mixer taps causes HW to turn on and off

[rfisher]

Hello all. This problem has only become noticeable since I installed a mixer tap shower in the bath.
Basically, when a hot tap is on, the combiboiler fires and hot water runs constantly (obviously at a reduced flow compared to the cold but still at what seems to me a reasonable rate). However, when the cold tap is added to this to get the desired temperature, the boiler begins to cycle on and off. It does not turn off for long, but easily long enough for the water to run cold. This also happens at my kitchens mixer tap but that obviously isn't nearly as noticeable as when the shower does it.
So I assume the addition of the cold is increasing the flow pressure against the hot water causing it to go through the boiler so slowly that the boilers thermostat turns the boiler off as the target temp has been hit. Well done, clever boiler. Not much good for me having a steady hot shower though is it?

So is it reasonable that the difference in flow rate/pressure between the hot and cold causes it or does that demonstrate a definite fault somewhere? Could there be a problem with how much water is going through the boiler? Any suggestions as to what my next steps to either fault finding or fixing might be?

I am a reasonably competent amateur in terms of the nuts and bolts of pipes and taps etc, but I'm definitely a rooky when it comes to this end of things so any hints and tips as to what to do next would be gratefully appreciated.
I'm not afraid to call in a grown-up if I have to, but I like to have a bit of a bash first myself!

Details:
Cold tap will do 2 litres in 10 secs
Hot tap will do 2 litres in 16 secs
Combiboiler is Worcester Bosch 24i Junior

Would any other info help?

Many thanks for your time!

 

[footprints]

As you suggest it is a difference in pressure across the boiler, high on the inlet and reduced due to an open tap on the outlet, that causes the boiler to fire for hot water.

Possible causes are.

The model of tap you have chosen has a very high resistance, unlikely on a combi system.

The boiler is faulty a job for a Gas Safe engineer only!

The cold water pressure is poor, maybe fingers crossed it may not have been turned back on fully after the new tap was fitted.

If it is poor pressure you might also find that while running a hot tap then turning on a cold tap elsewhere or flushing the loo (if it's mains) will have the same effect on the hot supply. It would at least exclude problems with the mixer.

 

[rfisher]

Yeah, putting a different tap on/flushing the loo will do it, but not as quickly as the other tap on the mixer.
I've no idea what my pressure is, but are the flow rates I posted an indication of that? Do they suggest poor pressure?

 

[footprints]

Trouble is flow rate and pressure are not the same thing although one can affect the other.

Imagine a 15mm pipe coming down two or three floors from the loft and a bucket of water quickly turned upside down, the bucket would discharge two gallons in about 2 seconds but the pipe which will have far more head of pressure will only discharge a half pint or so in the same time.

I would check all the stopcocks are fully on first, speak to neighbours to see if they have pressure problems and then get an engineer in to check the boiler in that order.

Sorry it's one of those problems with no nice quick answer to sort it.

 

[rfisher]

...many thanks for the replies, but I thought I'd put the resolution here in case it is of future use to any one.

I did have to get in a plumber and he realised that the gas was cutting out when the water reached temperature rather than just regulating down. He then found that there is a dial to adjust the gas flow (I'm sure I'm not using the correct technical terms, do forgive) between a minimum and maximum point. In my case they were both set to the same temperature (naughty original installer!) so when the water reached that temp it would shut off rather than regulate itself correctly. The dials actually are a fiddly pair of two different sized hex-key heads that sit within each other, so were annoying to adjust, but he did and it is all fixed now.
Hurrah!