Combi boiler (six months old) taps run hot to cold unless CH is on

sguilly

Hi,

Any help appreciated.

I have a 6month old Vaillant 837 combi boiler, was fitted end of November last year, system was power flushed before installation.

Since switching off the central heating a month and a half ago, the water runs from hot to cold after 5mins of use, if left to run the water heats up after a minute, but will run cold again in short period, this process repeats.

Viewing the boiler during this, you can see it switching off the delivery of the water whilst hot water is still being drawn. If I switch the central heating on the issue goes away, only to come back if the CH is turned off again.

This is currently being looked at under warranty, have a third visit organized for next Monday.

First visit was told my shower mixer was probably faulty, which was not the case. Second visit a new heat exchanger was fitted after a more thorough test of the boiler. This appeared to work as hot water ran for a prolonged period without issue. The following morning the problem had returned. Can only guess that during the previous days testing, whatever temporarily corrects the problem when the central heating is on was also in effect when the engineer was testing the boiler, he was maxing out the gas/hot water capabilities of the system.

Was almost an issue getting them out a third time as the boiler engineer had stated the fault was due to a blocked heat exchanger and recommend fitting a filter. They were not going to come out until I had a filter fitted but after escalating to a manager and then explaining the problem was still there, so more than likely not a blocked heat exchanger, and telling them I had a filter on order (which I do), they have agreed to come out.

Is it maybe a thermistor problem?

Thanks,

S

sguilly

PCB was replaced this week but problem was back the following day, have another a forth arranged for tomorrow. Even though it has been frustrating not having the issue diagnosed and fixed, I cannot complain about the promptness of the engineers visits, appointments are scheduled within 24hrs of a call being made.

jmdnnlln

APS switch...I have had a similar problem with a different boiler, It turned out to be a faulty APS switch, though to be honest this is usually the first thing an engineer will look at.
Does the unit provide any error codes?

sguilly

jmdnnlln wrote: »

APS switch...I have had a similar problem with a different boiler, It turned out to be a faulty APS switch, though to be honest this is usually the first thing an engineer will look at.
Does the unit provide any error codes?

They did not mention anything about the APS but will mention it tomorrow, they have scheduled visit to replace the wiring loom in the boiler as they think it is an electrical issue.

Last week the thermistors were replaced.

Update: My type of boiler does not have an APS.

Hoof_Hearted

A decent engineer wouldn't randomly replace parts. It needs proper diagnosing by somebody competent.

sguilly

Well it has been a week since the wiring loom and display were replaced and I am happy to say that I now have consistent hot water. It is hard to say if this was the issue all along as have had a number or parts replaced but hopefully this might help someone in the future.