Flow temperatures below 50deg with a hot water cylinder

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I have a Vaillant ecoTec plus 618 boiler and a Heatrae Sadia premier plus PP250B hot water cylinder.

I have central heating radiators and underfloor heating, I want to reduce my boiler flow temperatures to make it more efficient.

I understand hot water cylinders should store water at atleast 60 degrees to prevent legionnaires disease.

The hot water temperature for our cylinder is set on the cylinder itself and not controlled by the boiler.

Everywhere seems to suggest you can't set flow temps on the boiler below 60 degrees if you have a hot water cylinder due to legionaires, but is this still the case if the boiler doesn't control the hot water temperatures?

Our hot water cylinder is set to 65 degrees. Does this mean it requests water at this temperature from the boiler? I'm not really sure how that part works.

Out boiler flow temperature is currently set to 65 degrees, the max flow temperature set in boiler settings is the default 75 degrees.

if I set the boiler flow temperature to say 50 degrees, would this potentially lead to issues with the hot water cylinder even with that set to 65 degrees?
 
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The cylinder thermostat just switches the boiler off when the hot water reaches the set temperature.
If the cylinder is set to 65 and the boiler less than that, the hot water will never reach the set temperature - it can only be as hot as the boiler is set to.

Options - set the cylinder to 60 and the boiler to 65
Or go lower on both, but adjust the controls manually once a week to heat the cylinder over 60.
There are ways to automate that and have a different boiler temperature for hot water and heating, but new controls will be required.
 
If you have an elictrical immersion heater, set your boiler to the 50 degrees that you want and set the immersion to 65 and either use a timer or manually switch the immersion on once a week to act as a legionella prevention, as alread said some modern controls have this function built in, the small amount of electricity used to take from 50 to 65 once a week wont be very expensive and will be offset with the boiler running at a lower temperature
 
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We do have an elictrical immersion heater but no temperature controls on it, a simple on/off switch. But yes I could set the cylinder to around 65 with the boiler flow at 50. Then stick the immersion on once a week for a couple of hours, have looked at automated ways of turning the immersion on.

Might investigate this further and see what the easiest way to turn the immersion on/off automatically would be, will also look at the option of new controls on the boiler to set different flow temps for the hot water cylinder, central heating and UFH. Ideally I could get different flow temps for all three systems.
 
If you have an elictrical immersion heater, set your boiler to the 50 degrees that you want and set the immersion to 65 and either use a timer or manually switch the immersion on once a week to act as a legionella prevention, as alread said some modern controls have this function built in, the small amount of electricity used to take from 50 to 65 once a week wont be very expensive and will be offset with the boiler running at a lower temperature

I've been thinking about doing this, but my (top entry) immersion only seems to heat the top 40% or so of the cylinder. Do some immersions heat the whole cylinder? (Just asking about indirect cylinders here)
 
I've been thinking about doing this, but my (top entry) immersion only seems to heat the top 40% or so of the cylinder. Do some immersions heat the whole cylinder? (Just asking about indirect cylinders here)
your immersion will have a thermostat , you need to remove the cap ( with power off) and adjust it
 
your immersion will have a thermostat , you need to remove the cap ( with power off) and adjust it

The water gets really hot, but it only seem to be the top bit of the cylinder, however long it is on for. Can that be true?

EDIT: We've always assumed the immersion must be quite short.
 
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