Vaillant 838 wants to supply DHW during a cold water demand

Firstly many thanks to those who may be able to offer comment and my apologies in advance as a newcomer here, I am not a heating engineer but have tried to detail my experience as best as possible.


I have a recently installed Vaillant Exclusive 838.

I am generally very happy with the product and both the DHW supply and CH feature of the device work well as expected.

I have one minor problem though.

If the 838 is running in CH mode and cold water is run anywhere within the property when the cold water demand subsequently ends the boiler “believes” that there is a fresh DHW demand and switches to try and supply DHW. Almost as soon as this happens it then realises that there is in fact no demand and after a “DHW overrun” period, then re-starts the CH mode.

We have looked at a number of options:

With my installer we have lifted many floors to investigate the existing cold water system and pipework for any possible “dead legs” we have confirmed also that all of the redundant pipework from the old heating system was properly and fully removed.

In order to ensure there is no crossover at perhaps a faulty mixer, we have installed a full flow ball valve in the DHW supply pipe at the boiler and when closed (so there can be no DHW demand), it makes no difference.

The boiler had a single F.78 error – “Break ext dhw sensor” error logged and a Vaillant engineer has visited. I was told this was probably due to a faulty VRC 430 weather compensator that had initially been fitted (and since swapped) and that the problem I experience will be as a result of a probable “dead leg” somewhere in the cold water supply. No other components on the boiler have been replaced.

We have done some very quick tests of cold water pressure. There was a standing pressure of about 3.5 bar, which can drop to just over 1 bar with a high demand cold tap wide open and fluctuates briefly to just over 4 bar when the cold demand is removed and the boiler tries to switch to DHW supply mode.

The Vaillant 838 technical documentation quotes a requirement for 0.5 Bar mains water pressure for max flow rate.


My installer has worked tirelessly to try and correct the fault but weeks on we seem nowhere nearer to correcting the problem.

I would value any thoughts, similar experiences or suggestions.

Many thanks again

I would look for a dead leg on your old HW pipes rather than the cold mains pipe work.

gas4you said:

I would look for a dead leg on your old HW pipes rather than the cold mains pipe work.

Click to expand...

or you could fit a non return valve on the hw outlet from boiler.

Thanks all for your prompt feedback,

We have exposed what we believe to be most of the hot and cold pipework and found nothing as yet. There is one pipe run that we can't easily get to under the recently tiled floor of a bathroom. The services in here haven't been moved since they were installed new for the previous owner so we don't really anticipate a problem there?

If we close a ball valve at the boiler on the DHW supply outlet so there can be no hot flow the problem still shows itself, with this in mind do you think it would still be worth installing a one way valve on the hot outlet?

The vailliant engineer who was very helpful did suggest that we could consider fitting a one way valve on the incoming cold feed, but my installer thought this might make matters worse by potentially adding a slight additional restriction to the flow.

Thanks again for your help

1APM said:

Thanks all for your prompt feedback,

We have exposed what we believe to be most of the hot and cold pipework and found nothing as yet. There is one pipe run that we can't easily get to under the recently tiled floor of a bathroom. The services in here haven't been moved since they were installed new for the previous owner so we don't really anticipate a problem there?

If we close a ball valve at the boiler on the DHW supply outlet so there can be no hot flow the problem still shows itself, with this in mind do you think it would still be worth installing a one way valve on the hot outlet?

The vailliant engineer who was very helpful did suggest that we could consider fitting a one way valve on the incoming cold feed, but my installer thought this might make matters worse by potentially adding a slight additional restriction to the flow.

Thanks again for your help

Click to expand...

the vaillant engineer knows more than the installer, take his advice. by fitting it on the hw outlet you allow thermal expansion back down the cold main, on the inlet you don't but both will work. The restriction to flow will be neglible wherever you fit it.

I always recommend the non return valve is fitted on the DHW outlet to avoid any possibility of damage to the boiler ( which might not be covered by the warrantee.

Interesting question, does anyone know if the flow turbine will indicate a demand with a flow in EITHER direction?

Tony

Hi and thanks,

This afternoon we have installed a one way valve just after the DHW outlet and run water without air from every tap and every appliance.

Unfortunately this hasn't made any difference to the problem!

Any other ideas ?

Agile said:

Interesting question, does anyone know if the flow turbine will indicate a demand with a flow in EITHER direction?

Tony

Click to expand...

no it will not tony but if the warmstart is enabled you may get a slug of hw when the flow is reversed and the boiler will try and reheat the plate h/exch giving very poor hw.

This is not a problem we have seen on the many Vaillants we have installed.

It would appear that expansion is being taken up in a dead leg when the water supply fluctuates.

I would consider fitting a shock arrestor in the cold water main, if the dead leg on the hw cannot be found.

Hi

Look for dead legs (there will be one, check washing machine/dishwasher hoses for trapped air, especially hot fill hoses). If still no luck change D.88 diagnostic code to '1' from '0' (see installation book for instructions on how to do this).

This will raise the trigger point to 3.7 l/min with a short delay, to overcome this problem.

Cheers

The Bunny

I have changed the D.88 set point to 1 and it has indeed “hidden” the problem now with DHW never being called for during a cold-water demand. D.88 was undocumented in the manual that I have so thank you very much for this.

This has of course had the knock on effect of considerably desensitising the DHW mode to a real hot water demand although at present this is still much preferable.

We are still convinced there are no dead legs on the hot pipe work (having thoroughly checked and all appliances are cold fill only with no adjacent hot pipe work) and if we re-route the DHW outlet from the boiler on a short length of pipe to a single tap, bleed then close the tap the problem still presents itself whilst totally isolated from the house pipe work.

Would it still be worthwhile to fit a shock arrestor on the cold feed or perhaps move the one-way valve from the DHW side to the cold inlet?

From experience, is there any chance this could simply be a faulty or oversensitive DHW sensor?

Many thanks again for all of your advice.

Today we have fitted a Zilmet Z160 INOX mini-expansion vessel on the cold feed to the boiler. With the boiler D.88 DHW flow sense value set back at 0 which is the normal setting things are slightly improved however the DHW mode is still called when any cold water flow above a fast trickle is shutoff from either a tap, toilet or appliance.

Every tap and appliance has been thoroughly run free from air and having exposed virtually the entire CW and DHW pipe route there are no visible dead legs.

Would really appreciate any other advice or thoughts to solve this problem.

Thanks