Weird tap behaviour when flushing toilet

[Snacker]

I live in a mid 1970s detached house with a new combi boiler system fitted 5 years ago, but with what I suspect is the original bathroom suite and plumbing. I'd like to fix a really annoying problem with the taps in the bathroom sink.

If I flush the toilet then turn on the hot tap in the sink, nothing whatsoever comes out of it until the cistern has filled. If I turn on the hot tap first and then flush the loo, I get a much reduced flow from the hot tap and the cistern fills slightly more slowly as long as the hot tap is running.
The cold tap always works to some degree no matter which order you do things in, but the flow from it is significantly reduced while the cistern is filling.

The bath taps always work fine no matter what, but if you turn either of them on after flushing the loo, the cistern fills much more slowly.

The shower is a mixer shower and behaves similarly to the cold tap in the sink; it always works but flow is significantly reduced as long as the toilet cistern is filling or either the bath or sink taps are being run.

Does anybody know what's going on or am I going to have to map out the pipework first and post a diagram on here?
The behaviour is at least consistent, it's just confusing that certain taps take precedence over the toilet and others don't.

 

[John D v2.0]

Is it all from a tank in the loft? Sounds like the flow is quite restricted upstream of where the hot tank branch from the toilet fill.
Could be debris in the tank or the pipes, or a valve not turned on properly.

 

[terryplumb]

Is your boiler upstairs by any chance ?when converting a 1970 's gravity fed plumbing system ,to everything off the cold mains thru a combi boiler ,the plumber /s will not usually re plumb the whole house. Using as much as the original pipework as possible ,then connecting into it / ammending at the most convenient positions. Very often in loft and where the old hot water cylinder was. As the incoming mains cold has to now serve the hot as well ,on demand ,if you have poor mains pressure what you describe is typical of what you get ,together with convoluted pipe runs and sizes.

 

[JohnD]

for a better job, run a new pipe from the stopcock downstairs to the boiler. Measure the incoming water pipe to the stopcock. It may be blue or black plastic, or it might be copper. What is its diameter?

There may be other things you can do to help the flow.

Look out for any service valves, especially if they look like this

which is a very poor quality one and may leak.

they constrict the flow and can be replaced with better ones.

 

[Snacker]

Thanks for the replies. To answer the questions, the boiler is downstairs in the kitchen and it's a combi boiler with no header tank or hot water cylinder.

Out of interest I also checked flow rates last night and, in isolation, they're actually good. E.g. the water comes out of each bath tap at just under 19l/min if nothing else is on in the bathroom.

What's confusing me is how the toilet overrides the sink hot tap but not the sink cold tap, and both bath taps override the toilet! As far as I can see, both the sink and the bath are closer to the boiler and main stopcock than the toilet in terms of the path of the plumbing.

 

[Hot&Cold]

Have you got a cws on the the loft floor that supplies cold to your WC and a local basin ?

All info helps

 

[Snacker]

No, there's no plumbing in the loft at all. I stripped the last of it out when I did a partial loft conversion not long after moving in, and even that was just leftover bits of pipe that the plumber who fitted the boiler couldn't be bothered to remove, patch up holes in walls and ceilings etc. None of it was actually connected to the water supply.

 

[John D v2.0]

Maybe your stop tap isn't open properly/broken so even though the static pressure is ok, the flow isn't able to keep up?

 

[Snacker]

Maybe your stop tap isn't open properly/broken so even though the static pressure is ok, the flow isn't able to keep up?

I'll check that first then. Do you mean the main stop tap to the house or could there be others elsewhere in the system?

 

[John D v2.0]

I'll check that first then. Do you mean the main stop tap to the house or could there be others elsewhere in the system?

Definitely include the main one, but it's not unknown to have one separating the extension from the original, or outside in the street and under the sink. Or under the old sink location on the end of a lead pipe and under the new sink.