Possible problem with Worcester 28CDi boiler

Hi,
We had a new central heating system installed just over a year ago but the tempature of the hot water seems to be variable at different times of the day and in different parts of the house. For example yesterday evening the water was probably just better than warm from the tap in the bathroom but the kitchen tap this morning was reasonaly hot. But at other times the kitchen tap is not that hot either.
The boiler is a Worcester 28CDi combi.
There is a dial on the front of the boiler which allows for the tempature of the hot water to be increased, I have put this on max but it has made no percievable difference. The preasure shown on the dial is also quite low when there is no demand for water (about 0.5 bar), but this increases to well over 1 bar when there is demand.
One thought I had is that the boiler only kicks in when the demand gets to a certain preasure, is it possible to adjust the boiler to kick in when the demand is not quite so high?
The boiler has allways been like this and I've never realy bothered too much, it's more of an irritation that at some points the water is not that hot.
Thanks Andy
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You might find that when the temperature is cold, it is because the mains pressure is high and promoting a higher flow rate. Next time the water only gets tepid, take a container of a known size and time how long it takes to fill. Post back with the calculated flow rate.
Your combi boiler is capable rising water temperature by 35C at 11.4lpm. This time of year, your mains are probably around 5C. So, if you get around 11 lpm, the water temperature should be 40C, which is warm, but not exactly hot. If you expect water at 55C, this is an extra 15C rise, which will drop the flow rate to around 8 litres per minute. If you draw more water, the temperature drops. There is nothing that can be done to improve this, except by replacing the boiler with a much larger model, or installing a hot water storage system.
As for the pressure gauge, the water in the system will expand considerably when hot. This would cause a rapid pressure rise, but is absorbed by the expansion vessel instead. This is usually sized to limit the pressure rise to around 0.5 bar or less. When cold, the system should be pressurised to 1 bar. So, next time the system is cold, attach the filling loop hose and increase the pressure to 1 bar. Don't forget to disconnect the hose afterwards.
Christian.
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Chistian,
Thanks for your comments. I suspected much of what you said as it makes sense. I can easily refill the system to 1 bar, the plumbers fitted a pipe with two taps under the boiler to make this easier.
Thanks Andy
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Andy Coleman wrote:

AFAIK the flow rate is preset on the flow switch.

should be 1 bar. When hers drops to 0.5 bar the operation of the boiler becomes unreliable..
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