Vaillant Turbomax 824E fan running when cold water tap turns

[terrymac]

Hi

I have the above boler installed which has the following fault

It has developed a fault whereby everytime you turn off the cold water tap it sounds like a click and a relay closing and LED 3 on the front panel (warm start) comes on and the fan runs for a few seconds, the boiler does not ignite and then the system returns back to normal operation. The boiler pressure is about 1.5 bar

Can anyone shed any light on the reason for this. I assume that when you turn a cold tap on the cold water inlet pressure to the boiler will drop and the only item I can see in line to this flow is the aqua sensor

Regards

Terry

 

[Agile]

I think it may be the pump running to perform the warm start preheat.

Normally the boiler would fire up but perhaps theres enough latent heat in the primary heat exchanger to warm the water without needing to run the burner.

Tony

 

[terrymac]

Hi thanks for your reply the warm start is not selected on the boiler. I have since found out that a dead leg in the hot water pipework could cause this. I have just moved a washing machine from the garage and this was hot and cold fill. The hot water pipework was cut back up to above ceiling level as it is no longer needed but there is about a metre of dead leg pipework to a tee piece. I also did a bit of research and it said it is possible to selelct value 1 on D88 setting which should prevent this from happening. I don't know how trapped air in the DHW would affect the cold ater but it seems to be a common cause of the problem

I think it may be the pump running to perform the warm start preheat.

Normally the boiler would fire up but perhaps theres enough latent heat in the primary heat exchanger to warm the water without needing to run the burner.

Tony

 

[Agile]

If there is trapped AIR in the HW pipework AFTER the boiler.............

If you run a COLD water tap for a short time the water PRESSURE in the house will FALL.

The compresed AIR in the HOT pipe will expand and water will run BACK through the boiler towards the cold tap.

When you turn OFF the cold tap the pressure in the cold supply will increase.

Water will then flow THROUGH the boiler into the HW dead leg compressing the air. That flow will turn the boiler turbine.

The boiler turbine has a rotating permanent magnet which induces pulses of voltage in the pickup coil which is counted by the PCB and used to indicate a demand for hot water. Hence the pump and the fan start as the beginning of the ignition sequence.

To prevent, either wait a few days/weeks until the air dissolves or fit a non return valve AFTER the boiler on the DHW outlet pipe.

Tony Glazier

 

[Jinxy]

In other words you were probably right about the deadleg.There is me thinking the dead leg activated the flow switch...

 

[terrymac]

Thanks very much for a perfect explanation to the problem, I will try to cut the deadleg out next time I am home as I am out in Russia at the moment. If not I will fit an NRV to the boiler hot water outlet

If there is trapped AIR in the HW pipework AFTER the boiler.............

If you run a COLD water tap for a short time the water PRESSURE in the house will FALL.

The compresed AIR in the HOT pipe will expand and water will run BACK through the boiler towards the cold tap.

When you turn OFF the cold tap the pressure in the cold supply will increase.

Water will then flow THROUGH the boiler into the HW dead leg compressing the air. That flow will turn the boiler turbine.

The boiler turbine has a rotating permanent magnet which induces pulses of voltage in the pickup coil which is counted by the PCB and used to indicate a demand for hot water. Hence the pump and the fan start as the beginning of the ignition sequence.

To prevent, either wait a few days/weeks until the air dissolves or fit a non return valve AFTER the boiler on the DHW outlet pipe.

Tony Glazier