Vaillant Turbomax - odd behaviour

[Ben Mack]

Evening all

My Vaillant Turbomax Plus 828E seems to be behaving oddly (don't they
all when the cold weather hits)

When heating is on (i.e. the wallstat demand is on) it heats up until
the flow temp reaches the flow temp setting (currently 65 deg C), then
the burner turns off, the pump does its 5 minute overrun (during which
time the flow temp has dropped to around 30 deg C), then turns off, then
the boiler immediately starts up again, repeating this cycle ad nauseum.

Given that it is on for about 1 minute then off for 5, the house isn't
getting very warm :-(

I thought it should heat up to the target flow temp and then *modulate*
the burner to maintain that flow temp.

During the pump overrun time, if I either turn on a hot tap, or turn the
flow temp up to max (82 deg C) the burner kicks back in.

No fault codes are showing during this.

Is something broken?

I did put some X400 Sludge Remover in the system on Saturday, and though
the old contents drained out clear I guess it could have dislodged
something nasty, but I don't see how that could produce these symptoms.

Any thoughts? Help, I'm freezing......

Cheers

 

[Lee]

Evening all

My Vaillant Turbomax Plus 828E seems to be behaving oddly (don't they
all when the cold weather hits)

When heating is on (i.e. the wallstat demand is on) it heats up until
the flow temp reaches the flow temp setting (currently 65 deg C), then
the burner turns off, the pump does its 5 minute overrun (during which
time the flow temp has dropped to around 30 deg C), then turns off, then
the boiler immediately starts up again, repeating this cycle ad nauseum.

Given that it is on for about 1 minute then off for 5, the house isn't
getting very warm :-(

I thought it should heat up to the target flow temp and then *modulate*
the burner to maintain that flow temp.

During the pump overrun time, if I either turn on a hot tap, or turn the
flow temp up to max (82 deg C) the burner kicks back in.

No fault codes are showing during this.

Is something broken?

Do the radiators actually get hot?
Does a reset help?
It is possible, though maybe not likely, that the diverter valve is stuck.
Happened on our 828e once,with these exact symptoms, although it usually
gets stuck in the CH position rather than DHW.

I must get round to changing the valve, but it's easier to hit reset ,
than drain it down and replace it...

Lee

 

[Ed Sirett]

Do the radiators actually get hot?
Does a reset help?
It is possible, though maybe not likely, that the diverter valve is stuck.
Happened on our 828e once,with these exact symptoms, although it usually
gets stuck in the CH position rather than DHW.

I must get round to changing the valve, but it's easier to hit reset ,
than drain it down and replace it...

I agree there must be a substantial restriction on the flow around the
radiators and a stuck diverter valve could be a possibility.
Also check that both of the boiler flow/return isolators are open?
Also check that at least a few of the radiators are turned on.

 

[Ben Mack]

Thanks for the responses


The feed side does, but the boiler only fires for about a minute which
isn't enough time to heat all the water in the system

No, tried that

I don't think this is the diverter valve (even though I think we have
had trouble with this recently, that's what prompted me to flush the
system). I've tested it by feeling the pipes on each side of the
diverter valve:-
a) when heating circuit is cold, turn on hot tap - boiler fires and
feeds hot water to heat exchanger but not CH
b) when heat exchanger cold, turn on CH - boiler fires and feeds hot
water to CH but not heat exchanger

With what exact symptoms? As you can see above, it isn't stuck in the
DHW position

I've never understood why a reset unsticks a stuck valve - I suspect it
is something broken in the control board (maybe even a software bug)
that isn't telling the diverter valve to move when it should. If that's
the case, changing the valve isn't going to help IMO

I agree there must be a substantial restriction on the flow around the
radiators and a stuck diverter valve could be a possibility.

Why? The boiler heats the water, which circulates and reaches the
radiators.

Also check that both of the boiler flow/return isolators are open?
Checked

Also check that at least a few of the radiators are turned on.

Checked, and heat is getting to them

What I think is broken is that as soon as the boiler reaches the set
flow temperature it turns off the burner, rather than modulating back.

If I set the flow temp to max (82) then it takes a long time to get
there, long enough to heat the entire CH system, so we actually get a
warm house, but this doesn't seem right

The key question is - am I right in thinking that it should modulate the
burner when it reaches the flow temperature setpoint?

Cheers

 

[Lee]

With what exact symptoms? As you can see above, it isn't stuck in the
DHW position

The symptoms you first described, before you gave us more details

In our case the boiler was rushing up to 70C quickly and then shutting
down and firing back up for a minute before shutting down again.
The diverter valve was partially closed, so the flow was restricted.

I've never understood why a reset unsticks a stuck valve - I suspect it
is something broken in the control board (maybe even a software bug)
that isn't telling the diverter valve to move when it should. If that's
the case, changing the valve isn't going to help IMO

There is a rumour that there is/was a manufacturing or design flaw in
the valve.(see if it's marked A43...)
When the boiler is reset the valve is exercised to it's end limits in
both directions, then returned to it's normal position. This is usual
practice for non-feedback steppers, it's how the mpu calibrates the
"initial" position of the stepper.
In the "normal" operating mode, it does not appear to move as far
between the operating positions and apparently sticks.
(In the Turbomax, the stepper motor is linear rather than rotary, but
it's still a stepper motor.)
I used to repair equipment with stepper motors, and this failure mode
was quite common wherever a limited range of movement was employed.

snip

What I think is broken is that as soon as the boiler reaches the set
flow temperature it turns off the burner, rather than modulating back.

If I set the flow temp to max (82) then it takes a long time to get
there, long enough to heat the entire CH system, so we actually get a
warm house, but this doesn't seem right

The key question is - am I right in thinking that it should modulate the
burner when it reaches the flow temperature setpoint?

Cheers

Now, obviously I am not a Vaillant engineer, but
AIUI, it modulates based on the *difference* between flow temp and
return temp.
However, the minimum heat output is still 10.4Kw on the 828e, is the
loading less than that?
Remember that the loading changes with both flow and room temp
If it is, then the return temp may be too high to enable modulation,
based on the target flow temp setting.
Ours does not modulate if the target flow temp is set lower than 55C,
but then the loading from the rads is less than [email protected] which I always
assumed was the reason.

It modulates just fine if the flow temp is set higher

Vaillant's technical line is quite helpful (tell them you are an
installer), if you have the patience to be kept on hold for
an hour..

Lee

 

[Ben Mack]

The symptoms you first described, before you gave us more details

Oops, sorry ;-)

In our case the boiler was rushing up to 70C quickly and then shutting
down and firing back up for a minute before shutting down again.
The diverter valve was partially closed, so the flow was restricted.

Mmmm, although hot water is getting to the radiators, it could be slower
than normal - is there any way to test flow?

There is a rumour that there is/was a manufacturing or design flaw in
the valve.(see if it's marked A43...)

Yep A43 it is :-(

When the boiler is reset the valve is exercised to it's end limits in
both directions, then returned to it's normal position. This is usual
practice for non-feedback steppers, it's how the mpu calibrates the
"initial" position of the stepper.
In the "normal" operating mode, it does not appear to move as far
between the operating positions and apparently sticks.

Ah, suddenly makes sense

(In the Turbomax, the stepper motor is linear rather than rotary, but
it's still a stepper motor.)
I used to repair equipment with stepper motors, and this failure mode
was quite common wherever a limited range of movement was employed.

Yes, we design motor controllers with a similar 'datum' approach, I see
exactly what you mean

[snip]

Now, obviously I am not a Vaillant engineer, but
AIUI, it modulates based on the *difference* between flow temp and
return temp.

Mmm, it does indeed have a NTC sensor each side of the main heat
exchanger

However, the minimum heat output is still 10.4Kw on the 828e, is the
loading less than that?

It could be, I haven't worked it out, though it's cold weather, a poorly
insulated 4 bed terrace with all radiators on, so I doubt it. However at
the point at which it shuts down, the return is still cool (maybe
35degC), as it hasn't been on long enough to pump hot throughout the
system.

Ah, penny drops, it will modulate back the burner as the temperature
difference across the main heat exchanger *increases* - if flow around
the system is restricted the temperature difference will be too great so
it will just turn off the burner

Remember that the loading changes with both flow and room temp
If it is, then the return temp may be too high to enable modulation,
based on the target flow temp setting.

I think the return temp is too *low* for modulation, due to restricted
flow - does that sound feasible?

[snip]

Vaillant's technical line is quite helpful (tell them you are an
installer), if you have the patience to be kept on hold for
an hour..

Will do. I'll also see if I can get a replacement valve out of them as
the boiler is not long out of warranty

I think in the meantime I should drain out the X400 I put in there and
see how much crud it loosened

Thanks Lee

 

[Lee]

Mmmm, although hot water is getting to the radiators, it could be slower
than normal - is there any way to test flow?

I think the usual way is to stick you hand on the rad feeds and see how
quickly they get hot

Ah, penny drops, it will modulate back the burner as the temperature
difference across the main heat exchanger *increases* - if flow around
the system is restricted the temperature difference will be too great so
it will just turn off the burner

The difference in temp between flow and return will also be high if the
system has a large volume of particularly cold water, the software would
have to deal with this too.

I think the return temp is too *low* for modulation, due to restricted
flow - does that sound feasible?

I don't know how the software is written, but it sounds feasible.
It's also reasonable to assume that the allowable difference in temps
will be higher at a higher target flow temp setting, which might explain
the behaviour you are seeing.

Unless you have hundreds of litres of water at 4C, I'd check for a
partial blockage

Lee

 

[Ben Mack]

I think the usual way is to stick you hand on the rad feeds and see how
quickly they get hot

Within a minute, but not sure how that compares to a healthy system

The difference in temp between flow and return will also be high if the
system has a large volume of particularly cold water, the software would
have to deal with this too.

I imagine that the main heat exchanger will be designed so that at the
normal expected flow rate, with the burner full on, there will be a
nominal temperature rise across the exchanger of, say, 10 or 20 deg C,
so the output flow temp won't reach the setpoint (65 deg C in my case)
until the return starts warming up. Mine is getting a 30 to 40 deg C
rise across the exchanger, so I guess restricted flow is the problem

Unless you have hundreds of litres of water at 4C, I'd check for a
partial blockage

Quite

PS just ran the hot tap with the heating off, and noticed the CH output
pipe getting hot, so the diverter valve is definitely playing up, albeit
intermittently

I'm waiting for the Vaillant rep to call me back, to see if I can blag a
replacement valve

 

[Lee]

PS just ran the hot tap with the heating off, and noticed the CH output
pipe getting hot, so the diverter valve is definitely playing up, albeit
intermittently

I'm waiting for the Vaillant rep to call me back, to see if I can blag a
replacement valve

If you can't get a free one, then the cheapest I've seen them so far
(without discount) is £82.85 inc. @ Ezypart (part of Charles Hyde)
Unless someone knows somewhere cheaper....

Lee

 

[Ben Mack]

If you can't get a free one, then the cheapest I've seen them so far
(without discount) is £82.85 inc. @ Ezypart (part of Charles Hyde)

Thanks for that, not too painful, still no call back from Vaillant

 

[Bwel]

Thanks for that, not too painful, still no call back from Vaillant

Old post, but did you ever solve this problem ?

I have the same issue.