Viessmann Vitodens 100 WB1A Fault

[dawson6777]

Hi Peeps, I have a fault developing on the above boiler and have had expert advise from this forum in the past, hopefully someone can help me again.
The boiler DHW is playing up, (CH is fine though). We have noticed the hot tap (but ONLY on the Bath, which is a mixer/shower tap with wide aperture) is hot initially then luke warm. But if I throttle the tap down to a slower flow it seems to be hot again). All other taps and other shower are fine. Is this a fault developing with the flow switch possibly?

Regards, Dawson

 

[ollski]

What happens when you reduce the flow rate?

 

[JohnD]

And did this behaviour stat to occur when the cold weather came?

Typical of a combi.

 

[dawson6777]

What happens when you reduce the flow rate?

The water goes warmer.

 

[dawson6777]

And did this behaviour stat to occur when the cold weather came?

Typical of a combi.

Since around Christmas time.

 

[JohnD]

Is that a "yes?"

 

[Agile]

You need to measure the hot water flow and ideally the temperature of it.

Tony

 

[dawson6777]

And did this behaviour stat to occur when the cold weather came?

Typical of a combi.

No not really. It was cold before Christmas too, but the boiler seemed ok then. The missus like a bath you see, but I always shower.

 

[dawson6777]

You need to measure the hot water flow and ideally the temperature of it.

Tony

I have just run off 5L of hot from the kitchen tap in 20 secs from a cold start and ended at 32 deg C. Very basic method!

 

[JohnD]

A combi boiler can deliver X amount of heat. This means it can heat Y amount of water by Z degrees.

But if you increase Y, then Z will go down, and vice versa.

A bathtap delivers more water than a sink tap, so it will be less hot.

If Z is, say, 35 degrees for 11 litres per minute,
then in summer, tapwater might be (incoming main 12C plus 35C)= 47C
but in winter, tapwater might be (incoming main 4C plus 35C)= 39C

You are looking at a bathtap, in winter, so it is normal to be less hot.

If your figures are like that, sadly that's just what combis do.

However (you will have to turn the radiators off and let them go cold to check this) if the radiator pipes warm up when you run a bath, then you do have a fault.

 

[dawson6777]

A combi boiler can deliver X amount of heat. This means it can heat Y amount of water by Z degrees.

But if you increase Y, then Z will go down, and vice versa.

A bathtap delivers more water than a sink tap, so it will be less hot.

If Z is, say, 35 degrees for 11 litres per minute, with a 30kW boiler,
then in summer, tapwater might be (incoming main 12C plus 35C)= 47C
but in winter, tapwater might be (incoming main 4C plus 35C)= 39C

You are looking at a bathtap, in winter, so it is normal to be less hot.

If your figures are like that, sadly that's just what combis do.

However (you will have to turn the radiators off and let them go cold to check this) if the radiator pipes warm up when you run a bath, then you do have a fault.

Thanks John, I will just turn up the heat a little at the boiler and see how it goes.

 

[Agile]

You have not told us your boiler power.

You have 15 litres per minute and no normal combi boiler can heat so much water.

Reduce the flow rate by partially closing the water in valve under boiler.

Reduce to 9 li/min if 24 kW boiler and 11 li/min if 28 kW.

Then you may start to see warmer water.

During the winter you actually need a lower flow rate but then you would need to increase to these figures in the summer.

Tony

 

[dawson6777]

You have not told us your boiler power.

You have 15 litres per minute and no normal combi boiler can heat so much water.

Reduce the flow rate by partially closing the water in valve under boiler.

Reduce to 9 li/min if 24 kW boiler and 11 li/min if 28 kW.

Then you may start to see warmer water.

During the winter you actually need a lower flow rate but then you would need to increase to these figures in the summer.

Tony

Thanks Tony, I will try this.

 

[dawson6777]

You have not told us your boiler power.

You have 15 litres per minute and no normal combi boiler can heat so much water.

Reduce the flow rate by partially closing the water in valve under boiler.

Reduce to 9 li/min if 24 kW boiler and 11 li/min if 28 kW.

Then you may start to see warmer water.

During the winter you actually need a lower flow rate but then you would need to increase to these figures in the summer.

Tony

Hi Tony, Just to let you know I reduced the flow rate as advised and it see s good.
Thankyou for your advice and also to others for their inPut.
Regards, Dawson