Any ideas? Combi tries to supply DHW after a CW demand

[1APM]

Hi,

I posted here a few weeks ago, but unfortunately still have the same and ongoing problem.

We recently installed a Vaillant 838 combi to replace an aged Gloworm.
When almost any appliance / tap or toilet in the house ceases to uses cold water the boiler fires for about 10 seconds to try to supply DHW.

We have now exposed 95% of all the pipework throughout the house and there are no dead legs present.

We tried a one way valve in the DWH outlet at the boiler - this made no difference whatsoever.

We currently have a one way valve at the CW inlet to the boiler. This made a slight improvement with the boiler only coming on about 8 out of 10 times after a cold water demand although it has also introduced a very noisy water hammer at the stop tap, which I will replace shortly.

We installed a Zilmet water surge stopper in the CW feed at the boiler. This made almost no difference.

We installed a full flow valve in the DHW outlet at the boiler. With this closed i.e all of the house DHW pipework disconnected the problem is still present.

I can change the hot water flow sensitivity on the boiler D88, between two settings, but on the second less sensitive setting you have to demand "lots" of DHW before it will notice a genuine demand.

Standing CW pressure at the boiler is about 3 Bar and drops to about 1 Bar with a CW tap wide open elsewhere in the house.

Any other suggestions or thoughts would be most welcome as this is driving both myself and the original installer to distraction.

Many thanks

 

[Deleted member 105431]

set d88 to 1 using the diagnostics menu, this will sort it.


edit; sorry just read full post.

 

[gas4you]

Have you checked for dead legs on the cold and hot pipe work?

I have usually found that even if there is a dead leg, if it is fitted with a air bleed valve and fully bled it cause no problems.

I have to agree that yours does sound like a real pain in the arris and unfortunately sitting here I cannot offer any useful advise on this other than keep looking.

It does however sound like you have a good installer who is trying to help. That really does make a difference.

DM, as a side question, have there ever been any reports or instances of this problem being down to a faulty boiler or sensor anywhere?

I can't see how the boiler can react to this without the aquasensor spinning

 

[Deleted member 105431]

DM, as a side question, have there ever been any reports or instances of this problem being down to a faulty boiler or sensor anywhere?

I can't see how the boiler can react to this without the aquasensor spinning

I agree that the installer is making an effort to rectify the fault which shows that he is a good one. I can also see that the steps he has already taken make sense and I would follow a similar course of action myself. In answer to g4u I would say I have never had or heard of an aquasensor and/or boiler failing in this way. I have had faulty aquasensors not recognise the flow due to grit in them and more commonly hot and colds being reversed but never this. The dead leg inertia in the supply will cause the impeller to spin momentarily and trigger a demand, this is not a faulty sensor as it recognises the (momentarily)movement of water/pressure. I would suggest you leave as d88 set to 1 now with the warmstart off.


It may be worth checking any mixer taps are faulty and perhaps fitting check valves to them.

 

[1APM]

Hi Gents,

Thanks for the replies.
Setting D88 to 1 does hide the problem, but once changed, you have to open any hot tap wide open for ages before the boiler will supply DHW. A setting somewhere in the middle from Vaillant would have been great!

On the boiler, if you look at the real time value on the DHW flow sensor, it will briefly read 1 2 or up to 3 when any cold tap / appliance demand is ceased.

The installer has been superb and we have had half of the floors in the house up looking for redundant dead legs but this has been going on for weeks. On a positive note this means business has been good for local carpet fitters who frequent the house more than friends and family!

Do you think a substantial water pressure accumulator in the CW supply would fix it once and for all?

Thanks again

 

[Deleted member 105431]

Hi Gents,

Thanks for the replies.
Setting D88 to 1 does hide the problem, but once changed, you have to open any hot tap wide open for ages before the boiler will supply DHW. A setting somewhere in the middle from Vaillant would have been great!

On the boiler, if you look at the real time value on the DHW flow sensor, it will briefly read 1 2 or up to 3 when any cold tap / appliance demand is ceased.

The installer has been superb and we have had half of the floors in the house up looking for redundant dead legs but this has been going on for weeks. On a positive note this means business has been good for local carpet fitters who frequent the house more than friends and family!

Do you think a substantial water pressure accumulator in the CW supply would fix it once and for all?Thanks again

Accumalators are used to store and (tempoarily) boost water flow/pressure in applications where the cold main is poor so I can't see how that would help at all to be honest. I may be wrong though as I'm not an expert on these.

 

[gas4you]

I think you are correct DM, if anything, from my very limited knowledge of these, I would have to guess that higher pressure could make the problem worse

 

[simond]

If you have connected a stop end to the DHW outlet and the flow switch still operates when there is a fluctuation in the cold main, then something is amiss.

Stupid question, have you tried changing the aquasensor?

Have you tried returning the boiler PCB to defaults?

 

[1APM]

Hi Simon,

The boiler has been reset to factory defaults but the Aquasensor hasn't been swapped yet!

Vaillant were very good and did come out to have a quick look but wouldn't troubleshoot anything on the boiler saying that there would be an external install fault with a dead leg.

Three weeks later - there is no dead leg!

In order to eliminate another component I will ask my installer to order up a new aqua sensor to try at my own expense.

Thanks again.

 

[Deleted member 105431]

Hi Simon,

The boiler has been reset to factory defaults but the Aquasensor hasn't been swapped yet!

Vaillant were very good and did come out to have a quick look but wouldn't troubleshoot anything on the boiler saying that there would be an external install fault with a dead leg.

Three weeks later - there is no dead leg!

In order to eliminate another component I will ask my installer to order up a new aqua sensor to try at my own expense.

Thanks again.

I would be very, very surprised if a new aquasensor makes any difference at all.

 

[Bambergaspipe]

We recently installed a Vaillant 838 combi to replace an aged Gloworm.
When almost any appliance / tap or toilet in the house ceases to uses cold water the boiler fires for about 10 seconds to try to supply DHW.

We have now exposed 95% of all the pipework throughout the house and there are no dead legs present.

Did it do it before? have you only replaced combi one for one?

 

[simond]

I too would be surprised if it was an Aquasensor, but if the OP has isolated the HW output on a valve there is almost no chance of water movement within the boiler when a cold tap is opened.

I really can't see how the boiler could detect a cold water drawoff with the HW output isolated. In this scenario I think it should be a Vaillant problem and I would suggest he isolates the HW and gets Vaillant to start suggesting alternatives, and swapping the Aquasensor out may be one of the things they could try.

 

[Bambergaspipe]

I too would be surprised if it was an Aquasensor, but if the OP has isolated the HW output on a valve there is almost no chance of water movement within the boiler when a cold tap is opened.

I really can't see how the boiler could detect a cold water drawoff with the HW output isolated. In this scenario I think it should be a Vaillant problem and I would suggest he isolates the HW and gets Vaillant to start suggesting alternatives, and swapping the Aquasensor out may be one of the things they could try.

Boiler faulty then, said that, been there. possible diverter valve problem flow switch?

 

[gas4you]

Flow through aquasensor could still be there if plate HE was holed. Has the pressure been rising by itself?

Holed plate HE, even fairly new ones is not unheard of on these.

 

[1APM]

Hi All,

Thanks very much for you helpful comments and thoughts.

The previous set-up was a dated conventional boiler with an immersion cylinder.
When this was upgraded to the combi my installer took great care to delete all of the redundant pipe work from the original install and to and from the cylinder.

The Combi has only been fitted for a couple of months and has shown the same symptoms since day one.

In the early days the boiler was fitted with a VRC-430 weather compensator in the boiler. This looked superb, but despite reading the manual from cover to cover endless times and obtaining a training guide from Vaillant quite honestly I couldn’t get to grips with it and it would torment me by turning off the heat when required and spend unfathomable periods of time in over-run or just simply waiting.

The VRC-430 is now dismissed back to its box and I purchased a VRT-360f, which doesn’t look as flashy but does seem far less idiosyncratic.

The pressure on the panel has remained fairly constant throughout. It rises a little when the boiler in running, but not a great amount.

Cheers and thanks