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Hot Water, Then Cold, Then Hot, Cold and so on! Please help.

Postby JONEZZZ01 » Mon Jan 19, 2009 10:50 pm

I have a Ferroli Combi Boiler and for some time now i have been having the above problem. I wouldn`t be to bothered but we have a Thermostatic Shower coming off the boiler and its a right pain jumping in when its warm but then out when it goes cold. This problem seems to be getting worse and would really appreciate it if somebody could shed some light on the problem.
Many Thanks. JONEZZZ01.
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Simply Build It

Postby stoneyboy » Wed Jan 21, 2009 6:23 pm

If you turn on say the kitchen hot tap and the water runs hot and cold you need to get the boiler checked.
If its just the shower get this checked out.
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Postby TheDoctor5 » Thu Jan 29, 2009 2:17 pm

If you type the key words of your question into our search box to the left of the site you may find the answer is already posted or is in the DIY projects section of the website. Every post goes through a monitoring process and using the search box may speed up your answer.
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Postby marciasdad » Mon Feb 02, 2009 7:16 pm

I have exactly the same problem. i have a mixer shower connected to a combi boiler downstairs. when i turn the hot water kitchen tap on, the water comes out luke warm although i feel the hot water pipe leaving the boiler and the pipework is very hot.
when i operate the shower it works fine. I have worked out that when i turn on the kitchen sink hot tap, cold water is backfeeding from the shower to the kitchen tap making the water luke warm. To stop this from happening I could spend £74 and get a seal pack from Mira to prefect the seal in the shower, or spend £5 on a non return valve and fit it after the tee for the Kitchen tap.
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Postby marciasdad » Mon Feb 02, 2009 7:49 pm

Sorry I should have said I think the cold water supply to your shower is greater than the hot. As a result the cold water is restricting the hot water through the boiler.

This is causing the boiler to switch its self on and off as the boiler overheats and then cools.

This is why you are getting hot then cold water in the shower because the boiler is switching itself on and off throughout the water heating cycle.

You could try fitting a flow regulator to the cold water feed to the shower or a non return valve to the hot water feed or both.

I have a Mira Realm shower which i have had for nearly 15 years. I took the shower apart to clean it and the main fibre washer seal broke. I approached Mira for a spare and was told they do not sell these parts seperatly but as a service pack at £74. I didnt fancy paying £74 for a fibre washer (as the rest of the service pack is completely useless to me) so I made my own washer out of cardboard and used a silicone sealant to take up any gaps. It worked fine, no more leaky shower but it has caused the shower to let by creating the above problem at the kitchen sink.
I hope this helps and offers you a cheap fix.
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Postby rosebery » Tue Feb 03, 2009 1:52 pm


If you do stoneyboys's check and it's the shower you'll most likley need to replace the thermostatic cartridge.

Are you goinbg to say what's going on in reply to the posts already made?
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