Vaillant 937 Flow Sensor

Tipper · 17 Apr 2014

Hi, I'm still having issues with my 937 in that I only get HW at low flow rates. As soon as the flow reaches a higher level something restricts the flow and it all but stops particularly on the 3rd floor of the house!

I've checked both stopcock and boiler inlet filters and they are squeaky clean. I've disconnected the cold feed to check for inlet flow and that's OK too. But when the HW outlet is diverted direct to drain (used a piece of pipe to do so) the flow reduces to a dribble, barely 2.5 l/min!, so whats's going on?

Could the flow sensor be failing and causing a blockage at higher flow rates. Do they usually cause problems?

There's nothing else in the HW circuit that can restrict the flow as far as I can see on the diagram.

Agile · 17 Apr 2014

You need to measure the dynamic pressure at ground level before we can give you any useful advice.

It may not be a boiler issue at all.

Tony

Tipper · 17 Apr 2014

Thanks for your interest Tony.

5bar static pressure showing on the pressure reducing valve gauge. (PRV stripped and cleaned and is working fine.) Good strong CW flow all around the house. Dynamically pressure drops to about 3.5 to 4 bar when the adjacent tap is opened.

10 litre bucket fills in 15 seconds! So 40 l/min at tap adjacent to stopcock.

About 25-30 l/min at the boiler inlet - diverted inlet pipe direct into bucket.

Initially good flow, ie a spurt for a second or two, out of boiler when diverted into the bucket but then drops to around 3 l/min or less and stays there until flow shut off. If then reopened the flow repeats its high then low flow.

I am convinced it is a boiler issue and my question re the flow sensor is about the only thing I can think that is in the 'flow'. The rest is all pipes and the two backpack vessels. The heat exchanger is not directly in the HW flow circuit as the charging pump deals with that circuit and not directly with the HW.

Any thoughts?

Agile · 17 Apr 2014

It is not totally consistent what you have said.

Turn off mains feed and open CW tap and see if the gauge falls to zero.

Then open cold tap and set it to a flow rate that leaved 2.5 Bar on the gauge.

The repeat with hot tap!

It seems as if a boiler inlet filter is blocked.

But that would still allow a fair flow rate on the third floor.

Tony

Tipper · 18 Apr 2014

Agile said:
It is not totally consistent what you have said.

Turn off mains feed and open CW tap and see if the gauge falls to zero.

Then open cold tap and set it to a flow rate that leaved 2.5 Bar on the gauge.

The repeat with hot tap!

It seems as if a boiler inlet filter is blocked.

But that would still allow a fair flow rate on the third floor.

Tony

Thanks again but I'm not sure I understand your comments.

1 the gauge falls to zero when the stopcock is shut.

2 the prv setting gives more than 2.5 bar dynamically so can't see the point of reducing the pressure from that. It used to be set lower to protect the plumbing from high pressures from the town main as the house was a second home and frequently left empty for several weeks. I raised the set pressure to get better pressure and hence flow upstairs.

3 the pressure when the hot tap is opened stays high because something is stopping the flow through the boiler.

4 I spent hours recently getting to and checking an already clear boiler inlet filter so I know that one is clear. I also stripped put the prv and that too was squeaky clean.

Interestingly I have noticed at times that at lower flows (you can only get lower flow at the moment of course!) the HW flow pulses a little as if it has a pulsating restriction in the piping.
Is the flow sensor on these boilers a paddle wheel device? It's certainly shown as one on the diagram in the manual.

I've taken a punt at this being the problem and am picking one up this morning for the grand sum of £22. Only problem today will be getting to it to change it over!!!

Agile · 18 Apr 2014

Agile said:
Then open cold tap and set it to a flow rate that leaves 2.5 Bar on the gauge.

The repeat with hot tap!

Tony

The purpose of this is to estimate what dynamic flow rate is available from your supply!

The flow turbine has a very small plastic turbine disc which spins fast.

Little point in changing it !

Tony

ScottishGasMan · 18 Apr 2014

The 937 has a plastic dip tube in the tank which feeds the Hot water outlet of the boiler, over time these can collapse, causing low flow to the hot, the flow sensor is unlikely to be causing any issue

ScottishGasMan · 18 Apr 2014

I've taken a punt at this being the problem and am picking one up this morning for the grand sum of £22. Only problem today will be getting to it to change it over!!! icon_rolleyes.gif

Dont waste your time, its a nighmare job and wont help

Agile · 18 Apr 2014

We know the odds!

Inlet filter 3:1

Plastic tube 2:1 on.

Flow sensor 100:!

ScottishGasMan · 18 Apr 2014

actually theres one other for the odds, very silly question, but the two grey filling handles underneath, are they in the right position? if the one on the right is in the wrong position it shuts off flow through the boiler.

Still reckon it will be the dip tube

Tipper · 18 Apr 2014

ScottishGasMan said:
The 937 has a plastic dip tube in the tank which feeds the Hot water outlet of the boiler, over time these can collapse, causing low flow to the hot, the flow sensor is unlikely to be causing any issue

Well that looks promising...thanks!

ScottishGasMan · 18 Apr 2014

View media item 71892

Tipper · 18 Apr 2014

Thanks for the diagram.

Is that getatable from underneath or is it a full strip down job?

ScottishGasMan · 18 Apr 2014

can get it from underneath, relatively easy but you will get wet

Tipper · 18 Apr 2014

Thanks for your help.

One final question...my 937 parts manual does not have that part shown or listed so I can't find the Vaillant part number.

Can you assist with the part number from your diagram for part 41? Or a link to the parts manual as mine is obviously out of date?

Thanks

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Vaillant 937 Flow Sensor

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