"Ed Sirett" wrote in message
news
On Fri, 25 Mar 2005 23:55:06 +0000, andrewpreece wrote:


"mish" wrote in message
om...
Our combi boiler (Vaillant TURBOmax+ 28) has recently started playing
up. The CH is fine, but the DHW starts to run hot, then alternates
(just about -- nowhere near as hot as it was running a couple of weeks
ago) hot and cold.



Not sure about what the problem is, but I'm fairly confident that the
lcd
display
shows the boiler loop temperature, not the actual DHW temperature. The
DHW
temperature is controlled open-loop as it were, I measured the DHW
temperature
as being about 20C lower than the LCD display indicated temperature.
This
also means
the DHW temperature varies with the temperature of the groundwater, so
you
need to adjust the
front panel temperature knob a few times per year. The display gives you
the
impression
that the DHW temperature is settable to within a degree C if you choose
the
DHW menu,
but it's only indicative, I'd ignore that menu if I were you.
Your display temperature of 88C will translate to a DHW temp of
about 65C. In fact
fully anticlockwise the temperature knob should give you a temperature
rise
of 30C over the groundwater temperature, fully clockwise that rises to
an
increase of 47C, at least it does
on my Turbomax. I tell you this info in case it helps your diagnostics.


Whilst I agree that the display temp will not be the same as the HW outlet
temp. The variation will between the two will depend on the flow rate. The
water tempertaure knob sets the _maximum_ DHW temp. This will come into
play only if the flow rate is low enough. Also both sides of the secondary
heat exchanger need to be clear, as has been noted.

OK, I should qualfy that by saying that I had a cloakroom basin hot tap on
full kilter
during my experiments! Also I believe that there is a lower limit to flow
below which
the combi won't operate correctly, so the DHW temperature is never quite
going to
get up to the boiler loop temperature,

Andy.