"newshound" wrote in message
...
On 4/12/2016 12:25 PM, wrote:
On Tuesday, 12 April 2016 08:43:00 UTC+1, wrote:
Our hot water began to play-up when the boiler was just over a couple of
years old and the problem continues to this day (3 years down the line).
Annoyingly, we only changed the old one (10 year old Vailant combi)
after repeated attempts by BG service people, including replacement of
most major component, failed to fix exactly the same problem.

The problem is that the hot water is not reliably dispensed. When not
produced on demand, the boiler is not firing-up properly. It makes a
half-arsed attempt, with whirring of exhaust fan and a brief puff of
steam, but at the tap, the water is cold or lukewarm. Sometimes, though,
it plays ball and the hot water is fine.

Now here's the thing; if water is drawn simultaneously from a hot
outlet and certain cold outlets and/or a second hot outlet, the boiler
can usually be made to fire-up and away we go. In some cases, closing
off those additional outlets causes the hot water to wane.

In the light of the observations described above, it seems to me that
there might be an issue with water pressure, but it's not that the flow
rate is too high for the boiler to heat the volume passing through the
heat exchanger. I say this because i) draw from additional hot outlets
can remedy it, ii) when it happens, the boiler isn't firing properly and
iii) if anything, the problem tends to be worse in warmer weather.

I did, in fact, check our water pressure and found that while it was
rather high, it fell within acceptable operational range for the boiler.
Regardless, I wonder whether it's worth trying an adjustable pressure
limiting valve on the boiler inlet. I'm more open to a sporting chance
of a pragmatic solution rather than a dead certain lengthy
investigation!

What do you think? Worth a try, or total waste of time?

Cheers. Terry.

Combis can only modulate down so far, resulting in exactly the symptoms
you describe. It's one reason I'm not that keen on them.


NT


Agree, but I still think there is likely to be a fault in the control
system somewhere.

I suspect that gas repair guys increasingly follow automotive practice,
replacing one thing after another until they find the fix,

That isnt how automotive practice is done anymore.

The diagnostic system says what has failed and that is replaced.

then say that they all needed replacing. (Not to deny that this is
sometimes the cheapest strategy when you include labour costs, and *some*
of them probably understand the engineering).

Pressure limiting valves are not expensive, and useful if your cold water
is a bit "lively". I used to have one, but I'd guess it isn't the problem
here.