Water temperature being affected by central heating dial

7 Dec 2014
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United Kingdom
Hi all,

Apologies in advance if I am using incorrect terminology and for the long winded story. The short of it is, when I turn down the temperature of the central heating at the boiler, the water in the unvented cylinder does not heat to the temperature set at the boiler for the water.

A couple of years ago we had a new central heating and hot water system (vaillant ecotec boiler and 300L main unvented cylinder) installed as part of a full house refurb before we moved in.

I have never bothered changing the temperature dials on the boiler until a couple of days ago when a British Gas engineer came round to service the boiler. I got into a conversation with him and told him how our gas usage was much higher than in our last house and he suggested changing the temperature of the central heating at the boiler to a lower temperature.

The boiler has two temperature dials - one for heating and one for water. I know already that unless I set a timer (or set to continuous run) on the thermostat for the hot water (the thermostat has a control for hot water and a seperate control for heating), the water in the unvented cylinder will not heat upwill not heat at all. The temperature at the boiler was set at 70 degrees for both water and central heating.

Based on the conversation with the British Gas engineer, it got me thinking if we could benefit from turning the temperature down of the central heating at the boiler to see if we could still be comfortable by changing our room thermostat down by one degree which could be offset by the fact that there would be a more continuous heat when the hosue was occupied rather than blasts at a higher temperature.

After turning down the dial at the boiler for the central heating to just 45 degrees and dropping the temperature on my room thermostat by one degree, we were finding the house to be comfortable with the temperature in the house, however by the time it got to the evening, we found the temperature of the hot water was not as high as we would have expected. Wanting to bath the kids, we put the hour boost on but again this did not help the temperature rise any further. Only after upping the temperature at the boiler for the central heating up to 60 did the water become hot enough to bath the kids.

Is this standard behaviour we are seeing or is this a sign of a problem with the install or the boiler? Any help would be much appreciated.
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Good question that I would like to know the answer to as well.
I'm curious to know whether it is better to put the boiler switch on high for the central heating on the assumption that the room thermostat and individual TRVs will then control the heating.

We previously had it the other way around where I had the boiler setting low but my family members would complain it was still cold. Im not sure which one is better energy saving though.
That dial is the running temperature of the boiler. . Ignore the symbols. . Unless you have the full Vaillant control system fitted.

You'll need to set the dial to about 70 degrees.

Unless your new house is identical to the old one don't panic too much by comparing running costs.
You can't send water at 45 degrees into a cylinder coil and expect it to heat the contents beyond that.

Set your boiler water temperature (heating) to no less than 60C, preferrably 65C.
You should have gathered by now that the hot water knob on your boiler does sweet FA, so set it where you like.

It's the heating water that warms your domestic hot water (DHW), and it needs to be hotter than the highest temperature that you want your DHW to be.

EY143: the most economical mode is with the heating OFF, beyond that keep the house as cool as you can for as long as you can stand it for minimum bills....but 'er indoors (and teenagers) may object.
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Thanks guys for responding, much appreciated. At least I understand it now.
That wasn't humour that was stating an obvious fact. And your comment wasn't sarcasm - I think you're confused on the meaning of words.
And your comment wasn't sarcasm - I think you're confused on the meaning of words.
If I had genuinely thought that you didn't know the definition of the word "humour" then my statement would not have been sarcasm. As it is, I did, therefore it was "sarcastic". Unless of course you're saying you DON'T know the meaning of it, in which case I apologise profusely ;) :LOL:. Ask someone for a dictionary for Christmas, you'll come on leaps and bounds!

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