Hot water going hot and cold rapidly

[johnny2007]

So, before I throw money away on a new boiler that will put a dent in my finances, better ask here and then look for a boiler engineer willing to repair rather than replace.
Worcester Bosch Greenstar 28i junior, 17 years old.
Central heating works perfectly, all 10 radiators get very hot very quickly and boiler doesn't make any weird noise (in fact, hardly any noise) when running central heating.
However, turn hot water on and I get hot water for about a minute, then hot and cold rapidly, 10 seconds intervals.
Boiler makes a ticking noise when doing this.
This happens with every tap in the house.
7 out of 10 radiators are new, installed in March, flushed before installation and always worked perfectly.
When system was drained, there were no debris, water was clean and of a very pale yellowish colour (inhibitor?).
2 plumbers looked at it and suggested a new boiler due to age (they don't know each other), but as said, I would prefer to repair if possible.
What could it be?
The 2 plumbers wouldn't say.
Thanks

 

[terryplumb]

Blocked plate heat exchanger would be the first suspect.

 

[Johntheo5]

Rads should have been flushed out before and after installation, if not then possible PHE blockage on primary side as stated above, possibly PHE blockage on mains side but HW flow would then be very poor.

 

[johnny2007]

Thanks for your replies.
Central heating was run with fernox F3 for 4 days (4 hours a day) and than drained.
Refilled and emptied again, then finally filled and topped up with F1.
I haven't noticed any reduction in fb low, however this problem started when they fitted a water meter on the street.
Could it be that they dropped some rubbish in the pipes?
They're 1 metre down.

 

[amcluesent]

London is a hard-water area, do you have any water softening on the cold-water feed to the boiler? Erratic domestic hot water could be a plate heat-exchanger that's got scale build-up on the mains water channels causing warm/hot cycling. This is independent of the water in the CH circuits

 

[johnny2007]

Our area fortunately is not hard water.
We don't struggle to keep shower screen transparent, so I always assumed the water was ok compared to other nearby areas where I've seen 40mm pipe blocked by limescale.
The heat exchanger was replaced in 2019, when the boiler was roughly 13 years old.
Could it be blocked again?
Most importantly, can the fault be definitely found onsite?
Or is it a matter of trying to replace one part at the time and hope that the boiler works again?

 

[CBW]

Could be stepper motor on that model, do the rads or flow pipe heat up when running hot water?

 

[johnny2007]

Could be stepper motor on that model, do the rads of flow pipe heat up when running hot water?

Yes, on the first radiator, but that happened from day 1 when they installed the boiler 17 years ago.
I questioned it and was told it's fine

 

[CBW]

I wouldn’t know why it’s happened since day 1, but still could be the issue. A local independent engineer familiar with that boiler should be able to sort you, if you went fixed price repair, that could be expensive for what it might be.

 

[Johntheo5]

If boiler isn't modulating (on DHW) for any reason then flowrate ~ > 11LPM required to keep a 28kw boiler happy.

 

[johnny2007]

If boiler isn't modulating (on DHW) for any reason then flowrate ~ > 11LPM required to keep a 28kw boiler happy.

When they fitted the water meter outside I complained that this problem started.
They measured the flow and I get 22 litre/minutes at outside tap.
Flow seems to be same as before, or at least I haven't noticed any difference.

 

[Johntheo5]

As long as the HW flow from any HW tap is > 11LPM at 60/65C then no problems even if not modulating, but its more than likely that it is, down to ~ 6kw, so a flow rate of only 1.9LPM theoretically required then at 60C, you just might care to measure the flowrate from one HW tap to be sure to be sure and maybe open two HW taps to see if it keeps the boiler happy.

If you have DHW temperature control, turn it to max when testing.