I've got a fairly new combi boiler and about a month ago the shower started playing up... after about a minute of washing the water temperature goes freezing cold for about 15 seconds, and then it's back to normal before it does it again in another minute or so's time
At the moment it's fine cos of the hot weather, but Im not going to enjoy this during winter!
I've tried resetting the boiler, but that was to no avail.
Any ideas?
Yes. You have long pipe runs and the COLD run is HOT because its in a loft or somewhere..
That coupled with a low acting thermostatic shower mixer, will do exactly what you are seeing. If the cold is actually hot enough, there will be no demand on the combi and it it will NOT come on till the shower 'runs cold' and even then, it has to shunt all the not very hot water out of the pipework to get to the shower.
In winter the thing will simply run COLD until the combi water arrives.
ah ok, that makes sense
is there anything i can do to stop it?
And why didnt it do this last summer?
I have a similar problem with my combi. I've been told that it's due to the secondary heat exchanger being clogged up, either with scale on the DHW side, or with sludge/crap on the heating circuit side. The consequence of this is that heat is not transferred efficiently to the water flowing through the heat exchanger, the water in the heating circuit overheats and the boiler cuts out momentarily until everything cools down. The effect for the person in the shower is that the water runs cold for about five seconds in every sixty. As you say, it's actually quite pleasant at this time of year, but something needs to be done before November comes round.
Apparently it's not that hard to solve. You need to remove the secondary heat exchanger and soak it in the right stuff (Fernox DS3 I believe) to clean it up. If you can pump DS3 through it so much the better. There are some old threads on this which you can find via Google Groups.
If the problem is sludge and oxide in the heating circuit (rather than scale on the DHW side) that's symptomatic of general malaise in your system and you should chemically clean the whole system, flush it through and make sure it has adequate inhibitor in it. It may be that the original installer did not put inhibitor in. Apparently some heating engineers believe that it's not necessary in a sealed system because there is no way air can enter the system. In my limited experience this is rubbish which has more to do with saving them a few quid for the cost of inhibitor.
Anyway, all this advice is hypothetical at the moment. I still haven't got round to sorting mine out yet.
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