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]

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]

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

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

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]

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.

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

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 10Kw@55C 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]

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

Yep A43 it is :-(

Ah, suddenly makes sense :-)

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

[snip]

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

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

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

[snip]

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]

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

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 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]

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

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

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]

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]

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

[PaulTheOnlyOne]

replying to Ben Mack, PaulTheOnlyOne wrote: I know this was a long time ago but did the diverting valve sort out the pro lemon?

[The Natural Philosopher]

Only homehub users have te faintest idea to what you are referring.

And home hub users all have questions, but no answers.

So unless you care to say what on earth this is all about, you wont get many replies.

This is usenet, not google groups, not homehubs stolen access to usenet.

[Brian Gaff]

And I have no idea what a professional Lemon is anyway. Brian