CH Return Temp high, but Return pipe cold?

[asea]

Hi all,

I have a Vaillant EcoTec 837 that is about 4 years old.
HW operation is fine.
CH operation isn't.
I have the CH temp set to 75º.
When there is a demand for CH, the pump starts, then seconds later the burner ignites and the Flow Temp starts to rise quickly. After approx. 20-30 seconds, the Flow Temp has risen to 75-80º and the burner switches off. The pump enters 'over-run' mode and continues to operate for a further 4-5 minutes. Then the pump stops, and approx. 3-5 seconds later the whole cycle starts again. Pump on, burner on, temp rises rapidly, burner off, pump still running, pump off etc.etc.

I during this time, I noticed that the CH Return pipe was still cold, so couldn't work out why the burner was cutting out. I took a look at d.41 and noticed that it's temperature does not reflect the temp of the Return pipe.
When the burner is ignited, the difference between d.40 and d.41 is around 7-8º, ie. when d.40 gets to the desired temp (75º), d.41 is around 68º, but the pipe is cold.

The result is, that at the moment, the radiators in the house get to be 'warm' after about 3½ hours of the boiler being on. None are even remotely hot.

I have taken a look at someone else's Vaillant boiler (albeit a different model), and their burner stays on the whole time they have a request for heat. I believe that (from reading many forum posts) the burner should 'modulate down' the heat output to match the flow of the CH through the boiler, but I think my problem may be related to the fact the boiler thinks the return temp is higher than it actually is??

Any help or insights would be appreciated.

BTW, I have tried reducing the heat output in d.0 down from 28 to 12, but although it fired for longer, the same effect was happening. I have also tried telling the pump to run at top speed (2), but that had marginal effect as well.

Andy

 

[leakydave]

What is the system pressure? Is there any air in the radiators? is your zone valve (if you have one) stuck in the closed position?

Do you have an air lock at the pump which means thr boiler gets hot but hot water is not pumped around the system?

Quite quickly the boiler will reach temp and the burner cut out if you are not having circulation, won't it?

 

[asea]

What is the system pressure? Is there any air in the radiators? is your zone valve (if you have one) stuck in the closed position?

Do you have an air lock at the pump which means thr boiler gets hot but hot water is not pumped around the system?

Quite quickly the boiler will reach temp and the burner cut out if you are not having circulation, won't it?

Thanks LeakyDave!
Pressure is at 1.5, all radiators are bled (water is clear), and I don't believe there is a Zone v/v.
Not sure about the air lock at the pump, but would I be right in saying that I wouldn't get any hot water to the radiators if that was the case?
I have previously run the P.0 Bleed Test Program - would this eliminate an air lock at the pump if there was one?

 

[Terry lambert]

Are the iso valves under the boiler open?

If someone had worked on it they may have shut these and not re opened

 

[Terry lambert]

Have you got trv's everywhere? Are they open?

Lock shield side open?

 

[ALEC1]

the vaillant has an internal by pass and this is what happens some times...the by pass ensures the minimum flow rate through the heat exchanger.

Throttling that back should help...

 

[asea]

the vaillant has an internal by pass and this is what happens some times...the by pass ensures the minimum flow rate through the heat exchanger.

Throttling that back should help...

Thanks all for your responses. ISO values are both open, all rads have trvs and are open on 5, lock shields are open and set for a balanced system.
Only thing I'm not sure of is the comment above about the internal bypass. How do I 'throttle it back' ??
Cheers.

 

[ALEC1]

you shoildn't you need an RGI to do this..

On refelctionI am not so sure, though that it will solve the issueyou though

 

[asea]

Does anyone think that the symptoms I've described above, be caused by some grit or debris stuck in the Automatic Bypass?

I'm trying to understand why the boiler thinks the Return Temp is higher than the Return Pipe feels??

 

[hudzhd1]

Does anyone think that the symptoms I've described above, be caused by some grit or debris stuck in the Automatic Bypass?

I'm trying to understand why the boiler thinks the Return Temp is higher than the Return Pipe feels??

I have the same problem, I've used d.40 and d.41 to check the flow and return temperature and there is not much of a big difference? can anyone help? I will start my own thread!

 

[asea]

Sorted.
Turns out the filter in the Return Pipe was completely clogged, restricting the flow back to the boiler. Quick clean and 5 minutes later, full-on heat in all radiators! Burner modulating like it should and staying on continuously!

 

[hudzhd1]

fixed for me too! there was a problem with the diverter valve - acutator not pushing the rubber bung in the valve right back, so the domestic hot water was being heated rather than all the radiators, all working normally, even at the lowest temperature of 35 - 40 degress.

PS: my boiler is a vaillant ecoTEC Plus 831