in Celsius or Fahrenheit?

he says that I need to throttle back the taps as the flow is too much for the boiler to cope with

The rise in temperature of the hot water supply provided by a combi boiler is directly proportional to the power output of the boiler and inversely proportional to the flow rate. So doubling the power will double the temperature rise; halving the flow rate will double the temperature rise; though there may be a control on the boiler which limits the maximum flow temperature.

 

[Agile]

I expect that he is reading the boiler display which is in °C.

However, this displays the system temperature.

It sounds as if the plate heat exchanger is blocked by system debris.

Tony

 

[dimension999]

how much flow are you getting? Fill a bucket and time it, calculate how many litres per minute of hot water you are getting when it happens.

What is the temperature of the water out of the hot tap?

With regard to the litres per minute here are the results of that test:

Downstairs kitchen tap open full on hot: 10litres per minute.
Upstairs bathroom tab for shower open full on hot: 15litres per minute.

With regard to this temperature, I have to take the boilers word for it reading as 90 degrees celsius before cutting back/off to resume at 76 degrees celsius.

Does that help with diagnosing any potential issue?

The boiler is approx 3 years old and I think its the 30Kw version......

Your help is greatly appreciated......

 

[dimension999]

I expect that he is reading the boiler display which is in °C.

However, this displays the system temperature.

It sounds as if the plate heat exchanger is blocked by system debris.

Tony

Hi Tony,

Yes the temperature is in °C.......

Can you give me some more detail around the plate hear exchanger what does it look like? The plumber did dismantle the big bit in the middle (cylyinder like burner unit and hoover it out and brushed it out) when he was here......

 

[Agile]

The plate is a silver bit at the bottom back and about the size of a rather large pack of butter!

It needs to be replaced or specialist cleaned with acids.

The system may need to be cleaned if there is much dirt left.

The person who serviced the boiler should have been fully aware of the symptoms of blocked plates. Many dont though!

It results from the installer not properly power flushing the system!

Tony

 

[dimension999]

Is there anything else to check before I get this looked at?

It sounds expensive......I take it I will have to have the system power flushed as well as part of the resolution of the issue......?

Really appreciate the help here.

 

[Agile]

It needs to be confirmed that the plate is blocked but that sounds like a classic symptom.

What further treatment is needed to the system depends on how bad it is.

Its possible most of the dirt has been trapped in the plate and little is now left.

It might suffice to just fit a magnetic filter like a Magnaclean.

Tony

 

[dimension999]

Ahhhhhh, seems funny as the plumber said that he would have recommend that when the boiler was installed that a magnaclean should have been used........

And you guys don't think that the flow rate is too high for the boiler to cope with then?

 

[Agile]

A power flush removes dirt!

A Magnaclean traps dirt.

If the same person came who fitted it then he is hardly going to say the he failed to clean the system and its his fault !

Too much flow will just give cool water!

There is a flow restrictor fitted anyway.

Its simply a dirty system because it was not cleaned when installed!

Tony

 

[dimension999]

Just one note worth mentioning. When the boiler was installed, all the pipe work was changed for new pipework. Surely this would negate the fact that there should be dirt in the system?

 

[dimension999]

How can the plate being blocked be confirmed please Tony?

 

[Agile]

By measuring the temperatures on all four ports!

All new pipework still needs chemical treatment. Thats what X300 is designed for.

Tony

 

[dimension999]

Just checked and x400 was used to clean the system then x100 inhibitor is in the system.