Combi Boiler running colder?

I have an 18 month old Viessman Vitodens 100 Combi Boiler.

Recently it seems that the hot water coming ouyt of the taps is coooler than it used to be.

With the hot tap running and the heating off: Checking the display on the front of the boiler claims the burner is on full and that the output temperature is about 70 degrees. But water temp at tap is about 45 degrees. I am sure it used to be hotter (But I have no figures from then - I did not measure it)

Turning the boiler control up to maximum does not increase this any further (it is doing as much as it can already)

I can get the water at the tap a little hotter by reducing the flow a small amount. But reducing any further and the boiler cuts down the power, so I just get less water at the same, somewhat low temperature.

Question: Is this a common fault in Combis (my first). What is causing it?

Reply to
Neal
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Normal Combi behaviour in the winter when the mains water is very cold. The spec will give you a "temp rise at x litres/ min" If the water starts off very cold and the flow is high then the resulting outlet will be cooler. (Or you could have a problem!)

Reply to
John

Yes. That makes sense. Unfortunately this is a recent (as in the last few weeks) issues. So all during the cold weather in Jan and early Feb, the water was hotter than it is now. So that does not seem to explain it.

Reply to
Neal

Worth checking that no heat is escaping into the heating circuit whilst running H/W(symptom of diverter valve problems)

Reply to
John

In message , Neal writes

Sounds like its not modulating. You will have a main valve which provides a bulk of the heat , and a modulation valve which fine tunes the heat to suit the requirement

So - modulation valve not operating or the pcb not driving it

Turning the flow rate down too far means that you are probably not getting heat out of the heat exchanger fast enough and, reaching temp, its cutting out

each boiler, be it combi or not, has its own faults

Reply to
geoff

Are you in a hard water area? Scaling of the secondary heat exchanger could result in it being unable to extract the heat from the primary flow at the rate it used to.

(if this is the case, then it usually a fairly simple job to fix)

Reply to
John Rumm

Output DHW temp is never more than 60C. If it is 60C then it is delivering the DHW. Is the tap a mixer and passing? Cold water into the hot?

Reply to
Doctor Drivel

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It's possible that the water you were drawing in Jan / Feb was coming from a source that was heated (e.g. reservoir) in previous months and which was eventually topped up by rain water cold enough to bring the temperature down to the level you're getting now. It depends on how quickly water works its way through from source to user.

Give your system a chance to catch up before fearing the worst.

Cic.

Reply to
Cicero

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