Shower running cold when in use (Mira Element)

silvercrown

Ok long story: A few weeks ago we replaced the showerhead on our Mira Element which runs directly off the combi boiler. We have used this shower no problems for 5 years but suddenly when in the shower the hot water turns freezing cold but then back to hot again. This happened a few times until eventually it ran freezing cold and would not heat up. Called out plumber who advised whole new shower - like for like so anoher Mira Element was ordered off Amazon. He fitted this but the problem persisted. He said that the pressure was fine and he was completely baffled. We rang Mira and the plumber told the Mira guy he had considered a PRV and the Mira chap said yes that was his thought - so tiles were smashed up to gain acces to the pipes and a PRV and 2 non return valves fitted. Problem persisted. Basically the boiler wasn't kicking in when the shower was switched on - but the bath and sink taps were working fine and the plumber said all run off the same pipes. He then changed the showerhead - I thought he had replaced everything but he had apparently left the old showerhead on - now we had the new Mira showerhead on and suddenly the water was coming out hot - too hot actually. Plumber did some adjustments and all seemed well, water coming out of all taps was kind of scalding hot but at least the shower was also hot. 2 weeks later and the problem is back - we tried replacing the showerhead again but nothing. We took the showerhead off completely and the water just runs freezing cold. Boiler is either not kicking in at all when shower first turned on - or if it does kick in and you get your hair shampoo'd the boiler cuts out and water goes freezing cold. Running the hot tap in the sink helps keep the water warm but affects the flow. I feel like it has to be boiler issue as it's the boiler cutting out or not even kicking in when the shower is turned on. We have Corgi cover but when the problem was explained to them they said it's not the boiler but must be a shower problem. The plumber said the boiler was fine because we are getting hot water from the bath and sink taps (although I have noticed now that we need to run them on full blast to get the boiler to switch on and for a little while for water to heat up - feel like we are wasting a lot of water) Does anyone think they know the answer? Our only solution seems to be keeping the central heating switched on when we need to use the shower - this means that the water temp will be hot although it isn't a steady temp and does go cold but then hot again. Can't believe we have spent so much money on a new shower and labour but still none the wiser. Oh and the boiler is an Alpha and is almost 11 years old.

beaker141

So if you take the hot tap closest to the boiler, and only turn it on to say half flow - the water doesnt get hot ?

Surely that isolates the shower from the equation and says your boiler has a problem recognising heat demand unless its at quite a high flow rate ?

What would your plumbers response to that be?

I'd guess at the diaphramn in the boiler, or on an old Vaillant I once had the microswitch frequently failed.

Alex1983

What boiler is it, you could have a worn diaphragm which isn't lifting enough to operate the boiler. As just said turn a hot tap on slow and see if it fires the boiler

silvercrown

It's an Alpha combi boiler fitted in June 2006. I turned on the hot tap at the kitchen sink as that is next to the boiler - I did it slowly and the boiler did not kick in until the tap was running practically full force. The water is also taking some time (a minute or so) to heat up once it is running unless the heating is on or has just been on.

beaker141

silvercrown wrote: »

It's an Alpha combi boiler fitted in June 2006. I turned on the hot tap at the kitchen sink as that is next to the boiler - I did it slowly and the boiler did not kick in until the tap was running practically full force. The water is also taking some time (a minute or so) to heat up once it is running unless the heating is on or has just been on.

I'd get your plumber back - focus on the boiler and kitchen tap and once he manages to sort that problem out you'll probably find your shower is fine - other than all the money its cost you so far to fix a shower that wasnt broken !

The problem with some plumbers, is they dont deal in fixing boilers!

Have a google search - this sounds appropriate :-
http://www.boilerguide.co.uk/forum/boiler-central-heating-questions/alpha-cb24-hot-water-problem-332/

Alex1983

Most alpha have a diaphragm that operates a micro switch when you run hot water....it's worn or has a small tear in it and needs replacing

Alex1983

If you can get the boiler model number I could tell you the minimum flow rate the boiler requires to run for hot water

silvercrown

Alex1983 wrote: »

If you can get the boiler model number I could tell you the minimum flow rate the boiler requires to run for hot water

It's an Alpha HE CB33 :-)

Alex1983

Your boiler should fire for hot water with a minimum flow rate of 2.5 litres per minute and it does definitely use a diaphragm style hot water flow switch. You need to get a boiler engineer to replace the diaphragm.

silvercrown

Thank you Alex1983 and beaker141 for taking the time to read this and all your help - am going to get a plumber out and tell them what to look for!!