Question about a Combi-boiler

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I'm hoping that someone might have an idea in regards to what's going wrong with my combi boiler;

When the hot tap is running- especially with showers, i.e. the cold and hot running at the same time, the hot water will run cold intermittently. Any idea why? I don't think it's a water pressure issue.

Any way that I can try and identify what causes it? There aren't any household appliances that are using water from the mains when this occurs.
 
Same thing happened with my boiler a few months ago and I believe it was the heat exchanger that had to be replaced along with a temperature sensor. British Gas said the cost was going to be around £250, but I managed to convince them to repair it for free under the boiler cover we pay for.
 
No, it's not a thermostatic show mixer. Our shower literally consists of a pair of those taps where you pull up the plunger and it simply mixes the hot and cold water.

It happens even when there isn't demand for cold water i.e. if I run the hot tap for say 5 minutes it'll become cold/luke warm for 30 seconds then get warm again.

Thanks for the help so far!
 
My boiler does the same, I just figured it was because it's old/****. I do have that British Gas cover so might see if I can get them to sort it out.
 
Happens to my boiler every 6-9 months due to scale build up, it's a combination of heat exchanger and something like the "flow mixer valve"
 
I moved into a house just over a month ago with a combi boiler installed. The hot tap would run hot for a while then go cold, then hot etc.
Turns out the flow rate for the hot water was set to higher than the boiler could cope with.
Once the flow rate was reduced it worked perfectly.
 
Had something similar - turned out to be a broken temp sensor.

The boiler was getting as hot as it could and then safety mechs kicked in and turned it off - this cycle just repeated.
 
Thanks everyone.

We've just moved into this property, so I'm hoping I can get the landlord to fix it for free? I assume I'm within my rights to not have to pay for it to be repaired?
 
Also has the boiler been serviced before you moved in?

You should have a service form, showing it's had it's annual service & safety check.
 
Not sure about the annual service but we certainly saw a gas safety certificate. I'll get on to the agents then, argh!

On my combi boiler, there ain't a lot to service, the chap just connect a box of tricks to the service socket, gets a complete run down & then he makes any adjustments needed.:)
 
No idea... I'm going to get mine serviced every year, just to be sure. I hope that problems could be picked up before they become expensive, and I think it'd help with selling my place down the line if I had lots of neat documentation indicating I'd looked after everything. Additionally my 2 year guarantee depends on getting it serviced at the 1 year point. I'd rather spend the £60 a year and not regret it later, knowing the £60 is probably not money well spent.
 
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