Boiler short-cycling on HW

It suggests to me that your cylinder heat coil transfer rate is too low and probably your boiler is over rated for the house!

Do you really have a whole house heat loss of 22 kW ???

AND 22 kW of radiator output?

You say its a foamed cylinder but the old yellow ones had a pretty miserly heat coil and if thats a bit scaled up then its not surprising that it cannot absorb 8 kW when the differential is only 15°C.

Change cylinder, get a lower power boiler, increase the differential or just ignore the cycling!

Tony
 
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The question I would be asking is: why is the boiler going on/off and not modulating?

With an OV system you must have either Y-plan or S-plan, which do you have? I'm wondering if there could be an error in the wiring which is making the boiler go on/off instead of modulating.

I agree with others about HW temperature, it needs to be about 60°C.

How water temperatures affect Legionella
70°C to 80°C - Disinfection range
66°C - Legionella die within 2 minutes
60°C - Legionella die within 3 minutes
55°C - Legionella die within 5 to 6 hours
20°C to 45°C - Legionella multiply
20°C & below Legionella are dormant

One thing that intrigues me is: why have a temperature control on a modulating boiler? Surely the modulator should decide what the water temperature needs to be? I would be inclined to set the boiler stat to max and let it get on with it.
 
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D_Hailsham said:
One thing that intrigues me is: why have a temperature control on a modulating boiler? Surely the modulator should decide what the water temperature needs to be? I would be inclined to set the boiler stat to max and let it get on with it.
Click to expand...

The boiler provides whatever heat output is required to provide the user set flow temperature.

In Johns case that includes turning the boiler power down as it aproaches the set point and when the set temperature is reached and 8 kW of output is causing the set temperature to be exceeded then turning the boiler off and instigating the anti cycling delay.

Tony
 
JohnD said:
I turned down the cyl stat to increase the temp differential, I will turn it up again if it makes you happy :) though 60C feels scalding to me
Click to expand...

I agree, 60C will feel too hot so mixing it with cold will be necessary, but you won't have any bugs in it ;)
 
Agile said:
The boiler provides whatever heat output is required to provide the user set flow temperature.
Click to expand...
But how does the user know what is the correct temperature to set on the boiler. Joe Average will turn the stat down, thinking that will save gas; but it may not, if the boiler is now unable to produce the necessary heat.

In Johns case that includes turning the boiler power down as it aproaches the set point and when the set temperature is reached and 8 kW of output is causing the set temperature to be exceeded then turning the boiler off and instigating the anti cycling delay.
Click to expand...
I can understand that - the boiler can only modulate between a max and min level. When it has reached the bottom, it will then have to cycle.

But, if the boiler had been allowed to run at max temp, the hot water would have been heated up quicker and the cylinder stat stopped the boiler before the boiler had reached the lowest level of modulation. I expect there is a flaw in that argument. ;)
 
unless we know what the flow and return temps are, there's no answer for this problem, just alot of guessing.... :rolleyes:
 
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Do the Viessmann not have the F & R temps available in the on board diagnostics?
 
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JohnD said:
Boiler instrs say leave the boiler stat at the "normal" blob but you can turn it up if e.g. in exceptionally cold weather rads do not get house hot enough.
Click to expand...
The Installation manual says:
The boiler water temperature must be set at an adequate level to satisfy the requirements of the heating system.

Syst has a 3-port valve, I believe you call that Y-plan?
Click to expand...
What is the valve make/model?
Is this a new valve or was it there previously?
Some systems only work properly with a diverter (HW priority) and not with a mid-position. The Viessman Manual refers to diverter, so that needs to be checked out.

What controls (programmers, thermostats do you have)?
 
My personal view is either John has been messing about with the system/commissioning or he's looking for a problem that doesn't exist.

Take a holiday to Jamaica John and reeeeelaax man.
 
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