Here is my installation..
Vaillant ECOfit Pure 418 18Kw
VR 65 Control Module & VR10 cylinder sensor
VRC 470F Controller + VR 21 Outdoor sensor/DCF77 receiver + Radio
Receiver in boiler.
Sunvic SDM 1901 3-port
Hercol 117L 0.44 Sq M cylinder.
Grunfos Alpha + Circulation pump, variable speed
This is a regular occurrence...
Heating system enabled by time, but not the HW.
Boiler sometimes fires up when there no call for heat, runs for a very
short period, then shuts down. Usually radiators don't become warm,
just warmed up very slightly.
It happened this afternoon, 470 was registering 19.5C indoors, 12.1C
outdoors, with the desired set at 16.5C. Living room radiator
temperature increased to 23.1C.
In the boiler...
B. OFF terms are linked
RT terms are linked
BUS is wired via a two core up to my VR65
The system works perfectly doing all that it should, apart from this
occasional burn, when there seems to be no call or demand for any heat
from the boiler. Its not causing any real problems, it just a curiosity
I am keen to get to the bottom of..
Vaillant themselves seem to not have much idea.
Is it possible that the boiler has gone into "frost-protection" mode
(assuming it has such a mode)? Perhaps there is something faulty and the
temperature sensor believes that the temperature is approaching a level
at which frost might be a problem, and so switches on the boiler for a
short time at a low level.
It sounds like you need to instrument it enough to see what is actually
happening so you can narrow down where the "demand" is coming from (i.e.
somewhere in the external controls, or from within the boiler etc). You
may find you can get all the info you need from the ebus.
Perhaps it is as simple as a dripping hot tap somewhere, causing a
drop in the output water pressure which would signal demand to the boiler?
Or perchance a leaking hot pipe?
ie, the boiler may be working correctly but is indications a
problem somewhere else.
On 13/10/2019 19:01, Gareth's was W7 now W10 Downstairs Computer wrote:
Which part of the Vaillant 400 series *not being a combination boiler*
do you not understand?
It does not supply hot water on demand, it has no DHW flow switch, no
plate heat exchanger... Its a vented heating only boiler that can heat a
hot water cylinder when equipped with suitable external controls, pump,
Most modern combi boilers have a similar feature to keep a small
reservoir of water hot. It enables them to deliver hot water faster on
Harry's boiler is not a combi though, so has no store of tempered water
On Sat, 12 Oct 2019 22:16:34 +0100, Harry Bloomfield, Esq. wrote:
My Vaillant has a "pre-heat" feature which keeps the temperature in the
boiler up so that DHW is "instant" rather than taking a few seconds.
It's enabled by turning the temperature dial to the max, and leaving it
there. A light should come one after a few seconds. Once it's enabled you
can turn the temperature down if needed.
To disable it, you turn the temperature all the way down, and after a few
seconds the light goes off, and you can reset the temperature with the
On Sat, 12 Oct 2019 22:16:34 +0100, Harry Bloomfield, Esq.
I've given up trying to understand my new Vaillant. Twenty years ago,
in an earlier property, we had a Vaillant Ecomax (I think that's what
it was called), condensing boiler, S-plan system, with a simple house
thermostat, a simple programmer controlling on/off time of heating and
DHW, and TRV's on the radiators. When the house thermostat called for
heat, the boiler fired up and the water got hot to somewhere
approaching the maximum set temperature, the rads got similarly hot
until the house thermostat switched off. All very straightforward.
But in my new property, I have an up-to-date Vaillant, an EcoTEC Plus,
with a fancy house thermostat (Honeywell DT90E), a fancy programmer
(Honeywell ST9400C) and TRV's on the rads. It's an unvented DHW
system, unlike the previous property and an S-plan system again, so
only one hot water outlet pipe from the boiler.
The user controls of the Vaillant are limited simply to target water
temperature, set to 70C ATM, and one or two other things that I don't
change. The display shows water temperature, water pressure and what I
can best describe as flame size in the burner.
Only when the boiler is heating the DHW, either alone or together with
the rads, will the flame size show anything other than minimal size,
and the water temperature get anywhere the target maximum. When the
boiler is heating the rads alone, the flame size is close to minimum
all the time, the water temperature seldom gets above 40C and some of
the rads get warm but not hot (a couple hardly even get warm, but that
may be a layout problem).
I've bled all the rads, tried all the lock-shield valves at both fully
open and open just half a turn, but no way can I get the boiler to
raise the water temperature to anything like the target maximum,
except when it's heating the DHW.
It behaves completely differently to the old Vaillant I had in my
previous property. The only thing I fall back on is that it does seem
to work OK, and the bungalow is warm, although it does take a long
time to get up to temperature in the mornings.
Is there anything I can do to get the boiler to heat the water to
something approaching the target temperature when heating the rads, or
should I just be grateful it seems to work, despite it being different
to the one in the earlier property? But I do like to understand what's
That display shows the burner output. The boiler is a modulating boiler
and hence has the capability to match the power output to the actual
load. When running the heating this should result in longer lower
powered burns that more closely match the rate of heat loss from the
building. The intended result is a more constant temperature, and lower
fuel use since the boiler is running in an optimal condensing
temperature range more of the time. It can be particularly effective at
this time of year when you need some heating, but not much.
Vailliant boilers support split temperature operation. That allows them
to use different flow temperatures when running then heating, than those
required for heating DHW. So at this time of year, you may only need a
flow temp of 45 degrees to keep the house comfortable, but you will
still need a flow temp of 65+ to reheat the hot water cylinder. If the
boiler knows which task its doing, it can make the adjustment accordingly.
The variation will be down to system balancing. However the flow temp
limitation does not sound quite right.
On simpler systems, there are two set point temps, that for CH, and a
separate one for DHW. When running the CH, the boiler will be monitoring
the return temp. Should it "see" the temp rise to a point that suggests
not much heat is actually being dissipated by the rads, it will module
the power output down to compensate. On cold days where more heat is
being dumped into the house it should ramp up the output.
More sophisticated controls may include things like weather compensation
with external temp sensors, and also digitised reading of the actual
temp of the house which it can compare to the digitised requested set
temp. Then it will make flow temperature decisions based on all those
factors, as well as possibly including user configurable heat loss
curves that are tailored to the construction of the building.
(i.e. it will actually run the system they way many people think it
works - turning the stat up to 30 will actually make it heat up faster!)
ISTM that somewhere it got the maximum CH flow temp set too low. Some of
the boilers actually have two knobs for the purpose (one CH, one DHW).
Others may be configured in the installer mode from the programmable
stat / controller.
Yup it should be possible to get it to work better.
Your thermostat while flash and digital, is pretty dumb as far as the
boiler is concerned - i.e. it just tells it to turn on or off depending
on the temperature.
The posher vaillant controls sit on the boilers ebus, and tell it the
actual temperature, the desired temp, and also let you access the
boilers configuration and set all the parameters (or which there are
*lots*). This may just be a case of needing to tell the boiler it can
use a higher max flow temp for heating.
What model boiler is it?
Does it have the little square LCD display on the front?
Have you tried switching into installer mode from the front panel to see
what options you have (BTW, the default pass code for installer mode is
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