Is there an obvious explanation to boiler problem?

25 Nov 2006
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United Kingdom
I have a combi boiler which is becoming erratic - hot water downstairs is okay but the upstairs shower doesn't always trigger the hot water demand. When that happens I've been turning on the downstairs hot tap first, then the shower and once it's heated up, slowly turning off the downstairs one. Sometimes the boiler stays on, sometimes it doesn't. I wondered if it was the shower thermostat but it does happen from time to time with the bath (which is separate), although not as often. The mains pressure and the boiler pressure are okay and so is the central heating. Is it more likely a boiler problem or could it be a pipe problem? Thanks.
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What shower is installed?

Do you get hot water from all other taps without issue?
It's a Mira 415. Downstairs is ok, it's usually the shower but it has happened before with the bath. Not as often though.
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Couple of things to start you thinking: -

Is it a new install, I guess not?
What's your flow rate?
Is a flow regulator fitted?
Are the isolating valves fully open?
Is the hot water inlet strainer blocked?
Is the cold water in the bathroom from the mains or is it still fed from a tank?
I suspect this is an older combi and that the water flow detection system is ailing. If so then I have been driving the part around London today!

Good fun these puzzles, much more vague and fun then the Times crossword!

Thanks for the replies, very helpfiul.
The boiler is an Ideal Combi Logic 35.
It was a new replacement boiler when we moved in nearly 3 years ago.
The cold water is mains fed.
Flow rate - approx 13 l/m on cold, 11/m on hot
No flow regulator
Isolating valves are fully open

Teatime said
Is the hot water inlet strainer blocked?
That's new to me. I found the Installation and User Guide on the Mira website and removed the shower cartridge and there isn't one (hot or cold) I can see where they're meant to go but there's nothing there.

So is that likely to be the problem - maybe there's crud inside the cartridge and it's time for a new cartridge (and a service pack for the strainers by the look of it?)
sometimes shower heads get scaled up and reduce the flow too much.

How many litres per minute come out of the shower head?
Approx 9 l/m. Doesn't look or feel like there's anything restricting it, it's quite powerful.

I've just removed the cartridge and turned the water back on to let it flow freely out of the pipes. I don't want to keep doing it because water may get behind the tiles but out of the 3 times I tried it, the boiler only fired up once. So would that suggest it's not a shower problem and it's back to looking at the boiler /pipes?
Flow turbine is a common fault on these, without seeing/testing its what I'd put my money on :mrgreen:

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