No constant hot water

[mileso101]

Hi
Combi Greenstar 24i had a new heat exchanger and an expansion vessel replaced 6 months ago. Today the CH is fine, pressure looks fine about 1.5bar
but the HW when running gets really hot then stops and you can hear the boiler sort of cycling then stop and then it sound like the fan goes off and on repeatedly for a 30 seconds if you stop the HW, after a few seconds the boiler goes quiet and when you run the HW tap again after about 20 seconds you get the same really hot water suddenly then it goes cold then the boiler goes through the same cycle again.

Any idea would be appreciated

 

[Johntheo5]

If the HW flow is greater than say 3 LPM and just going hot and cold then possibly a problem with xover between cold and hot in a tap or shower mixer, you could shut the mains supply at the boiler then open a mixer HW tap or shower and check that no water flows when hot setting selected.

 

[robinbanks]

So you open just the hot tap (on, say, the kitchen sink or the bath) and you open it fully, and you see the temperature cycle between hot and cold?

How many litres per minute do you get when it is doing this? (Time it into a jug, pouring away as necessary to carry on measuring, doesn't have to be hugely accurate)

 

[ianmcd]

open two hot taps running at full flow, at the same time and see if the boiler still cycles

 

[mileso101]

even using two taps it does the same thing takes ages to heat gets very hot then the boiler then you hear the boiler stop burning gas and till it quieten down and there's a sort of cycling and clicking noise noise and the water goes cold and it stays like this until you have stopped running the taps.
you also have to wait for the boiler to stop making a noise completely this takes about 1 minute or reset it, then you can run the hot tap again, other wise you just get a constant clicking noise
see video below

IMG_E5121.MOV

Shared with Dropbox

www.dropbox.com

 

[CBW]

Was it main heat exchanger or plate heat exchanger that was replaced?

 

[Johntheo5]

Could be the diverter valve or as stated above Xover (short circuiting) between hot/cold in a mixer, especially if the plate heat exchanger was changed, shut off the cold mains at the boiler and see if any water still comes from the hot tap(s).

 

[mileso101]

Yes the plate heat exchanger was changed a couple of months ago

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[Johntheo5]

Was the PHE changed because of the same problem?, is so, then IMO either diverter valve
Or mixer problem.

 

[mileso101]

All the hot taps, kitchen, bathroom and the shower show the same issue. the PHE was changed as well as the expansion vessel for a the same issue
we kept the old one just to make sure they were changed.

I'll try the HW with the main feed closed when i get home