Vaillant Combi boiler hot water problems

[Broadback]

My Vaillant Turbomax Plus boiler is having problems supplying hot water. The central heating is fine but the hot water,though it works,intermittently goes cold. I first noticed it when using the shower, I thought at first that the shower was at fault, however on checking it also happens when running the hot taps. I have checked that no one else is calling for hot water at the same time and have tried adjusting the water temperature to no avail. At a guess it runs hot ok for about 20 seconds then goes cold for 10 seconds. Any ideas please?

[A.Lee]

Does it continually go hot then cold then hot etc? If so, possible hot water heat exchanger blockage - quite common apparently.. Or, Primary heat exchanger fault, or possible pump failing - both less likely, but possible..

Or does it run hot for 20 seconds, the stays luke warm? That would point toward a divertor valve fault.

Alan.

[Broadback]

It alternates hot and cold.

[A.Lee]

I had the same thing, starting around this time last year. DHW Heat Exchanger was cleaned out, then changed after a few months, still no good, turned out to be either the primary heat exchanger or the pump. Power flushing and chemical treatment did not cure the fault.

It was 10 years old, had never been very reliable, so I bit the bullet and had a new boiler. It is possible that a flush out of the DHW HE would cure your fault. On mine, it took 10 minutes or so to get the HE out, so a fairly easy task, no idea what it would be like on your Valiant. Alan.

[Ed Sirett]

One or both sides of the 2ndary heatX. Either Replace with a new unit and then descale "the spare" at your leisure. Or. If you can do without heating and HW for a day then descale the existing unit. You will not get good results trying to descale either side in situ. You may, if very unlucky spring a leak between the primary and domestic sides when you descale.

[geoff]

possibly, but then the flow rate would drop

possibly, but it would also present itself in the CH mode too

I can't remember, but, if it opens the gas valve and then has a certain time window for flame sensing to occur, and its not, that would account for the behaviour he's seeing