Worcester 35CDi Mk 1 - Intermittent cold water.

[XmisterIS]

We bought the house in Dec 2013, it has a Worcester 35CDi boiler providing hot water on demand (i.e. no water tank because of loft extension).

Over the last week or so, we have noticed the following behaviour when taking a shower in the morning:

1) Turn on shower.
2) Boiler switches on burner.
3) Water starts to run hot.
4) Water stays constant temperature for a while.
5) Boiler switches off burner.
6) Water continues to run hot.
7) Boiler switches on burner.
8) Water suddenly runs cold for a few seconds.
9) Water runs hot again.
10) Boiler switches off burner.
11) etc (the hot-cold cycle continues).

The shower is on one side of the bathroom wall with the boiler on the other. (So the shower is very close to the boiler).

Any ideas as to what the problem is and how to fix it?

 

[petit_pablo]

Sounds like a classic blocked HW plate heat exchanger problem.

If you run another tap does that remain constant or does it also behave similarly?

 

[XmisterIS]

The bath tap remains constant - boiler stays on constant too all the while that the bath tap is running. It only does the on/off hot/cold thing with the shower.

 

[petit_pablo]

Pop the shower head off and turn the shower on, lack of flow through the shower head could cause it.

 

[Resident]

Hi,

I just started a thread about the same problem and also got the advice to check if the "plate heat exchange is partially blocked".

The answer is detailed and can be found here:
//www.diynot.com/forums/plumbing/combination-boiler-ghost-like-hot-water-fluctuations.407894/