Radiators heating when hot water tap is on

[snowpenguin]

A couple of months ago our Worcester Bosch boiler had a leak on the right hand side. We had the right hand block changed.
After this was done, we noticed that our radiators were heating up any time we ran the hot water. Plumber came back out and checked the diverter valve was working, which it was. Then said a new plate heat exchanger should fix the issue.
Had the plate heat exchanger fitted this week, but the issue is still ongoing.
They came back out today to fit a new diverter valve just in case that was the issue, but found that it made no difference.
They've then said it may be a block on the left hand side, so have replaced the pipe there. Just used the hot tap tonight and the radiators are still firing up.
We've currently paid £500 to fix this issue that wasn't there before their original visit, and no one can work out what the issue actually is.
I guess my questions are - any ideas on what could be causing this if not the diverter/plate exchanger/blockage on the right? And where do I go with continuing to pay for all the new parts and fitting as it's at the point of putting us in hardship!

 

[Johntheo5]

Are you still getting full DHW flowrate/temperature with the rads heating as well?, if the CH is programmed off do you get better DHW performance?

 

[snowpenguin]

Are you still getting full DHW flowrate/temperature with the rads heating as well?, if the CH is programmed off do you get better DHW performance?

We are, but its taking longer to heat water when on so better performance when CH is off

 

[Johntheo5]

If you post the boiler model/year someone on here may be able to advise.

 

[Madrab]

If there is enough flow out to the radiators to heat them up when the HW was on, then there's really only one problem that could cause that with a combi and that's a problem with the diverter. Basically the diverter isn't switching over properly and is still allowing flow out to the rads when it should all be diverted through the plate HEX, that's its only job.

So the only thing it can be is the diverter, the diverter motor or the signal the motor is getting from the PCB. A problem with the plate HEX shouldn't cause that to happen.

If they changed the HEX and there was nothing wrong with the old one then I'd ask for the old one to be put back and any money you paid out to be refunded. Same with the diverter. There shouldn't be any cost to you until the problem's fixed! Did they not test that their repair actually sorted the issue, obviously not? It's such an easy test > turn on the HW and does the flow pipe start to heat up, if so problem's still there!!

This is a problem with some so called boiler engineers, they don't have any real diagnostic experience. Proper fault finding needs to be performed before throwing expensive parts at a boiler just to see what sorts it.