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 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